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Moving Platform Processing Tutorial using Sea-g (C27)


In addition to this Cookbook, it is advisable to visit Moving platform data reduction (C10)

And also view a webinar online:

Intrepid Marine Processing Webinar: http://www.intrepid-geophysics.com/ig/index.php?page=videos


Tutorial Overview

Marine Gravity Survey Processing by 'Sea-g' is composed of 10 Stages. Each Stage can be advanced or revisited by clicking < Back or Next >


Gravity surveys are typically conducted as part of a seismic acquisition program. We describe how to process marine gravity data which has been collected on-board a marine cruise using a LaCoste and Romberg (L&R) meters.

Data formats from Two Spring-Based Relative Gravimeters are supported in 'Sea-g':

Marine Gravity System-6 (MGS-6) with reduced monitors, and its predecessor,

Air-Sea System II (AS-2)


These gravity meters are essentially electro-mechanical devices with the following modifications from the original land based meter: The damping is much higher and the unit is on a gyro-stabilized platform to level the sensor on a moving vehicle. The AS-2 has an auto-feedback system to keep the meter spring tension tending to zero. The newer meter (MGS-6) has a full-force feedback which eliminates most of the effects. Monitors of acceleration and velocity of the motion of the stabilized platform are recorded along with a logged, filtered observed gravity, a spring tension and cross-coupling.

The processing of marine or aircraft gravity data from these LaCoste and Romberg meters follows the process derived from papers published by LaCoste (1967) and Valliant (1986). The main objective is to use the monitor data to correct for the motion of the meter caused by turbulence or ocean waves.

Factory calibration of the meters (AS-2/MGS-6) gives 7/2 monitor gain coefficients used to compute zero-correlation between the observed gravity and the monitors.

Tutorial Dataset

The data we use for this tutorial comprises a cruise with three N-S lines crossing three E-W lines, for nine intersections or cross-overs. While monitor gains are recorded many times, they do not change during a survey. The data is in Air-Sea System II format (AS-2). AS-2 produces two types of data files, with file extensions: ".DAT" (data file, observed for a cruise) and ".ENV" (environment file, for coefficients).

The data file has 21 comma separated records, recorded once per second. The environment file, recorded once every 10 seconds, contains information about the system conditions during data acquisition, and includes the k-factor (to convert volts to gravity meter units), and gain coefficients for the monitors.


This tool is designed to handle a single cruise of data, i.e., see the Cruise definition within the Flowchart below. At the end of marine gravity survey processing in 'Sea-g' the output is: FreeAir, Levelled FreeAir and difference grids. Stepping through this Tutorial will allow you to see a demonstration on how to turn your gravity profiles into a Free Air gravity grid. The processed gravity databases include versions for split-lines, cross-over error analysis, and levelled data. The database format is either in ..DIR (Intrepid format) or .gdb (Oasis format). A gravity processing report is also generated (.rpt).

The generated database(s) can be loaded again at another time, in processing software such as INTREPID.


At the conclusion of the 11 Stages of ‘Sea-g’, you can launch the INTREPID project manager, and use INTREPID tools to undertake further data QC, Levelling, Gridding and Editing.

System Requirements

This Sea-g tool is designed to work on a cruise of gravity data lasting for up to 15 days. The tool should easily process, filter and display that amount of one second data in various graphical contexts on a standard laptop. The 15 days could simply be extended by using more memory, if this proves an issue.

Our software runs on most “traditional” x64 machines.


64bit: support x86_64 instruction set;

AMD K8 series, K10 series and above, include all Athlon, Sampron, Opteron, Turion, Phenom etc.Intel Dual Core CPU or above, 2GHz, with more than 3 MB of L2/L3 Cache;


At least 8GB, the more the better.


This depends mainly on the size of your dataset. our software requires about 1GB disk space.

Graphics Card

A dedicated graphics card from nVidia (GeForce 7xxx series or above) or AMD (Radeon 4xxx series or above) is required. We do not recommend any form of embedded Intel GPUs.

Operating Systems

We currently support Microsoft Windows 7 / 8 / 8.1/10 64-bit (Please request 32-bit if required).

For Microsoft 8.1 and above, please install in Compatibility Mode (*usually: right-mouse-click from the .msi installer file to access Compatibility Mode choices)

Also, common LINUX operating systems are supported and should show no appreciable differences in how the tool operates.


Sea-g_icon.pngOnce the software is installed from the .msi file and licensed, an Icon appears on
your desktop. Double-click the icon to launch 'Sea-g'.

Alternatively, you can launch the software from the installation directory, for example

C:/Intrepid/Sea-g5.6.1_x64 is the {install_path} this notation is consistent through INTREPID Documentation

The data for this tutorial are located in the installer files directory

(or your alternatively selected 'Sea-g' installation location)

They are called S-133_subset.DAT and S-133_subset.ENV

  1. Start ‘Sea-g’ Software:
  2. Click on NEXT
  3. MGLC4_001.png

    Stage 1. Select Project

    Stage 1 of the tutorial firstly allows the user to Select a Recent Project (a recent taskfile) or Create a New Project (a new Working Directory) for automatically saving all of your processing sub-products and products: Databases, Grids, Reports and Taskfiles.

    For example, default Report files are named:




    Warning: No spaces ! Generally, spaces in a working directory path or dataset filename may cause failures, so avoid these.

    Reports will pop-up at the end of the Stage 11 of the wizard and will also be saved permanently in the working directory.

    Taskfiles allow you to save and load choices that you made whilst going through the guided workflow of the wizard in a previous session. They are saved automatically at the end of a completed session. Taskfiles are very useful for repeating workflows.

    Import Formats. ‘Sea-g’ supports two current meter types from the Micro-g LaCoste range.

    Output Gravity Units can be selected.

  4. Create New Project (select button from top right).
  5. Choose a default processing directory Choose the Folder, and then type a Folder Name. For example C:/.../*/output Save
  6. Note a default task file will be created:
  7. C:/.../*/output/gravity-import.task
  8. Next
  9. Choose your Meter Type from a drop-down menu. For this Tutorial we will select Micro-g LaCoste Air Sea Gravity System II corresponding with the gravity meter used in this survey.
  10. MGLC4_002.png
  11. Choose your Units. For this tutorial leave 'Output Gravity Units' : mGal
  12. When finished select NEXT

< Return to Stage 1. Select Project to explore other stages.

Stage 2. AirSea System II Import Settings

This Stage of the wizard is for importing the marine data and specifying some important settings.

  1. Use the Add Files button to input ASCII files S-133_subset.DAT (observed raw data) located in the installer directory : {install_path}/sample_data/cookbooks/gravity/moving_platform/MP_4_Rawdata (or your alternatively selected 'Sea-g' installation location)
  2. Note the S-133_subset.ENV (environment meta data file) located in the same directory, will load automatically.

    All spatial data remains in GEODETIC projection (i.e. Latitude and Longitude)

    Typically, each day of data comes in a separate ASCII file, and you can add or remove the data for a cruise, on a day by day basis as you choose. Partial cruise processing is supported.

    [Use the Add Files button, to add additional days of data to composite, if required. This tutorial has only one file.]

  3. Set the 4 Settings that are survey specific items:
  4. (Note the following values are specific to the data of this tutorial. You must determine your own unique values prior to processing your own data.)

    • UTC Offset (Hours offset from GMT used for tidal correction): 8.0 Hours
    • Meter Absolute Value (Reference absolute gravity, transfer from the onshore value):
      979824.4 mGal
    • Meter Still Value (Still gravity reading, at the dockside): 10589.2 mGal
    • DateTime as per date file start date/time. For this tutorial, assume just one still reading, so leave this as the default value.

    The theoretical gravity is based on the WGS 84 ellipsoid and uses Somigliana closed form equation. The user should make sure that the land absolute meter values are given in the same system.

    When only one still meter reading is known then the DateTime is not required and Meter readings are converted to absolute gravity by adding (Absolute - Still) to each reading. When a second still meter reading is taken at the end of the cruise then the two Still Readings and their corresponding DateTime’s are used to convert each Meter reading to absolute gravity, using linear interpolation (based on the DateTime of each reading). The second Still Reading does not need to be same location as the first Still Reading.

  5. When finished select NEXT
  6. MGLC4_005.png

    < Return to Stage 1. Select Project

    Stage 3. Gravity Meter Settings

    This Stage is for review or edit of the calibration data for the tutorial dataset acquired with:
    gravity meter S-133/V1.0.

    In this case, the calibrations of the gravity meters settings already exist in the .ENV file and were imported in Stage 2. We can keep the default values. However there is the possibility to enter them manually (over-ride them) if necessary at this stage.

    Notes for MGS-6 data

    • Only one monitor is needed for MGS-6
    • The global Scale Factor can be used to fine-tune a screw table look-up
    • The ‘Sea-g’ tool shows differing screens, depending upon meter type. Whilst many of the screens are the same as shown in this tutorial, there are some differences for MGS-6 usage.

    The MGS-6 meter has a design feature, to allow the operator on the ship to see a Raw Gravity reading, that mimics the actual gravity. This is done by setting a Gravity Reference field during operations. This offset is removed during processing, to recover the meter reading, processed in the normal manner, then restored after corrections and scaling are applied.

    The Air-Sea 2 meter setting screen

  7. Calibration Settings: Global Scale Factor defaults to 1.0, change to 0.9978
  8. (This is a calibration number particular to each meter, available from the manufacturer.)

  9. Meter settings: AL, AX, AX2, VCC etc., leave the factory values unchanged.
  10. Select NEXT

< Return to Stage 1. Select Project to explore other stages.

Stage 4. Output Settings

This Stage is for selecting the gravity output directory and the output database name. At the end of the Marine Gravity Survey Processing (Stages 1 to 11) the output will be FreeAir (FA) gravity, Levelled FA, and difference grids. The processed gravity databases will include versions for split-lines, cross-over error analysis, and levelled data. The grid file formats will be user-specified. The database format is either ..DIR (Intrepid format) or .gdb (Oasis format).

The database(s) can be loaded again at another time, in processing software such as INTREPID, allowing the user to perform other processing, filtering or gridding workflows. (See APPENDIX 2 -Task Files Created by the ‘Sea-g’ Wizard that may be used directly in underlying INTREPID software)

Export of reporting style graphics are available from a number of the wizard pages. These enable items such as a Colour Table and Title to be embedded in a Map or Profile View for graphic export.

  1. Leave defaults, select NEXT
  2. Alternatively, Browse to another output directory and enter filenames for Report File and Output Database, then select NEXT.


    +te: The output database is the cruise database prior to splitting. The stub of this name is also used, with suffixes, to capture the split of the cruise into lines, to null the values at the cruise turns, and the Cross-over database.


    < Return to Stage 1. Select Project to explore other stages.

    Stage 5. Profiles

    When you click Next in Stage 4 above, the software imports the raw marine cruise data and automatically displays the profiles of the Spr_Ten (Spring Tension), the Beam (Beam position) and the VCC (Cross Coupling) Monitors.



    a) The units of Spring Tension are in "meter units" of relative mGals

    b) The raw Beam and the VCC are very noisy


    From this Profile Display Window, it is possible to choose which monitor to visualise.

  3. Change the display of the profiles by clicking Inspector icon (top-right). The control display parameter list appears on the right. MGLC4_013c.png
  4. Warning: You may need to resize this other display in order to see the controls.

  5. Click on Show Map to display the survey Map or Cruise Path.
  6. By default, the map is shown when entering this stage.

  7. Standard shortcut keys (relevant to the Map View) can also be used to adjust views. These correspond to the menus appearing in the top-left hand corner:
    • 0 = Reset View
    • s = Select
    • d = Digitise
    • p = Pan
    • z = Zoom
    • r = Rotate (when available)

    Extra shortcut on Map View only: Holding down Space and Left Mouse performs a Pan and over-rides the current mode. (Releasing Space goes back to previous mode.)

  8. Experiment with selecting different profile views by selecting a different 'Data' choice from a drop-down menu inside the 'Data' box, for any number of “Plots’ within available ‘Groups’, hence displaying the data for a different monitor. You can also add more ‘Plots’ and/or add more ‘Groups’ on the same page ('+' ‘Plot’ or '+' ‘Group’). Or change the colour of a Plot (select 'Colour').
  9. Tick on ‘Show Legend’ in the top, upper right-hand corner.
  10. When finished, select NEXT.

< Return to Stage 1. Select Project to explore other stages.


Stage 6. Data Filtering

The monitors contain a lot of high frequency noise. You must filter this signal so that the frequency content matches the observed raw gravity, prior to the corrections steps.

When selecting NEXT at the conclusion of Stage 5 above, a Fuller Filter (by default) is automatically applied to all the monitors and the software displays the results of this filter for:

Spr_Ten, Beam and VCC (in red) are displayed above the same profiles of Raw data (blue)

Tip: It is possible to change the displays using the same options (inspector at top-right), as available in Stage 5 above.

  1. In this Stage there are choices of 3 Filters that can be applied to the monitors: Exact Blackman, Fuller and RC. From the lower-left of the Stage-Window, select the Filter Type drop-down menu.
  2. Choose a new filter (RC or Exact Blackman), then select Reapply Filter from the button, to the left. (You will notice a very slight change in the display, when re-filtered, this change is most visible in the VCC profile.)
  3. Return the Filter-Type to "Fuller" and again select Reapply Filter from the button, to the left.
  4. Leave the default Fuller Filter of 120(s) and Press NEXT

Background reading - filters

Exact Blackman Filter

This is a Finite Impulse Response (FIR) low-pass filter (FFT method) with a cut-off frequency of 1/t Hz, where t is the filter length in seconds. For marine applications, the filter length is usually set to 120s or 180s.

Fuller Filter

This is a classic geophysical FIR low-pass filter (Spatial convolution method) used in various scenarios. Similar to the Exact Blackman filter, the Fuller filter has a cut-off frequency of 1/t Hz, where t is the filter length in seconds. For marine applications, the filter length is usually set to 120s or 180s.

RC Filter

This is an Infinite Impulse Response (IIR) low-pass filter. This is a digital implementation of a hardware filter circuit consisting of a resistor and a capacitor. Contrary to the Exact Blackman and the Fuller filter, the RC filter is not symmetric. This leads to phase shifts in the signal, i.e. the signal peaks are shifted after filtering. To remove this phase shift the filter should be applied in several stages (ideally a multiple of two) with each stage going backwards or forwards through the signal. Any phase shift introduced by this filter will be corrected in the final processing step. The cut-off frequency is 1/(t*n) Hz, where t is the filter length in seconds and n the number of filter stages. For marine applications, the filter length is usually set to 20s or 30s and the number of filter stages is set to 6.

Filter comparison


Figure 1 The illustration above shows the spatial impulse response of the 120s Exact Blackman filter (red line), the 120s Fuller filter (blue line) and an RC filter (green line) with 6 stages of 20s each. The impulse response drops to zero for both the Exact Blackman and the Fuller filter, whereas the impulse response of the RC filter goes towards zero asymptotically. Note that the impulse response of the 6 stage 20s RC filter is symmetric, i.e. no phase shifts are introduced in this case.


Figure 2 (below right) shows the Fourier transform of the spatial impulse response. All three filters have the same cut-off frequency of 1/120 Hz, but differ in the way they suppress high-frequency noise. The Exact Blackman filter (red line) suppresses frequencies in the range 0.01 Hz to 0.06 Hz more strongly than the Fuller (blue line) and RC filter (green line). Conversely, the Fuller and RC filters suppress frequencies above 0.6 Hz more strongly than the Exact Blackman filter.


Figure 3 (below) shows a typical output of the three filters after processing noisy marine data. The RC filter output (green line) is smooth, but generally shows the smallest amplitude range, i.e. exhibits the strongest damping of the signal. The Exact Blackman filter (red line) has the largest amplitude range, but some high-frequency noise is still detectable in the signal. The output of the Fuller filter (blue line) lies mostly between the RC and Exact Blackman fil­ter. Note that the differences between the filters are relatively small (of the order of 0.1-0.2 mGal) and that the high frequency noise detectable in the signal filtered with the Exact Blackman filter is of the order of 0.01 mGal.

< Return to Stage 1. Select Project to explore other stages.

Stage 7. Data Corrections

This section of the tutorial covers corrections for Eötvös, Earth tide, Platform motion and Drift.

The Stabilized Platform Correction performs several processes, as follows:


‘Sea-g’ converts the raw gravity reading as measured in meter counter units to relative mGal using a lookup table and interpolating between ranges.

Earth tide correction

The Longman Earth Tide model approximately describes the Sun and Moon tidal forces at any point and time on Earth. It is not numerically a large error correction - typically 0.1 mGal applies.

Eötvös correction

The largest source of error is caused by the Eötvös effect. This correction compensates for the velocity of the gravimeter across the rotating Earth, and it can typically be ±15 mGal. ‘Sea-g’ uses the calculated ship or aircraft velocity, and cruise-bearing to make this correction.

Steps performed are as follows:

  • Application of factory supplied coefficients to adjust for the effects of the filtered monitor readings of the gravity meter movements on the observations.
  • Eötvös correction.

The profiles automatically displayed are the Gravity Reconstructed, the Eötvös Correction and the final Gravity Corrected.

  1. Plot the Spring Tension in Red over the same profile for the Gravity Reconstructed (top profile)
    Select displayed data icon (top-right corner)
  2. In the Group 1: Click on the + below Plot 1 (allowing additional data in the same profile).
  3. MGLC4_030a.png

    (A Plot 2 is created in Group 1)

  4. In the 'Data' field, select Spring Tension Spr_Ten and make the Colour field Red.
  5. When finished select NEXT

< Return to Stage 1. Select Project to explore other stages.



With this profile view in Stage 7, you can now compare all results on a Profile Plot of observed gravity for the cruise (blue) compared to the spring-tension (red). This shows that the NS lines and the EW Lines have a 70 mGal offset due to the Eötvös correction, which is significant in this dataset.

Note: No calibration functions, such as checking the factory settings for the meter, are offered in ‘Sea-g’ as these checks would usually be undertaken back in the factory.

Stage 8. Edit out spikes in corrected gravity caused by cruise turns or other spurious readings

This stage of the Tutorial is for the manual removal of spikes in the corrected Gravity profiles, which may correspond to a “turn” of the ship, or a spurious reading to be nulled (eg., a cable on the sea-floor), or an unusual ship acceleration. The process can be accomplished in possible 3 ways.

A - Profile View method

B - Map View methods

C - Table View method

Each method is discussed in turn below.

Warning: In Stage 9 another editing style is available: for the purpose of “Ignoring” entire lines. This editing style cannot occur until after Stage 8, because new lines numbers are created at “null” breaks, at the completion of Stage 8 editing.

A - Profile View method for editing

The first way is to mouse-select the spike in the Profile View, verify it is a "turn" of the ship by simultaneously viewing the same selection in the Map View, and then by applying "Null Selected Data" (from the bottom left-hand button).

In the figure below: Right side: Presents a Map View of the Marine Survey and the trace of the ship. Selecting the required mode from the top-left, you can Zoom ‘in’ and ‘out’ in the Map View by using the mouse wheel. Pan the survey by drag and drop, using left click of the Mouse. Select in the Map View is used to locate turns in the profile view.

Left-side: Presents the Corrected Gravity Profile for the entire Survey. You can Zoom ‘in’ and ‘out’ in the Profile View on an area selected, with the left-mouse as a pointer, and then using the wheel of the mouse. Use Reset View button or press “0” to return to default view. To help identify ships turns, the velocity North and East fields are also shown. (See Section Below, on Managing the Velocity Component Display to place these together on the same Profile.)

Null Selected Data edits can be reversed or re-instated using the Undo or Redo buttons (all buttons are located lower, left). This goes back through all changes of the current session.

Warning: The Undo and Redo buttons will operate only in the current session. If you perform a Save & Exit to finish the current session, this action will modify and save your databases.


Summary Edit Controls - Profile View Method

  1. In the gravity_corrected channel of Profile View, select a zone of a spike in the signal, by dragging of the left-mouse cursor over the spike (drag select). Zoom In for more accuracy. A blue polygon will generate, on the profile for the gravity_corrected channel, see image. Yellow polygons generate on the same line-location of other channels).
  2. MGLC4_03600700.pngMGLC4_03500701.png
  3. Releasing the left-mouse cursor fixes the selection polygon. To re-select a different polygon, simply repeat the Drag-select in Step 1 (do not choose Null Selected Data).
  4. Observe and check the corresponding selected "turn" in Map view, select Show Map from the bottom-right side button of the Profile View.
  5. MGLC4_03700702.png
  6. When you are happy with your polygon choice, choose Null Selected Data on the Stage 8 profile panel (bottom-left).MGLC4_03800703.png
  7. Reproduce this process for all spikes that correspond with turns in the ship cruise path, or other spurious spikes in the data, you may elect to clean. This process has to be done manually with the expertise of the user. Undo and Redo buttons are available in the current session (only) to assist.
  8. MGLC4_037a.pngMGLC4_039.png

    The gravity_corrected channel is the only field in the database which is affected. You should verify this by also popping up the Table View, locating the gravity_corrected field, and noting NULLS where your edits have been made.

    Warning: Do not NULL the velocity field profile data as this does not influence the subsequent required processing step of splitting the cruise into discrete lines.

  9. Standard shortcut keys are also available for use during editing in the Table and Map Views:
    • del = Null Selected Data
    • ctrl+z = Undo
    • ctrl+y = Redo
  10. If you cannot get the job done in one session, the Save & Exit button records the edit history to date, allowing you to re-run the session up to this point and re-call (but not undo) all the committed edits so that you can continue. The edit history is recorded in the Taskfile.
  11. On re-launching ‘Sea-g’ you are asked if you wish to resume your last session, where you left the editing. If you agree, Stage 8 profile view returns with all current edits highlighted. Continue as before.
  12. When all the spikes are nulled press NEXT.

Managing the Velocity Component Display

An convenient layout for doing the editing work, is suggested below, for when the velocity components are the primary way to find the cruise turns. This allows both velocity directions to be displayed on a single profile panel.

  1. Delete the Group 3 profile plot
  2. ‘+’ Add a plot to Group 2 (add Velocity_north)
  3. Change the channel being displayed to Velocity_east
  4. Change the colour to RED

B - Map View Methods - using ‘Select’ mode or ‘Digitise Polygon’ mode

Other ways to edit out turns are available using the Map View. Either use Digitse Polygon mode to create a polygon, or use Select mode with shift-select, or drag-select.

Summary Edit Controls - Map View Method: Select

Select mode

Click on the Select mode (top left, Map) and either select, shift-select, or drag-select signal records in the Map View.

  1. You can click select any reading. You will see your selection reflected in the other 2 views. This is a typical way of visually locating a turn, and then finding the corresponding portion of the line data you need to NULL.
  2. You can shift-select in Map View. Do this by left-clicking on the start of a selection and then shift-left-clicking on the end of the selection. The selected signal records are then highlighted. This method allows you to Zoom or Pan, mid-way through your selection.
  3. You can drag-select in Map View. Do this by holding down the left mouse cursor on a trace, and dragging the mouse around a turn. The start of the selection is shown as a dashed crosshair and the end of the selection is dragged as you move the mouse and is also shown as a dashed crosshair. When you release left mouse button the selected signal records are highlighted.
  4. Use the Null Selected Data key to NULL the signal records selected. The marker size of your selected portion also dynamically scales when you zoom.

  5. Multi-select: In Map View (and also Table View) you can also make a multiple set of selections before applying the Null Selected Data key. Multi-select works with:
    • the Left-mouse-cursor with the Control Key (for a point)
    • the Left-mouse-cursor with the Shift Keys (click twice for a range)
  6. Standard shortcut keys (to Map View) can also be used. These correspond to the menus appearing in the top-left corner of the Map window:
    • s = Select
    • d = Digitise
    • p = Pan
    • z = Zoom
    • 0 = Reset View
    • r = Rotate (when available)
  7. Another short-cut: Holding down Space and Left Mouse performs a Pan and over-rides the current mode. (Release of Space goes back to previous mode.)

Summary Edit Controls - Map View Method: Digitise Polygon

Digitise Polygon Mode to select points inside or outside a polygon

Click on the Digitise mode (top left, Map) and start digitising points in a masking polygon. Use the left click of the mouse to add a point to the polygon. Use the right click popup to finish the polygon. There are two ways to close the polygon which allow you to select points either inside or outside the polygon.

  1. Right click using the mouse-cursor to select the “Close & Select Inside” option from the popup. This closes the polygon and selects all points of the displayed dataset that lie inside the polygon.
  2. Right click using the mouse-cursor to select the “Close & Select Outside” option from the popup. This closes the polygon and selects all points of the displayed dataset that lie outside the polygon.
  3. Using this way to select a set of points then lets you NULL those that are either inside the polygon or outside the polygon. Use the Null Selected Data key to do this once your choice is selected.

The next image shows that the polygon has been closed by selecting the Close & Select Outside option from the right click mouse popup options. All the points outside the polygon have been selected in this case.


Next, NULL-ing the selection using the Null Selected Data key leaves only the points that where inside the polygon. This outcome is shown on the next image below.


C - Table View Method for editing

Warning: Not a recommend method for the current Tutorial exercise with S133_subset.* file

Table View is the third main method for visualizing/editing your database, giving you access to all the fields/channels and all the values. There can be situations where the main method of editing the marine gravity signal using the vessel velocity as a guide, does not work, or is inappropriate. One such situation is when the vessel is stationary, or, for what ever reason, the GPS data is clearly wrong or faulty. In this case, using the Table View to edit your data may be preferred.

Warning: Be Careful! You are able to do bulk / multi-select nulls and edits on any of the fields, in Table View, so take care.

Multi-select: In Table View (and also Map View) you can make a multiple set of selections before applying the Null Selected Data key. Multi-select works with:

-the Left-mouse-cursor with the ctrl Key (for a point)

-the Left-mouse-cursor with the ctrl + shift Keys (for a range)

Summary Edit Controls - Table View Method

  1. You need to ensure you are editing the active signal channel (column) i.e., gravity_corrected field, even if you are using the GPS location records as the guide for the reason why you are nulling the records.
  2. You can drag-select readings down a column, and simultaneously see your selection reflected in the Map View.
  3. You can also click on the first record in a series, then shift-select a second record, to complete the range of data in your selection of a column.
  4. Mostly the keyboard shortcuts follow standard conventions. For example, use the del key while in the Table View to NULL a portion of the signal.
  • del = Null Selected Data
  • ctrl+z = Undo
  • ctrl+y = Redo

Click on Show Table and the following view will appear.


Optional Exercise with an alternative data set (NOT for this Tutorial session)

In another session, when you alternatively load S133.dat file, use the Summary Edit Controls above to edit the data in Table View.

We provide an example dataset with some of the issues mentioned above (e.g., the vessel is stationery).

The S133.dat file, not the subset, contains more than 100 bad GPS records at around 5900. This causes a bad Map View, as the Latitude/Longitude extents now include (0,0). If you go through the tutorial again, but start with this alternative dataset, you will encounter this issue.

The fields or channels in the database, as it now stands, are shown across the top of the page.

Many extra fields are introduced, as part of the underlying conventions for treating Gravity data in a unified and systematic way by Geoscience Australia. These extra fields should not concern you in any way, but they should also make sense and their purpose should be more or less obvious.

The fields we need to concern ourselves with are Longitude, Latitude and gravity_corrected.

All the intermediate fields used to capture the raw, filtered, correction factors are still present. In the figure shown above, all near zero and zero values for Latitude and Longitude are located, and a selection of the gravity_corrected field for these records is made. The Null Selected Data key is then used to NULL out these records, so that they are permanently removed from the reduced gravity data to Free Air processing.

The full dataset contains a few other small navigational artefacts that can now be trimmed, so that it looks very similar to the S133_subset dataset used in the main tutorial.

Select NEXT

< Return to Stage 1. Select Project to explore other stages.

Stage 9. Line Analysis

This stage of the wizard allows the user to actively compare up to 7 lines on the same profile view.

Actions from Stage 8 to Stage 9:

  • A new version of the dataset has now been written out which contains:
  • Splits at null locations, and
  • Ordering by Line numbers (and Group numbers) *split..DIR
  • A split cruise table of all lines now appears left.
  • By Default, the Minimum Samples setting is 10. This refers to 10 samples making up a line segment. Line segments which are very short (comprising less than 10 samples) will not be used in Leveling, as they will be deemed spurious, perhaps the left-over data after nulling cruise-turns. The Minimum Samples setting can be changed, and the Reset Table button applied.
  1. First obtain a spatial view (in Maps view) of the Line numbers, choose SHOW MAP.
  2. Select Inspector (upper right of Map panel)
  3. Select Configure Presentation. Modify the presentation as required, we recommend adjusting the font-size of the line numbers.
  4. MGLC4_LineAnalysis-1.png

    Choose lines numbers to compare that are parallel, ideally in repeat locations (This tutorial dataset does not contain repeat lines, but this feature is demonstrated at this stage, nonetheless.).

    The Primary Line of the Repeat Set

  5. Back on the Profile View’s Split Cruise Table: Select the row to set the Primary Line of the “Repeat Set” (first selected is the Primary, in red). The first and last points of the Primary Line define a common axis for overlaying other lines in the Repeat Set.
  6. The x axis on this Profile View is distance along the mean bearing of the line segment. From this perspective, Zero(0) distance is shown on the left-side, the start of the Primary Line.

  7. Tolerance in metres This is a setting found at the bottom left of the Stage 9 page. It sets up a tolerance corridor (or bounding box) around the mean bearing of the Primary Line segment. The distance is set to either side of the mean bearing line, as viewed in Map section. Samples/readings outside of this tolerance zone will not be shown. Tolerance can be set in metres or DMS (degrees, minutes, seconds).
  8. Build a Repeat Line Set for analysis, and then select lines to “Ignore” from Leveling

  9. On the Profile View’s Split Cruise table: Add or Remove lines from the Repeat Set by selecting them while holding down the ctrl key.
  10. Based on your line comparison analysis, tick Ignore for lines that you do not require to be included in the levelling and gridding.

Select NEXT

< Return to Stage 1. Select Project to explore other stages.

Stage 10. Levelling

When the Stage 10 wizard page launches, the levelling is now completed.

In this Stage we have an opportunity to review the levelling results or re-level after adjusting the polynomial degree.

An average misclosure report for the entire database is reported - with and without levelling - and stated, top left of the page.

  • These are processes that have just occurred:
  • applying Free Air correction on the corrected gravity data which are not-nulled;
  • splitting the cruise data into a new lines database, using the NULLED gravity_corrected channel
  • locating all the cross-overs between one line and another
  • estimating the mis-closures of the survey at the cross-overs for the Free Air channel
  • saving a cross-over database
  • applying a line profile adjustment

Optionally you may now choose to:

  • See views of before and after line profile adjustments
  • Adjusting the polynomial degree and re-level.
  • Examine the cross-over point on a profile.
  • Ignore selected lines for the levelling process (ignore, and re-level)

In this tutorial, at this Stage, the Group and Line profile controls now become active, because the splits have occurred, and we now have multiple lines in the database.

Use the Group toggle button to progress back and forward through the line database.

In the profile view below, the blue is un-levelled and the red is levelled.

The Minimum Angle control button weeds out near parallel lines when cross overs are being calculated. 30 degrees is the default minimum incident angle between lines. Repeat lines therefore do not contribute cross-overs between themselves. Repeat lines are recorded like any other line in the database.

Adjusting the Polynomial degree and re-leveling

By default, a zero order line profile adjust is performed ie DC only.

  1. The Polynomial Degree can be set to 1 (lower-left options).
  2. Apply Relevel (lower-left options) and the whole survey will be re-levelled.

All the above listed processes are repeated when you do this.


In this profile view, the blue is unlevelled and the red is levelled.

Examine the cross-over point on a profile

You may have to toggle forward and backwards through your lines to find each of these (toggle the Group, upper right)

Zoom into a portion of a profile where a Cross-over is indicated by an x on the profile plot.

See image below for the Zoomed display:

The convention used in Profile display is that the ‘x’ value indicated is from the crossing line (before levelling), the black line shows the magnitude of the mis-closure, with a tie into its actual point on the unlevelled blue profile.

When the adjusted red line lies in the range between the two observed Free Air values it means that levelling has improved that particular crossover.

Ignore selected lines for the levelling process (Ignore, and Re-level)

You may decide to ignore a line (and re-level), based on poor cross-overs, or because the line is not within your survey design area.

  1. Toggle forward and backwards through your lines to navigate to your target line (toggle the Line, upper right)
  2. Check your line selection in the linked Map View (Show Map, lower right)
  3. Return to Profile View with the target line number selected, top right -
  4. Then tick-on the Ignore this line box (middle-lower Dialogue box). And apply the Re-level.

A number of lines can be selected for “Ignore this line”. Re-levelling will be applied to the whole survey, without the ignored lines from your current session.

Again blue is unlevelled and the red is levelled, in the image below (See Cross-overs discussion above)


Select NEXT

< Return to Stage 1. Select Project to explore other stages.

Stage 11. Free-Air Anomaly Grid and other products

The next stage of 'Sea-g' will:

  • Create a Grid of the unlevelled Free Air channel, at a default cell size (no Nulls/ ignored Lines).
  • Create a Grid of the levelled Free Air channel, at a default cell size (no Nulls/ ignored Lines).
  • Create the difference grid between these two above.

These default grids are now visualised, and also saved to your Working Directory.

Should you relevel?

If after the first grid creation, it is apparent that spikes are still present in your data, or cross-over errors are still present, then you can:

  1. Select the "Back" button to return to Stage 10, where you can revise the levelling steps.

recommence to regenerate your grid.

Should you re-grid?

If you want to over-write the default grids, and repeat with your chosen cell size, use the steps below:

  1. Set the cell size required for the grid (in metres or DMS).
  2. Then select the Regenerate Grid button.
  3. MGLC4_043a.png
  4. Select the Show Diff Grid button (lower right) to also see the levelled Free-Air minus the unlevelled Free Air. This shows in a grid view, what adjustment has been made to your survey data by the levelling process. The unlevelled Free Air grid is also present, but not currently examinable within the wizard environment.
  5. When finished click NEXT

Configuring Presentation Maps

Map View comes with the option to Configure Presentation thus create presentation maps including such items as a Colour bar, Title, Line numbers etc.

  1. Open the “Inspector (top right corner) to allow selection of these extra features in your display.
  2. You may need to Pan the layout, to position the grid so that elements such as the Colour bar and Title are well positioned. Use a combination of the Pan and Zoom features to achieve this.
  3. You can also Show line numbers (tick) for the newly created line database. A default font size, which is suitable for a more typical survey size is set. You may want to change it for this Tutoral Dataset which is based on a relatively a small survey size. Choose to reduce the Font size in the popup menu.
  4. You may also wish to adjust the image enhancement via the Colour lookup, Data clip controller. These controls are located in the far right panel selections.

See image below.


The inspector (top right corner) will launch the popup menu shown, for adjusting Presentation Maps

There are other small adjustments possible via the controls, e.g., background colour.

Print - Save

Use the Print button (upper Menus) to save images of your composed map in RGB GeoTiff, jpeg or png format.

Note, this is a quick method of gathering a report plot, there is no control of set scale, and what you see on the screen is what will be displayed in the image.

Appendix 3 contains samples of how to do a full cartographic workflow, using some scripting. However, this older style of Map Composition is soon to be replaced (late 2016) in both ‘sea-g’ and INTREPID.

In the interim, we suggest that your high quality, high resolution grid files (saved in your Working Directory) are suitable for map-compilation in 3rd party software if specialised large scale maps are required.


Finalization Stage - Processing Completed

All the stages to process a cruise of marine gravity data to a Free Air grid are now complete.

  1. The Final Window provides the 'Sea-g' user the opportunity to:
    • View the Final Report,
    • Launch the INTREPID Project Manager, to review the final database, cross-over database. This is also the recommended approach for applying more processes, including QC using Editing and Gridding menus which are part of the 'Sea-g' product.
    • Export the Corrected and Levelled Free Air grids in a number of formats
    • Export the Corrected and Levelled Free Air datasets in a number of formats
  2. We recommend you check-on the first two options, as a minimum choice.
  3. MGLC4_044.png
  4. The grid formats are just a selection, which can be used to bring the results into any downstream interpretation or mapping environment.
  5. The created datasets can be exported to Geosoft GDB. The datasets can also be exported to other formats later, using the Intrepid Export tool.
  6. Select FINISH


After completion of the 11 stages of 'Sea-g' a Marine Processing Report (gravity_processing.rpt) and a QC Report (<survey>_splitQC_Report.txt) are optionally popped-up in a Notepad for Review. You can save or immediately review the contents of the reports to recall all the steps and settings that have been performed. You can repeat these saved processes that you are licensed for using the INTREPID tools directly. The report from each one of these processes shows up in the combined Marine Processing Report file, neatly separated. The image below shows the start of the import process report.

All details and settings are recorded for the benefit of the user.


The levelling report is also very informative.

**************************************************** Intrepid Marine Level v5.6.0 INTBTA-3098 for Windows (x64)
by PELAGIAN optimised build 857f5cf6a9e6
Start processing - 21/05/2015 10:51:52 **************************************************** Initial check for required inputs to do a legal run doing LevelLines... Primary input dataset C:/Intrepid/Sea-g5.6.0_x64/sample_data/cookbooks/gravity/moving_platform/Rawdata/S-133_subset_rev2_split, input Z field
Apply Corrections to rest of line and saving to
Calculating INTERNAL SURVEY CROSSOVERS Start group 0 surveyA is fid 0.000 dist 0.00000, start X,Y 120.45297, 35.87522, rec 0
Start group 1 surveyA is fid 8.000 dist 0.00047, start X,Y 120.45325, 35.87457, rec 3
Start group 2 surveyA is fid 73.000 dist 0.00394, start X,Y 120.45541, 35.86312, rec 139
Start group 3 surveyA is fid 693.000 dist 0.03152, start X,Y 120.46763, 35.83036, rec 156
Start group 4 surveyA is fid 1183.000 dist 0.05561, start X,Y 120.50487, 35.83237, rec 254
Start group 5 surveyA is fid 3486.000 dist 0.18123, start X,Y 120.63837, 35.82864, rec 201
Start group 6 surveyA is fid 4156.000 dist 0.20584, start X,Y 120.63393, 35.80078, rec 261
Start group 7 surveyA is fid 6759.000 dist 0.33885, start X,Y 120.49571, 35.78821, rec 357
Start group 8 surveyA is fid 7426.000 dist 0.35456, start X,Y 120.53364, 35.76715, rec 713
Start group 9 surveyA is fid 9416.000 dist 0.44419, start X,Y 120.63098, 35.75806, rec 396
Start group 10 surveyA is fid 10294.000 dist 0.46862, start X,Y 120.60885, 35.74006, rec 221
Start group 11 surveyA is fid 12731.000 dist 0.58840, start X,Y 120.60225, 35.86621, rec 190
Start group 12 surveyA is fid 13391.000 dist 0.61536, start X,Y 120.56704, 35.85853, rec 302
Start group 13 surveyA is fid 16074.000 dist 0.73128, start X,Y 120.55795, 35.73361, rec 333
Start group 14 surveyA is fid 16776.000 dist 0.75327, start X,Y 120.52747, 35.74484, rec 314
Start group 15 surveyA is fid 19126.000 dist 0.86562, start X,Y 120.51911, 35.86716, rec 243
Start group 16 surveyA is fid 19697.000 dist 0.88680, start X,Y 120.48906, 35.89037, rec 269
For this levelling problem - TOTAL GROUPS = 17 Calc for SurveyA ngroups=17 against reference FreeAir doing LevelLines internal only... Average Shift (Average Adjustment) -0.313228 Saving Calculated Marine cross-overs to
Cross over save for surveyA 0 Apply Z Corrections to rest of line and saving to
Note that an output field
C:/Intrepid/Sea-g5.6.0_x64/sample_data/cookbooks/gravity/moving_platform/Rawdata/S-133_subset_rev2_split..DIR/FreeAir_level_nearest is created. This is a distance of observed point to nearest crossover
*****No crossovers for line 0 *****No crossovers for line 1 *****No crossovers for line 2 *****No crossovers for line 3 *****No crossovers for line 5 *****No crossovers for line 7 *****No crossovers for line 9 *****No crossovers for line 11 *****No crossovers for line 13 *****No crossovers for line 15 *****No crossovers for line 16 All finished ok **************************************************** End processing - 21/05/2015 10:51:53 - C:/Users/Documents/gravity_processing.rpt ****************************************************

The QC Report gives a statistical analysis of the input marine gravity data, calibrations, and levelling processes. An example is shown below:

GQC Report General Survey Stats Processed Data Files: C:\Intrepid\Sea-g5.6.2_x64\sample_data\cookbooks\gravity\moving_platform\Rawdata\S-133_subset.DAT (30/09/2003 1:56:39) (30/09/2003 7:29:58) Extent of raw data: Longitude: 120:27:10.7 120:38:29.1 Latitude: 35:43:58.5 35:53:30.1 Line km: 0 Number of Samples: 0 Number of Nulls: 0 Extent of levelled data: Longitude: 120:28:11.9 120:38:02.7 Latitude: 35:44:13.5 35:53:13.6 Line km: 78 Number of Samples: 13661 Number of Nulls: 0 AirSeaII Gravity Meter Calibrations Meter S/N: S-133/V1.0 Meter GlobalScaleFactor: 0.9978 Meter BeamScaleFactor: 0.09636 Meter AL_MON: -0.0458 Meter AX_MON: 0.05 Meter AX2_MON: 0.33 Meter VCC_MON: -0.525 Meter VE_MON: 1.064 Meter XACC_MON: 0.0 Meter XACC2_MON: 0.0 Meter LACC_MON: 0.0 Meter LACC2_MON: 0.0 Meter sampling rate: 1.0 Processing Parameters Still Readings: 979824.4 10589.2 30/09/2003 1:56:39 UTC_Offset: 8.0 Filter Type: FULLER_FIR Filter Length: 120 Levelling method: LevelSurface Polynomial Degree: 0 Minimum Angle: 30.0 Gridding Cellsize: 0:00:05.50 degrees (138.1 metres at 35:48:05.4 degrees latitude) Line Stats Table Line Bearing Line-km Start Lat/Lon End Lat/Lon Min FA Max FA Mean FA Stddev FA Ignored (mgal) (mgal) (mgal) (mgal) 1 90 1.6 35:49:49.5/120:28:11.9 35:49:49.8/120:29:15.0 -3.7167 9.3275 5.0831 3.8549 2 90 10.1 35:49:58.3/120:31:02.3 35:49:58.8/120:37:43.7 2.3143 7.2016 5.9352 1.1428 3 270 7.9 35:48:02.5/120:37:46.3 35:47:59.9/120:32:32.7 -3.2288 10.4931 3.5450 3.4162 4 269 1.5 35:47:59.8/120:31:05.5 35:47:59.1/120:30:08.2 7.7926 9.2487 8.6873 0.3522 5 174 2.3 35:47:32.6/120:29:43.0 35:46:17.6/120:29:51.3 5.1870 12.4530 8.6893 1.6929 6 90 9.1 35:46:01.4/120:31:45.1 35:46:00.6/120:37:48.4 -2.5960 7.9056 3.4327 3.4575 7 219 3.3 35:45:41.2/120:38:02.7 35:44:13.5/120:36:50.5 -6.5973 10.0395 -4.5184 2.4655 8 359 12.8 35:44:30.9/120:36:31.3 35:51:24.2/120:36:26.6 -6.4432 7.0422 2.2737 3.1637 9 274 1.6 35:52:00.7/120:35:39.9 35:52:05.8/120:34:35.3 1.7232 2.7711 2.2828 0.3435 10 180 12.2 35:51:03.8/120:33:59.7 35:44:28.3/120:34:00.3 0.2556 8.6974 5.6389 1.8333 11 359 12.8 35:44:16.8/120:31:42.8 35:51:10.1/120:31:33.7 4.3921 10.9553 8.0523 1.9136 * 12 308 3.3 35:52:00.8/120:31:10.1 35:53:13.6/120:29:36.6 -4.3512 2.4867 -1.9096 1.7674 Total Lines: 12 Valid Lines: 11 Ignored Lines: 1 Levelling Residuals CrossoverID Lines A/B Lat/Lon Closure Before Level Closer After Level Ignored 1 2/ 11 35:49:58.8/120:31:33.6 -0.05483301 -0.05483301 * 2 2/ 10 35:49:58.8/120:34:00.5 0.43376139 -0.04269681 3 2/ 8 35:49:58.7/120:36:26.7 -0.22272069 0.18608457 4 3/ 8 35:47:59.8/120:36:26.9 -1.23998118 -0.12156035 5 3/ 10 35:48:00.0/120:34:01.6 -0.41663329 -0.18347590 6 6/ 10 35:46:01.3/120:34:00.3 0.96807517 0.60179534 7 6/ 8 35:46:00.9/120:36:26.9 -1.04701569 -0.52803207 Total Crossovers: 7 Active Crossovers: 6 Ignored Crossovers: 1 Misclosure before Levelling Min Closure: -1.23998118 Max Closure: 0.96807517 Mean Closure: -0.25408572 Stdev Closure: 0.84793209 Misclosure after Levelling Min Closure: -0.52803207 Max Closure: 0.60179534 Mean Closure: -0.01464754 Stdev Closure: 0.38067808

Outputs viewed from the INTREPID Project Manager

After completion of the 10 stages of 'Sea-g', the INTREPID Project Manager tool pops-up. It defaults to the directory in which the user has created their new database.The final outputs are a database file (..DIR) and a grid file (.ers). These, and the report (.rpt) are located in the directory set in Stage 4 of 'Sea-g'.

  1. Within the top-panel of the INTREPID Project Manager, mouse-select the final database created from the 'Sea-g' tutorial above (..DIR file).
  2. The Lower Panel shows an entry for all fields in the database.

  3. Next, select the 'Free Air' channel in 'Fields' within the Lower Panel.
  4. Go through the menu tabs above, in turn: Statistics, Histogram, MetaData and Thumbnail. Note which database and field you are currently quering.
  5. The first time user of this tool should also select one of the output grids and view the available Statistics, Histogram, MetaData and Thumbnail.

Line Statistics

The Intrepid project manager also gives easy access to Line based statistics for any field in a line or point based dataset, such as S-133_subset_rev2..DIR.

Each newly created line, is given a number starting from 1.0, in the field SPLITLINE ( note its alias LineNumber). The original cruise Line Number is also still available.

  1. Choose the gravity_reconstruct field, click on the Statistics tab, and at the bottom, you can see a Button for LineStats. This reports on the statistics for this field on a line by line basis. The report can then be easily exported to ASCII CSV, using the button provided.
  2. MGLC4_067.png
  3. A more comprehensive Line Statistics report, also suitable for direct use as a CSV file, can be accomplished using the same Project Manager tool, but by accessing this required function via a command line to access the batch function -lstats
  4. MGLC4_063.png
  5. Launch the command window this way, and the DOS prompt inherits access to all the Intrepid tools you are currently licensed to have.
  6. Type fmanager -lstats S-133_subset_rev2..DIR, and a report file entitled fmanager.rpt is created that contains the required comprehensive line statistics for every field in the database. The file is specially formatted to allow easy import into such tools as Microsoft Excel. The report for the first line, after all the processing from this tutorial is below. The Group-by fields are reported first.

Cross-Over Database


There is a newly created cross-over points database created by the marine level tool. There are many fields in this database, and the important measure of the error in the repeatability of the marine gravity measurement, is the Misclosure field.

You can use 3D Explore to open this database and visualize it. The figure above shows the cross-over database being chosen and the Misclosure field statistics.

Note: INTREPID has the convention that each line shares the misclosure with the crossing line, so that the mean is always zero i.e. for every positive misclosure, there is a negative matching one. The units below are in mGal


Histogram of Misclosure, showing the symmetry of the values.


The above illustrates a table view of the cross-over database, with the Misclosure field highlighted.

There may be a novel Vector field also calculated named Plunge, which captures the 3D surface plunge of the field at the cross-over point, in a vectorial sense (unit direction cosines).


The capacity to remember what you have done earlier is stored in either the main task file ( edit history) or in a user preference file ( dataset, directories, layouts)

At the end of the Sea-g processing, the database and grid files, should produce data that is close to EGM08 gravity modelling.

There is a chance that you may have to increase the heap space for Sea-g. This need arises when you have limited memory and you wish to process a large cruise of data, say > >15 days. By default, the operating system and JAVA allocate 1/4 of the available physical memory to allowable heap space. You can increase this limit by setting an environment variable INTREPID_JVM_XMX.

For example, set INTREPID_JVM_XMX=4096,

This sets the heap space to 4 GigaBytes, and on an 8 GByte memory machine, this would double your default allowance. This must be done prior to starting a session.

APPENDIX 1 -Task Files Created by the ‘Sea-g’ Wizard that record the session

At the heart of the “command set” that binds ‘Sea-g’ wizard together are task files. Each task file that is created and executed by the wizard at the various stages, remains behind, after successful processing.

The principal work is accomplished with the gravity-import.task. This file saves the state of the session (all settings, parameters and operations). A selection from this file is reproduced below. This includes the instructions to NULL portions of the cruise signal at the turns. It is a useful reference.

Additionally, a batch processing re-enactment of all of the steps in the gravity-import.task file can also be accomplished by the power user, once familiarity is achieved. However, this is only possible after the purchase of additional INTREPID software modules (other than ‘Sea-g’). See Appendix 2 for task files which CAN be re-run in the INTREPID software from the Project Manager, without purchasing additional INTREPID software modules.

Example Task File (part of the full gravity-import.task file):

IntrepidTask { Gravity { RunType: AIRSEAIIField GravityDatabase: "C:\\Tutorial\\outputs\\S-133_subset..DIR" TerrainType: OCEAN_SURFACE CorrectedGravity: "gravity_corrected" ReconstructedCC: "CC_reconstruct" FreeAir: "FreeAir" Eotvos: "eotvos_correction" EarthTide: "tide_correction" ReconstructedGravity: "gravity_reconstruct" Marine { MeterNumber: "S-133/V1.0" UTC_Offset: 8.0 AbsoluteValue: 978523.41 StillValue: 10017.2 StillDateTime: 2003.7454274162862 Sampling_Period: 1.0 IgnoreEotvos: false IgnoreDecorrelation: false FilterParameters { FilterType: FULLER_FIR FilterWidth: 120 FilterSteps: 6 } ReplaceData { DataField: "gravity_corrected" Group: 0 Range { From: 705.0 To: 939.0 Value: -5.0E75 } } ... } OutputUnits: MILLIGALS DatumType: WGS84 OutputDatum: "WGS84" ReportFile: "C:\\Tutorial\\outputs\\gravity_processing.rpt" SavedStage: 0 Reduction { InputFieldData: "C:\\Tutorial\\AIRSEAII\\S-133_subset.DAT" InputENVFile: "C:\\Tutorial\\AIRSEAII\\S-133_subset.ENV" SkipEarthTideCorrection: false GPSProjection { map_projection: "GEODETIC" spatial_datum: "WGS84" }

APPENDIX 2 -Task Files Created by the ‘Sea-g’ Wizard that may be used directly in underlying INTREPID software

Task files which can be re-run in the INTREPID software from the Project Manager, without purchasing additional INTREPID software modules (other than ‘Sea-g’), are listed below. They include: line-splitting, levelling, and gridding:

These sub-processes are also controlled by task files.

To illustrate this: some of the Steps from Stage 9 that are required to level a cruise are accomplished by using two Task Files, executed one at a time:



A - Split Cruise Task

  1. From the INTREPID Project Manager, select (left-mouse-click):
  2. S-133_subset_split.task, then right-click > Run Job File
  3. The task specification (.task) will perform the operation for the newly created cruise dataset in a single batch task.

  4. Note that the NULLS manually created in 'Sea-g' are used to guide the line splitting. A new output line database is created (s-133_subset_split..DIR)
  5. After completion of the task, a 'HISTORY' file will be added to the directory (above).
IntrepidTask { SplitCruise { Input: "C:\\Intrepid\\Sea-g5.6.0_x64\\sample_data\\cookbooks\\gravity\\moving_platform\\Rawdata\\S-133_subset..DIR" ZIN: "FreeAir" Output:
SplitOnNullsOnly: true } }

The above picture shows the success pop-up after running the split cruise task file.


Highlight, by mouse-click, the newly created split lines database, and choose the Metadata tab. There are 17 lines created by the process.


Now choose the Thumbnail tab, and choose the FreeAir field. You can see the lines have been successfully broken at the turns.

B - Re-Gridding

The Gridding step can also be redone, using the task file that is generated by the ‘Sea-g’ wizard.

Grid the data and review the grid. The task specification (.task) file S-133_subset_gridding.task will perform the operation using an algorithm suitable for variable density data. Examine the grid using a sun angle view to locate levelling errors. You may wish to grid the unlevelled data and compare the two grids side by side. If you find errors, you may need to examine the raw data more closely. Further processing options would be to

  • Examine the raw data for errors;
  • Adjust the levelling parameters;
  • Remove the line or lines causing the levelling error;
  • Use the INTREPID Flight Path Editor (from the Main Menu Editing in the INTREPID Project Manager) to clip sections of lines at ship turning points. The acceleration caused by turning may cause gravity errors. (There is a HELP button in this area providing a pdf training manual to guide you in this workflow.)

< Return to Stage 1. Select Project.

APPENDIX 3 - Introduction to Intrepid Tools Used for Supplementary Reporting and Gravity Processing

There are over 50 separate tools in the maintained solutions offered by Intrepid. Each tool is fully documented and comes with sample datasets and task files to illustrate how to transform your geophysical data, to process or aid interpretation. For the marine gravity application, about 5 of these tools may be needed to continue the gravity processing to the Complete Bouguer data reduction stage. The only tool not included in the basic Sea-g package that is demonstrated in the following discussion, is the main Gravity Tool.

Intrepid Gravity tool

For extra gravity functionality, there is a main tool, that supports all the standard transforms eg Bouguer, terrain corrections. Sea-g accesses some of this functionality, but is designed specifically for marine gravity reduction to a Free Air gridded outcome. The INTREPID Gravity tool is general in nature, supporting land, marine and airborne surveys. It has the capability of estimating various levels of instrument and operator errors in large and complex merging of many prior gravity surveys. It also comes with support for gravity gradiometry. When you have good bathymetry data available, you may wish to also undertake the marine Bouguer correction, and the INTREPID Gravity tool can help with that. Isostatic corrections using the USGS Parker method is in a separate tool.

3DExplore tool

Sea-g has a wizard design and it is based upon using plug-in JAVA tools that are also in the 3DExplore tool. (Project Manager > Display > 3DExplore). You have access to this general purpose dataset viewer as well.

3DExplore gives you many more options to view and edit simultaneously, grids, datasets, voxets, surfaces. It also offers special purpose plots for vector, tensor and angular data fields, spectral plots for Gamma Ray data etc.

Intrepid SpreadSheet tool - Repeat Lines

The data for this tutorial does not contain any repeat ship track lines, over the same space.

However, this is not an uncommon situation, especially when you wish to check for errors and repeatability of your measured geophysical signal.

You cannot easily get access to the required portions of the commonly observed signal, until all the processing is complete, and there are 2 lines in your final database, that approximately cover the same profile.

One suggested workflow that is available to you, is to create a new field in your database, that is the re-sampled observation from the “OTHER” line, onto the current line. Re-sampling is required, because the XY location information is not the same, and only approximately equivalent.

We provide a Spreadsheet formula example from another dataset, to accomplish this re-sampling. This function can be used interactively or in batch mode. The following example is from a batch file invocation of the Spreadsheet Editor (dbedit).

Process Begin Name = dbedit Comments = “current line is 11, reference line is 15 for the repeat” Parameters Begin Action Begin Type = OpenField Name = “2015_05_23_MGS6_198_split..DIR” Action End Action Begin Type = DeleteField Name = “v1” Action End Action Begin Type = CreateField Dtype = IEEE8ByteReal Width = 0 Bands = 1 GroupBy = No Initial = “repeatlinear(FreeAir,Longitude,Latitude,15)” Name = “v1” CoordinateSystemType = LOCAL Action End Parameters End Process End

The formula function used is REPEATLINEAR(A,Xf,Yf,ReferenceLineNumber). The formula text is case insensitive.

The two lines you wish to compare are known as the current line and the reference line. The current line Xf & Yf fields are used to calculate a distance along. The A field of the reference line is used to form a local Y axis, or offset distance ( imagine a peice of graph paper with the 2 axis).

A piecewise linear interpolation function is used to estimate the “Reference” observed data, away from where it was sampled. A new field is added to the database. In the above batch example, it is named v1. This function is queried to populate the reference line signal values at the required X & Y location.


Once this process is calculated, standard 3DExplore profile plot methods can be used to visualise the registration of the 2 signal fields at any distance along the common profile. The screen grab for the above case is shown below and there is quite a good repeatability of the measured gravity signal.


You should not expect perfect correspondence as the repeat lines are almost never exactly co-located. The Red profile is just an estimate of the signal, away from its actual location. The error shown in this case is around 0.25 mGal.

Map Composition tool

It is expected that the simple maps available within Sea-g, will serve most purposes. You have access to a very comprehensive way of producing detailed and flexible, cartographic style paper products, if you need to take this extra step. These tools and methods are described in Map composition (G17). This is a comprehensive capability, that manages in a flexible way, any 2D located data, logos, text, contours, polygons etc etc

Sample Cruise Path Map

A sample Map file for plotting the cruise path data of the Split Cruise data set from processing the S-133_subset.DAT dataset is provided.

The map for this tutorial are located in the installer files directory


Copy this map file to the same directory containing your outputs from the SEA-G tutorial steps. To customise the map you can edit it with any ASCII editor and refer to the Map Composition manual on the syntax.

To preview the map select it from the INTREPID Project Manager and launch Visual > Print Map. You will see a preview map as shown below


Success requires that the map file be correctly located and that the database name used is S-133_subset_split..DIR. If you have changed your naming style, you need to use a text editor to amend the hardcoded name in the first line of the map template file.

Dataset = ./S-133_subset_split..DIR FreeAirGridInUTM=./S-133_subset_split_FreeAir_level.out.ers UTM=NUTM51 Margin Begin Detail = Full Internal = No Border Begin Detail = Full Thickness = 1 Colour = Black Style = Solid Page Begin Width = 197 Height = 287 Detail = Full HCentre Begin Width = 197 Data Begin Width = 170 Height = 170 Detail = Full MapProjection Begin Projection = $UTM Datum = WGS84 MapProjection End PseudoColour Begin Image = $FreeAirGridInUTM PseudoColour End PathPlot Begin Dataset = $Dataset TraverseLine Begin Detail = Full Colour = Black Thickness = 0 Style = Solid SubSample = 1 ShowDirection = Yes ShowLineToBase = No ShowLineToFiducial = No LineNumber Begin Z = $Dataset/SPLITLINE LineNumber End ShowFiducials = No RecoveryType = FiducialInterval FiducialInterval = 100 FiducialLabelRate = 10 LineNumberText Begin Detail = Full String = "Line" Colour = Black Size = 1 Font = 0 Angle = 0 Justify = lb Gap = 0 VGap = 0 TextThickness = 0 LineNumberText End LineNumber2Text Begin Detail = Full String = "�" Colour = Black Size = 2 Font = 0 Angle = 0 Justify = lb Gap = 0 VGap = 0 TextThickness = 0 LineNumber2Text End FiducialText Begin Detail = Full String = "�" Colour = Black Size = 2 Font = 0 Angle = 0 Justify = lb Gap = 0 VGap = 0 TextThickness = 0 FiducialText End FiducialMarker Begin Detail = Full Colour = Black Size = 2 Thickness = 0 Symbol = Cross Angle = 0.00000000 FiducialMarker End TraverseLine End PathPlot End Ticks Begin Detail = Full MetreGrid = Yes EastInterval = 1000000 NorthInterval = 1000000 Format = NESW LabelAtTop = Yes Style = Border Internal = No TextSize = 2 TextFont = 0 TextThickness = 0 TickSize = 2 TickThickness = 0 LabelOffset = 0 Ticks End Ticks Begin Detail = Full MetreGrid = No LongInterval = 0:10:0 LatInterval = 0:10:0 Format = DMS LabelAtBottom = Yes LabelAtLeft = Yes Style = Border Internal = No TextSize = 1 TextFont = 0 TextThickness = 0 TickSize = 1 TickThickness = 0 LabelOffset = 0 Ticks End Data End VSpace Begin Height = 10 VSpace End HBox Begin ScaleBar Begin Length = 50 Interval = 10 Unit = Metres YSize = 1 ShowScale = Yes Style = 0 TextFont = 1 TextSize = 1 TextThickness = 0 ScaleBar End HSpace Begin Width = 10 HSpace End HCentre Begin VSpace Begin Space = 3 VSpace End Text Begin String = "Sea Acceptance Test Line Path" Colour = Black Size = 2 Font = 2 Angle = Arial Justify = lb Gap = 0 VGap = 1 TextThickness = 0.2 Text End VSpace Begin Space = 1 VSpace End Text Begin String = "AIRSEA-II S-133" Colour = Black Size = 2 Font = 2 Angle = 0 Justify = lb Gap = 0 VGap = 1 TextThickness = 0 Text End VSpace Begin Space = 1 VSpace End Text Begin String = "AIRSEA-II Test Set, Sep 30, 2003" Colour = Black Size = 2 Font = 2 Angle = 0 Justify = lb Gap = 0 VGap = 1 TextThickness = 0.2 Text End VSpace Begin Space = 3 VSpace End Text Begin String = "*** Sample ***" Colour = Black Size = 2 Font = 2 Angle = 0 Justify = lb Gap = 0 VGap = 1 TextThickness = 0 Text End VSpace Begin Space = 10 VSpace End HCentre End HSpace Begin Width = 10 HSpace End NorthArrow Begin Length = 20 GridNorth = 0 TrueNorth = 0 MagneticNorth = 0 ShowProjection = Yes TextFont = 1 TextSize = 1 TextThickness = 0 VGap = 1 NorthArrow End HBox End VSpace Begin Height = 10 VSpace End HCentre End Page End Border End Margin End

A second example map composition file is also shipped, that includes the gridded representation of your Free Air field, as a back drop. This opens up a conversation about Geodetic and projected coordinates.

Projection Conversion tool

The Sea-g tool works strictly in GEODETIC coordinates, to achieve its goal of a simple Free Air grid from a cruise of newly acquired marine gravity data.

The paper map above, shows the sea trial lines projected onto a UTM reference frame, and also shows the original GEODETIC tick marks. Warping of the line data is done while producing the map. This is not recommended for grids, as this can suddenly be a big job. For this reason, if you wish to add a backdrop grid to your map, you also need to run a projection conversion job. The aim is to warp your Free Air grid to a NUTM51 projection.

To warp the FreeAir grid select it from the INTREPID Project Manager and launch Utilities > ProjectionConversion.


The screen grab shows the process of changing the required output projection to NUTM51. You then follow this with setting a cell size in meters, rather than decimal degrees. Choose 200m. Change the name of the output grid to S-133_subset_split_FreeAir_levelNUTM, and press OK

The number of rows stays much the same, but the number of columns reduces quite a bit, as this dataset has a high latitude. (117 down to 88) . You now have a projected grid that can be used directly as a backdrop, for a projected map product.

Export tool - Exporting to ASCII

Any dataset from Intrepid can be easily transformed to a range of other industry standard formats, including standard ASCII CSV files. You have direct access to this functionality via the INTREPID Project Manager. Under the top menu item Dataset, choose Export


The separate Export tool will pop-up.

The steps to use this tool to create an ASCII CSV file are-

  1. Under File, choose Open Input Dataset, choose S-133_subset_rev2..DIR
  2. Specify Output File, choose test
  3. Under Menu Item Output vector Format, choose ASCII Columns
  4. You then get a field chooser, with all fields chosen by default. Choose those you are interested in.
  5. Select Apply
  6. Job done, examine the output ASCII file

< Return to Stage 1. Select Project to explore other stages.