valem

Oasismontaj Guided Learning Paths

Voxel Assisted Layered EarthModellingGeosoft’s Voxel Assisted Layered Earth Modelling (VALEM) solution is a ‘hybrid inversion’ approach that combines thebest from different modelling approaches and calculation algorithms, and seamlessly integrates with GM-SYS 3D, toprovide an improved, interactive solution for base of salt and sub-salt interpretation.

VALEM enables you to perform voxel based gravity inversion to improve the delineation of base of salt/sub-salt boundarieswhere interpretation based on seismic data alone is challenging or impossible. These techniques help to eliminateuncertainty during the interpretation and target selection process, reducing risk and cost when advancing oil and gasexploration programs in sub-salt regions.

This guide lesson outlines some tips to getting started with VALEM and does not provide a comprehensive How-ToGuideonmodel building with GM-SYS 3D. Please visit http://my.geosoft.com/supportcentre for additional information or contactyour assigned Technical Analyst for help with any of the included topics.

In order to use VALEM you need to:

Have a basic knowledge of gravity modelling and experience using GM-SYS 3D.

Have a working project where a sub-salt problem needs to be solved.

Have the ability to connect to the VALEM service.

A Geosoft ID and a reasonable broadband internet connection are required to connect to Geosoft's cloud service.

Understanding VALEMThe VALEM inversion workflow runs within GM-SYS 3D. Once amodel is created within GM-SYS 3D, VALEM can beused to run an inversion on the base of salt over the entire model area, or a selected region of interest. The computation isperformed using the VALEM cloud service – a version of the Geosoft VOXI Earth Modelling service that is optimized forsolving the base of salt problem in layered earth modelling. A refined base of salt layer is returned by the inversion serviceto GM-SYS 3D and is incorporated into themodel.

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Since VALEM is integrated wholly within GM-SYS 3D, there are some default behaviours that must be considered beforeperforming a VALEM inversion.

Important Considerations in GM-SYS 3D

Automatic Grid FillGM-SYS 3D will automatically fill dummy values within a grid in order to ensure that it matches the extent of themodelspace. This default behaviour makes model building easy but may create regions in your model that are not optimal forinversion. It is important to be cognisant of these areas in your model, since if selected in your region of interest, theinversion algorithm will attempt to fit these erroneous data points. We recommend that you use real data in your observedsurvey or at the very least a geologically reasonable interpolation in order to focus the inversion's data fit on reasonablevalues.

In previous versions of GM-SYS 3D, rotatedmodels were not permitted and so grid filling was required at the corners of themodel. With the VALEMworkflow inmind, GM-SYS 3D now supports rotatedmodels.

Automatic Voxel FillEach property voxel imported into themodel must be devoid of dummies to the spatial extent of the 3D model. This ensuresthat moving or deleting layers does not create voids in themodel. GM-SYS 3D will automatically fill dummies in voxels, butthere are tools within Oasis montaj to do this manually. For VALEM, it is best to take care when filling voxel dummies – aswith grid filling, themore geologically sensible the fill, the better the inversion result.

VALEMmodels must include a hypothesised density distribution for thematerial that can be used to fill themodel wheresalt is removed. The inversion algorithm will then choose between a constant salt density and the background fill density,and then add salt to themodel until an optimum data fit is achieved. The success of the VALEMmodel will depend greatlyon the choice of background fill density, therefore it is recommended to use geologically reasonable values.

Oasis montaj Guided Learning Paths



Figure 1: East-West cross section through the SEAM density volumewith the allochthonous salt body removed. Thesurface identifies the known top of allochthonous salt. The salt was replaced by interpolating from the sides in aconformablemanner. This was the startingmodel for VALEM inversion.

Piercing InformationYoumay wish to specify layers that can be pierced (altered) by themovement of the salt/sediment boundary. The defaultfor the lowest extent of the active volume is the first non-piercable layer.

DC ShiftIdeally, you'll want themisfit between the observed and calculated gravity response to be due only to themissing salt. Thiswill not always be the case (in the real world) as there will be uncertainty in the remainder of themodel that will affect themisfit. It is therefore crucial that the backgroundmodel and DC shift be set, to the users' best ability, to ensure the bestpossible VALEM result. If the DC shift is incorrect the VALEM result can end up having toomuch salt, or none at all.

Vertical Expansion of CellsIn cases where your base of salt extends to significant depth, you'll want to maintain the resolution of your model. In theseinstances, it is recommended to reduce the vertical expansion of your cells to ~1. Try values between 1 and 1.05 for besteffect.

FFT Expansion PercentageIt is recommended to set your expansion percentage such that the total model extent is twice the length of the largestwavelength you'd like to be able to recover in themodel. For example, for amodel that is 50mx50m, youmust set your FFTexpansion percentage to at least 100% to recover a wavelength of 50m in your forward calculation. This is analogous to theuse of padding cells in forwardmodelling within the spatial domain.

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The VALEM Inversion ServiceVALEM uses theMicrosoft Azure cloud service to perform the inversion calculation. The optimisation of themodel forinversion is performed in the background and is consequently not user adjustable. The inversion service is optimized forfast and effective hybrid layered earth modelling.

Themodel created for inversion is "residualised" – that is, we've subtracted what is known leaving the kernel of what isunknown.The following paragraphs explain how theGM-SYS 3D model is prepared for inversion and are for informationpurposes only.

Starting and Parameter Reference ModelThe startingmodel is a residualised version of the GM-SYS 3D model. The densities in the startingmodel are relative to thehighest density in the volume.The VALEM inversion will add salt below the top of the salt layer until it satisfies the observeddata. In the absence of influence from observed data, themodel will converge towards the startingmodel, as it is also usedas a parameter referencemodel. The parameter referencemodel is weighted with a high degree of confidence in regionsclose to the top of salt – this ensures that the inversion recovers the observed salt.

Lower and Upper BoundThe lower bound is the residualised density of salt (defined as a constant), the upper bound is the residualised sedimentdensity (defined as a voxel).

Active AreaThe active area defines the horizontal extent of cells that are allowed to change in the inversion. It is defined by an area ofinterest (AOI) polygon; the size of which determines the size of the VALEMmodel and the resources needed to run it.

1. The user explicitly defines an AOI to limit the inversion to a subset of the 3D model;2. The user elects to invert the entire 3D model; the AOI is effectively 1 cell smaller than the 3D model (required for

padding).

Constraint GridUsingGM-SYS 3D type constraint grids can restrict movement of the salt to specific regions in the X- and Y-dimensions. Ifthe constraint grid value is exactly 0, the 3D model cells within that column are held fixed during the inversion; any valueother than 0 allows the cell value to vary during inversion.

Active VolumeVertical movement of the salt interface is bounded by the top of salt and a user-selected boundary below. When a userselects the inversion layer, the top of the active area is automatically populated. By default, the lower boundary is the firstnon-pierceable layer below the inversion layer, but the user may select any layer below the inversion layer or optionallyselect a constant elevation beneath the inversion layer (as long as it is within themodel space).

The user has the option tomaintain the horizontal extent of salt. That is, salt will not be completely removed from any(horizontal) location where salt exists in the startingmodel.

Themesh created for the inversionmodel using VALEM is bounded laterally by the active area/constraint grid; andvertically by the top of salt layer and the user selected lower boundary. This minimizes the computational requirement forthe inversion calculation but maintains the samemesh geometry as the GM-SYS 3D model.

Model ResolutionVALEM is currently limited tomodels up to 300x300x128 cells. Model resolution can be altered to fit within this resolutionlimit or to perform a quick, inexpensive test run.




Iterative Reweighting Inversion (IRI) FocusingThe default VALEM inversion uses several iterations of IRI focusing to generate a layered earth solution with the discretedensities (salt & background fill) that are in the original GM-SYS 3D model.

Extraction and Display in GM-SYS 3DThe inversion results in a 3D distribution of residualised density. The improved salt-sediment interface is then extractedfrom the voxel and displayed automatically in the GM-SYS 3D model as an "Alternate Relief Surface" for the base of saltlayer. The extraction process involves several forwardmodels of various density relief surfaces to determine the one givingthe best fit. The result is displayed as an alternate relief surface in GM-SYS 3D without altering the original model. You canthen define the VALEM extracted relief surface as the active relief surface – themodel will subsequently update using thisnew boundary (this includes clipping of crossed layers).

Running a VALEM InversionThe following is an example of a general procedure for running a VALEM inversion.

Preparing your model for inversionYou should prepare your GM-SYS 3D model with the following inmind.

1. Fill grids and voxels with geologically reasonable values for both dummy areas and background fill density to ensuremisfit is minimized in non-salt regions.

2. Load your survey, and ensure that it is defined from a constant elevation and not a grid (youmay have to continueyour data to a constant elevation if not already done).

3. Define pierceable layers below the top of salt.

4. Adjust DC shift as needed to ensuremisfit is minimized in non-salt regions.

Run a VALEM inversionOnce themodel is prepared, youmay select the base of salt layer for VALEM inversion.

1. Select the base of salt layer from within theModel Explorer, right-click and select VALEM Structural Inversion fromthe pop-upmenu.TheVALEM Structural Inversion dialog appears.

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Note that youmust be connected to the internet and signed in with your Geosoft ID to access the VALEMservice.

2. Specify the Inversion name. Note that a unique inversionmodel name is required.

3. Select whether to perform the inversion on a specific Area-Of-Interest (which requires a polygon file) or over the entiremodel.

4. If you chose to perform the inversion on a specific AOI, select theAOI polygon file to use.

5. Optionally, you can use aConstraint grid to refine the active area.

With a constraint grid, a grid cell value of 0 defines inactive parts of themodel; any other cell value defines the activeparts of themodel. A constraint grid, compared to an AOI polygon, may more easily define the active area for someusers/projects.

6. The Top of active area is a read-only field that displays name of the layer above the selected inversion layer, whichdefines the top of the active layer.

7. Select the Inversion layer, if the default is not your layer of interest. If you change this, the Top of active area field willalso change.

8. Select whether you'd like toMaintain horizontal extent of salt.

9. Select theBase of active area using either a constant elevation (within themodel space) or a layer within themodel.

If you chose to base the active area on a layer, select the Layer from the dropdown list.

10. Click theMore button to display more options.




11. TheNumber of cells (x,y,z) is a read-only field that displays the estimate of the size of the VALEM job, from theinputs provided above.

12. Select the IRI focus factor, 3 is the current default. IRI focusing allows for discreet densities, therefore setting thisfactor to None is not presently recommended.

13. Once you are satisfied with your selections, click theCreate button.Themodel is optimized for inversion and sent to the cloud service for computation. A process log and progress barare displayed within the GM-SYS 3D window. The result of a successful inversion is then added to the GM-SYS 3Dmodel as an Alternative Relief Surface.

14. You can abort the inversion by clicking theStop button beside the progress bar once the inversion begins.

During the inversion, you cannot change/edit your GM-SYS 3D model.

Interpreting and Iterating VALEM ResultsThe inversion process in VALEM can end with 3 possible messages.

Inversion Completion Messages1. Failure.

This means that the problem was ill posed in someway (perhaps input parameters) and did not finish in any usefulway.

2. Check result.This means that the inversion did not converge towards the selected data fit (default 1) but the results may still beuseful. In this mode, VALEM gives you the option to incorporate the results back into your GM-SYS 3D model. If youselect not to, you still have the option to "Assimilate VALEM Result" from the VALEMmenu.

3. Success. This means that your inversion converged near the selected data fit (default 1) and the result isautomatically assimilated into your model as an "alternate relief surface" in the 3D display. You will need tomake thesurface "active" in order to resolve conflicts with other surfaces and calculate the new response.

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EAGE-SEG Model for VALEMGeosoft provides sample data for you to use when working through these How-ToGuides. To access this EAGE-SEGdata, speak to your Technical Analyst (TA) and they will provide you with the required data sets.

The EAGE-SEGmodel is derived from the seismic dataset described here:

http://wiki.seg.org/wiki/Open_data#SEG.2FEAGE_3D_modeling_Salt_Model_Phase-C_1996Aminzadeh, F., et al., 1995, 3-D modeling project: 3rd report: The Leading Edge, 14, 125–128.Aminzadeh, F., et al., 1996, Three dimensional SEG/EAGE models —An update: The Leading Edge, 15, 131–134.Aminzadeh, F., et al., 1994, SEG/EAGE 3-D modeling project: 2nd update: The Leading Edge, 13, 949–952.

The supplied GM-SYS 3D model is modified in that the true top of salt and true base of salt grids were extracted usingOasis montaj (convert voxel to database then direct grid database usingmax or min elevation). This process produced gridsthat estimate, to a good degree, the top and bottom of the true salt body.

Below is a list of files and some background information about them as well as guidance on obtaining a good first result fromVALEM.

Model FilesThe coordinate system of the entire project was arbitrarily set to NAD 83UTM 17N. The choice of coordinate system hasno effect on the density model – however the project must have a projected coordinate system attached to it.

Survey DataGzObs-4.grd

This is the observedGz.

Survey height is constant at 0m with no background density removed.

Definition of the True ModelTrueModel_GMSYS3D.geosoft_voxel

This is the voxel exported from the truemodel in GM-SYS 3D (defined by the grids below)

Top_Salt_True80.grdThis is the top of salt grid extracted from the true voxel via direct gridding at 80m cells

Top_Salt_filled80.grdThis is the above grid extended using GM-SYS 3D default behavior

Base_salt_true.grdThis is the base of salt extracted from the true voxel via direct gridding at 80m cells

Base_true_filled80.grdThis is the base of salt extended to the extents of the GM-SYS 3D model such that the extended portions are equal to theextended portions in the top of salt layer. This effectively pinches the salt body out where it is not explicitly defined in thetrue voxel model.

Bathy_z.grdThis is the grid defining the bathymetry relief extracted from the true voxel model. It will automatically be filled to the extentsof the GM-SYS 3D model when added.

Background_DensityFlood.geosoft_voxelThis is the sediment fill voxel.




Top_Salt.plyThis is the polygon that defines the lateral extents of the top of salt.

VALEM ResultsRecovered_Model_DCShift_Minus4.geosoft_voxelModel result of VALEM with DC shift of -4 as a voxel.Base_Salt_Recovered_DCShift_Minus4.grdResult of VALEM with DC shift of -4 as a relief grid

Setting up theGM-SYS 3DModel1. Create a new model from either the bathymetry grid or the background_densityflood voxel using a constant top of

model 0m and a constant bottom of model -5180m.

2. Themodel cell size should be 80x80x80m with an expansion factor of 1 – edit this in the index voxel geometry dialog.

3. Set the density below the constant top layer to 1.03 g/cm^3 for sea water

4. Add bathymetry layer (bathy_z.grd) and use Background_DensityFlood.geosoft voxel for density distribution.

5. Add top of salt layer (top_salt_filled80.grd) and use constant density of 2.2 g/cm^3.

6. Add base of salt layer (top_salt_filled80.grd – effectively no salt) and use Background_DensityFlood.geosoft_voxelfor density distribution.

If you have a hypothesized base of salt relief surface, youmay use it, but note that you shouldmanuallymatch relief in regions of FFT expansion to that of your expanded top of salt – this effectively keeps the saltwithin your salt body and prevents it from leaking into the remainder of your model due to FFT expansion.

7. Basement need not be defined as the density is contained in the voxel distribution – unless you want to set thebasement to Not Pierceable, in which case you'll have to define its relief surface.

8. AddGz survey data (Gz_Obs-4.grd) with constant elevation of 0m and no background density.

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Checking the GM-SYS 3D Model before VALEM1. Forward calculate Gz response (for this example use a 100% expansion to resolve basement wavelengths).

2. Look at misfit grid.

Do you have a negative anomaly where you hypothesize the salt to be below the Nil Zone?Do you have a positive anomaly where you hypothesize the salt to be above the Nil Zone?Is the error small and fairly consistent elsewhere?

3. Adjust DC shift, forward calculate, and check misfit grid for the notes in step 2.

Try values 0, -2, -4, etc. or use the auto best-fit and digitize options (digitize in regions outside the top of saltextents – youmay want to plot the top of salt polygon to help).In this example, the DC Shift (-4) is due solely to an offset in the observed data. With the DC shift setappropriately, the remainingmisfit is due only to themissing salt. This will not always be the case (in the realworld) as there will be uncertainty in the remainder of themodel that will affect themisfit. It is therefore crucial thatthe backgroundmodel and DC shift be set to your best ability to ensure the best possible VALEM result. If the DCshift is insufficient the VALEM result can end up having toomuch salt, or none at all.

Running VALEM on the EAGE-SEGModel1. Run a VALEM inversion on the Base of Salt layer with default settings. Use the top_salt.ply file to define the AOI.

Recommend a 100% expansion here, but be sure to use the same as you did in the forward calculation.

With an appropriate DC shift (-4), you should get an inversion datamisfit on the order of 1 (between 1 and 5 isexpected for this example).Did your inversion datamisfit improve between the first and last iterations?How high was your inversion datamisfit in the first iteration?If your inversion datamisfit was too high (>10-100) then try a different DC shift or adjust the remainder of yourlayers for a better backgroundmodel.

2. Assimilate the results and analyse relief surface.

Is the result qualitatively reasonable?3. Make the result relief active and run a forward calculation.




Did themodel misfit improve?

4. If the result is good, export to a voxel and compare to the truemodel.

5. Experiment with different DC shifts, see what impact it has on the result and the inversion datamisfit.

How-ToGuide Publication Date: 09/01/2015

Copyright 2015Geosoft Inc. All rights reserved.

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