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GMSYS-3D 3D Gravity and Magnetic Modeling for Oasis montaj USER GUIDE www.geosoft.com

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Page 1: GMSYS-3D Modeling User Guidedocshare01.docshare.tips/files/26937/269375967.pdf · 2016. 12. 16. · GMSYS-3D utilizes some of the newest functionality in Oasis montaj, particularly

GMSYS-3D 3D Gravity and Magnetic Modeling

for Oasis montaj

USER GUIDE

w w w . g e o s o f t . c o m

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The software described in this manual is furnished under license and may only be used or copied in accordance with the terms of the license.

Manual release date: 6/22/2009.

Written by, Tom Popowski, Gerry Connard, and Rowland French. Please send comments or questions to [email protected]

Copyright © 2009 Geosoft Inc. All rights reserved. Geosoft is a registered trademark, and Oasis montaj is a registered trademark of Geosoft Inc. Other brand and product names mentioned herein are properties of their respective trademark owners. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form, or by any means, electronic, mechanical, photo-copying, reading, or otherwise, without prior consent from Geosoft Inc.

The software described in this manual is furnished under license and may only be used or copied in accordance with the terms of the license. GMS3D.ug.2009.06

Windows, and Windows NT™ are either registered trademarks or trademarks of Microsoft Corporation.

Geosoft Inc. Queens Quay Terminal 207 Queens Quay West Suite 810, PO Box 131 Toronto, ON Canada M5J 1A7 Tel: (416) 369-0111 Fax: (416) 369-9599

Web Site: www.geosoft.com

E-mail: [email protected]

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GMSYS-3D

User’s Guide

Release 1.3

March 20, 2006

TABLE OF CONTENTS

INTRODUCTION TO GMSYS-3D ..............................................................................3

RELEASE 1.3 .................................................................................................................4

INSTALLATION ............................................................................................................5

3D MODELS ...................................................................................................................6

BUILDING MODELS ....................................................................................................7

1. Consistancy with the “index” grid 7

2. Grid expansion and “fill” 7

3. Periodic Grids and Edge Effects 8

4. Observed and Calculated Data Plane 8

5. Regional Removal and DC Offset 8

6. Magnetic properties require micro-cgs units 9

7. Inversion constraints 9

GMSYS-3D MENU .......................................................................................................10

Model Definition submenu 10

Grid Editing submenu 12

Display submenu 14

2D Cross Sections submenu 16

Grid Preparation 16

Manual FFT Preparation submenu 17

Calculate submenu 18

GMSYS-3D TOOLBAR ...............................................................................................22

DEMO MODELS ..........................................................................................................23

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Salt 23

Death Valley 24

REFERENCES ..............................................................................................................25

APPENDIX A MODEL FILE (*.g3d) FORMAT ........................................................1

APPENDIX B NEW FEATURES IN VERSION 1.3 ...................................................1

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Page 3

INTRODUCTION TO GMSYS-3D

GMSYS-3D is a 3D gravity and magnetic modeling package for surface-oriented

models. A model is defined by a number of stacked surface grids with density-,

susceptibility-, and remanent magnetization-distributions specified for the layer

below each surface.

Calculations are performed in the wave number domain and are based on Dr. Bill

Pearson’s implementation of R. L. Parker’s algorithm (1972). The program is

designed to run within Oasis montaj, making available all of the familiar grid

manipulation and display routines in addition to application-specific routines.

The GMSYS-3D menu, gmsys-3d.omn, provides to access GMSYS-3D functions.

A GMSYS-3D Toolbar also allows quick access to some of the more commonly

used functions. Models are defined by specifying the grids that define the

surfaces, and the layer properties. “Model definitions” are saved in a *.g3d file.

The model definition can be edited through GMS3MDEF.GX, which is accessed

through the Edit Model menu item. Grid editing functions, grid preparation

routines, model display routines, and forward and inverse calculations also are

provided through the menu structure. You can also make use of many additional

Oasis GX routines for grid manipulation and grid display.

GMSYS-3D requires Geosoft's MAGMAP 2D-FFT system to operate.

GMSYS-3D utilizes some of the newest functionality in Oasis montaj,

particularly in the 3D visualization routines, so some features will not work

correctly if you are running an older version of Oasis montaj. For more

information on the 3D display operations, Geosoft has a technical note, Working

with 3D Views in Oasis montaj,

(http://www.geosoft.com/support/technote/pdf/3DViewsOasismontaj.pdf) which

is useful for understanding the 3D visualization tools.

This User’s Guide provides an overview of the GMSYS-3D approach to 3D

gravity and magnetic modeling and the GMSYS and the menu structure. Help

files for all the dialogs provide detailed information on the parameters used for

each procedure. Companion Tutorials step the user through most of the functions

of GMSYS-3D. New users are strongly encouraged to work through the

Tutorials.

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Page 4

RELEASE 1.3

Release 1.3 adds the capability to convert a GMSYS-3D model to a Geosoft 3D

voxel grid and the capability to calculate the gravity response from a 3D voxel

grid.

The new “Model Explorer” tool is a “tree-structure” summary of the current

model that is similar to the Oasis “Project Explorer” tool. It remains visible for

reference while using all of the other GX’s. If any of the model parameters are

changed by other GX’s, the Model Explorer is automatically updated.

The Time-to-Depth conversion allows the user to build a model in time and then

use velocities to convert to a depth model.

A new backup scheme has been incorporated into the inversion process. Users

may choose between the old method (a single step undo), an automatic naming

scheme to save each step in the inversion process, or explicitly choose a name for

each backup grid. Users may also “checkpoint” (i.e., make a complete copy of

the model in a separate directory) a model with one click from the inversion

dialogs.

A new function allows users to resample a model to half-size (i.e., double the grid

cell size) and save it to a new model name. Calculations with the half-size model

will be four times as fast as with the original model. A similar new function will

create an “expanded” model that is fully “prepped” for the FFT. An expanded

model speeds up the calculation process.

The GMSYS-3D help is now integrated with the Oasis montaj help system. This

integration allows hyperlinks with other help topics.

A new “Auto DC-shift” button will automatically adjust the DC-shift of the

calculated gravity and magnetics grids to force the mean error to zero.

All of the text strings now support UNICODE

The Release Notes and APPENDIX B list the new features added since release

1.2.

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Page 5

INSTALLATION

GMSYS-3D is distributed and installed with Oasis montaj™. To run

GMSYS-3D, open Oasis and load the menu gmsys-3d.omn (GX | Load menu…).

If you received updated software directly from Northwest Geophysical

Associates, follow the instructions provided with that release.

GMSYS-3D requires the following Oasis montaj licenses:

10105 montaj™ MAGMAP Filtering

10523 montaj plus™ Modeling 3D

You can view your licensed products using File / License / View License…

GMSYS-3D version 1.3 requires Oasis montaj™ version 6.3 or above.

The GMSYS-3D toolbar, providing some of the more commonly used

functionality, will load automatically with the menu. By default, the tool bar

appears on the left edge of the Oasis window but the user may drag it to other

locations. You may toggle any toolbar off or on by selecting the Tools button in

the Project Explorer and right-clicking the toolbar name.

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3D MODELS

3D models are defined by a number of surface grids with a density distribution

(and/or susceptibility & remanent magnetization distribution) assigned to each

layer. Layer density may be specified using either a constant density, a vertical

density-depth profile relative to a “reference surface”, or a laterally-varying

density distribution defined by a grid. Layer susceptibility may be specified using

either a constant susceptibility and remanent magnetization, or a laterally-varying

susceptibility distribution. Figure 1 illustrates a three layer model where the

parameters “DEN_#” are constant densities for each layer and “SURF_#” are

filenames for grids defining those surfaces. Model parameters are edited using

Model Definition | Edit Model from the GMSYS-3D menu and are saved in a

model definition file (*.g3d).

Figure 1: Model Example

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BUILDING MODELS

As indicated above, models consist of layers defined by a stack of horizon grids

and parameters assigned to each layer. The grids may be created outside of

GMSYS-3D, imported from another application (e.g. seismic interpretation

software), or built "from scratch" using the tools within Oasis (e.g.

NGAGEDIT.GX, GMS3FGRD.GX) or some other application. Because

GMSYS-3D uses frequency-domain (i.e., FFT-based) calculation routines, the

grids need to be specially prepared to be compatible with FFTs.

1. Consistancy with the “index” grid

All grids in the model must be consistent with respect to:

Grid size (number of rows and columns)

Cell size and units

No dummies in the grid

Grid origin

No crossing surfaces

GMSYS-3D provides a function “Model Definition | Check Model”

(GMS3CHECK.GX) to check a model for all of these problems and adjust grids

to eliminate crossing surfaces. Another function “Make Grids Coincident”

(NGAGRDMCH.GX) is provided to force a grid to match an “index” grid.

GMS3FGRD.GX may be used to build a new, flat grid that matches the

parameters of the "index" grid for the currently loaded model. The standard grid

preparation routines (GMS3XPND, GRIDFILL, or NGAPFILL) will fill in the

dummies during normal grid preparation steps. However, users should be aware

that this happens to all of the grids in the model and that unwanted artifacts may

be introduced in critical areas of their models.

2. Grid expansion and “fill”

All the grids must be expanded in size and “filled” so that they are “periodic” to

eliminate edge effects. The amount of expansion required depends on the shape

of the model horizons near the edges of the original grid. The final, expanded

dimensions of the grid must also be “good FFT” dimensions. If you use %’s to

specify expansion amounts, all of the routines described below and the Oasis

automatic routines expand to “good FFT” dimensions.

The user has two options with regard to the “expand and fill” preparation. The

simplest is to use the “Setup Automatic FFT Preparation” option

(GMS3XPND.GX). With this option, all grids are expanded inside the

calculation routine each time it runs and then “masked” back to the original size at

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Page 8

the end of the calculation. This option is easier to use but gives the user less

control and requires significantly more computation time with larger grids. The

second option is to prepare each grid separately before building the model using

the procedures in the Manual FFT Preparation submenu. This option allows more

control and saves time during the calculations.

3. Periodic Grids and Edge Effects

Periodic grids “wrap” seamlessly left-to-right and top to bottom. This can also be

visualized by placing copies of the grid on all 4 sides, and continuing the process,

so the periodic grid of grids continues to infinity. When the grid is “filled

periodically” a smooth interpolation is used to wrap the grid from one edge of the

“real data”, off the edge of the expanded grid, and back from the opposite side of

the expanded grid, to the “real data” on the other side. The analogous process

wraps the top and bottom edges and corners of the grid.

Calculations are carried out on the artificial array of adjacent, repeating grids.

Hence, you must expand the grids sufficiently so that at the edge of the “real data”

the gravity and/or magnetics are not unduly influenced by the geology in the

adjacent, repeated grid.

The grid filling is in fact a geologic interpretation. If you have a deep basin on

one edge of the survey area, the gravity on the edge will be affected by the

extension of the basin, beyond the survey area. The basin could truncate abruptly,

or could continue for a considerable distance. You, as an interpreter, must make a

judgement on how you want to continue it. To continue the basin for longer

distances, increase the size (percent expansion) of the expanded grid. For smaller

grids of smaller areas, expansions of 100% or more are not unreasonable.

You have the option of saving intermediate expanded and filled grids and viewing

them to assure that the extrapolated geology is reasonable. By “manually”

expanding and filling the grids, you have much more control of these edge effects.

4. Observed and Calculated Data Plane

The response of the model is calculated on a flat plane. If your observed data are

not on a flat plane, you need to upward continue your observed data grids to the

“ZREF” elevation. Previous versions required that ZREF be chosen to be above

the top of the topography. GMSYS-3D provides a “draped to level” continuation

function “Model Definition | Upward Continue Observed Grid” (GMS3VUP.GX).

Version 1.3 can calculate gravity at any level within the model (as long as it is

constant, i.e., flat). The model check routine (GMS3CHECK.GX) still reports

ZREF below the top of the topography as error

5. Regional Removal and DC Offset

In general we recommend removing any regional field before loading the

observed data grid into GMSYS-3D. If the regional was removed by using an

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Page 9

Oasis filter, you should include the same filter control file (*.con) as an “inline

filter” in the GMSYS-3D forward calculations (GMS3GRAVF.GX and

GMS3MAGF.GX).

GMSYS-3D allows you to remove a “DC-shift” (bulk shift) from the calculated

response in the forward calculations and inversion. The DC-shift is critical to the

inversion process.

6. Magnetic properties require micro-cgs units

Note that the susceptibility and remanent magnetization need to be input in

micro-cgs units. In the future, input in SI and cgs units will be supported. When

modeling vertical-sided simple magnetic bodies (e.g., vertical prisms), users may

see small oscillations at the Nyquist frequency superimposed on the calculated

magnetic anomalies in the areas with low gradients in the calculated field.

Although this “noise” is an artifact of the frequency-domain calculation

technique, the amplitude is relatively small (~.01 nT) and typically does not

appear when modeling real-world geology.

7. Inversion constraints

GMSYS-3D allows the use of “constraints” grids to constrain or restrict some of

the inversion options. A “constraints” grid is a grid that matches the “index” grid

for the model and contains values between 0 and 1. Where the value of a grid

node in the constraints grid is set to 0, the corresponding location in the grid being

inverted will remain unchanged. Where the value of a grid node in the constraints

grid is set to 1, the corresponding location in the grid being inverted is totally

unconstrained. If the value in the constraints grid is greater than 0 and less than 1,

it acts as a weighting factor to slow down the amount of movement during

inversion. There are four tools for creating a constraints grid in the “Grid

Editing” menu.

For example, if you are inverting on depth to basement in an area surrounded by

outcropping basement, you might build a constraints grid that is set to 0 in the

areas of basement outcrop and 1 in the center of the basin. The transition from 0

to 1 would determine how sharply the edges of the basin could drop off.

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GMSYS-3D MENU

This section of the User’s Guide introduces the reader to each menu item

available in the GMSYS-3D menu. In Oasis montaj most menu items invoke a

GX, which is a routine to perform a given function. More complete descriptions

of the parameters and operations of each GX are included in the Help for that GX.

GXs may also be invoked from the GX menu or from the GX tool on the standard

Oasis toolbar.

Model Definition submenu

Menu item, “Model definition | Upward Continue Observed Grid to Flat” – GMS3VUP.GX

The GMS3VUP GX upward continues a data grid from a “draped” surface to a

level surface.

Menu item, “Model definition | Upward Continue Observed Grid to Draped” – GMS3VUPD.GX

The GMS3VUP GX upward continues a data grid from a “level” surface to a

draped surface.

Menu item, “Model definition | Show Current Model” – GMS3SHOW.GX

The GMS3SHOW GX displays a summary of the current model in an information

box.

Menu item, “Model definition | Model Explorer” – GMS3XTOOL.GX

The GMS3XTOOL GX loads a “tree-structure”

summary of the current model that is similar to the

Oasis “Project Explorer” tool. It remains visible for

reference while using all of the other GX’s. If any of

the model parameters are changed by other GX’s, the

Model Explorer is automatically updated.

Menu item, “Model definition | New Model” – GMS3NEW.GX

The GMS3NEW GX clears the previous model from

memory, prompts the user for a new model file

(*.g3d), asks for new display parameters, and then

runs GMS3MDEF (Edit Model). The grids that you

plan to use for the new model should be in the

working directory prior to running the New Model

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procedure. The model can be edited later using the Edit Model procedure to

insert or remove surfaces.

Menu item, “Model definition | Open Model” – GMS3OPEN.GX

The GMS3OPEN GX clears the previous model from memory, prompts the user

for an existing model file (*.g3d), retrieves that model, and then runs

GMS3MDEF (Edit Model)..

Menu item “Model definition | Edit Model” – GMS3MDEF.GX

The GMS3MDEF GX opens a dialog box for entering the grids and properties

that define a model. This “model definition” may be saved in a file (*.g3d).

Whether or not the model is saved it will remain in the OASIS workspace as

system parameters until a new model is opened.

GMS3MDEF also reads the “index grid” parameters (grid size, cell size, origin,

etc.) and stores them with the model. The index grid may be any one of the grids

in the model and is used to define the grid parameters and grid location, which

will be required for the other grids. You may specify gravity or magnetic DC-

Shifts to be added to the respective model response grids.

If “Auto Check Model” is selected, GMS3CHECK.GX (see below) will be run

automatically each time the model editing dialog is closed.

Menu item, “Model definition | Check Model” – GMS3CHECK.GX

The GMS3CHECK GX checks all the grids used in the model definition for

consistency with the index grid, with the option of running NGAGRDMCH.GX

to correct the non-conforming grids, then rechecks the model after any

adjustments have been made. Any time you change or add grids within a model,

you should re-run the check. If “Auto Check Model” is selected, the check is run

at the end of Edit Model.

Parameters checked in GMS3CHECK are as follows:

Grid size (number of rows and columns)

Cell size and units

Grid Origin

Presence of dummies in the grid

Crossing surfaces

If GMS3CHECK detects crossed surfaces, you will be offered the option to adjust

the upper or lower grid. If crossed surfaces are detected and fixed, GMS3CHECK

runs again until no crossed surfaces are detected.

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Menu item, “Model definition | Close Model” – GMS3CLOSE.GX

The GMS3CLOSE GX saves changes to the current model and clears the model

parameters in the workspace.

Menu item, “Model definition | Copy Model” – GMS3XHALF.GX

The GMS3XHALF GX duplicates of all the files required for the current model at

half scale (i.e., 2X the original grid spacing), preferably in a new subdirectory.

This is to ensure that file names common to both models will be protected from

modifications of the other model. This proves useful in creating faster-calculating

versions of large models.

Menu item, “Model definition | Copy Model” – GMS3MDUP.GX

The GMS3MDUP GX creates a duplicate of the current model in another

directory including all grids. This differs from the “Save As…” option in the Edit

Model dialog, which merely saves a new .g3d file with pointers to the old grids.

Menu item, “Model definition | Copy Model” – GMS32VOX.GX

The GMS32VOX GX creates a 3D density or susceptibility grid (geosoft-voxel)

from the current GMSYS-3D model. The resulting 3D grid can be displayed and

manipulated using the standard Oasis GX’s in the “Voxel” menu.

-- end of Model Definition submenu--

Menu Item, “New Constant Grid" - GMS3FGRD.GX

The GMS3FGRD GX creates a new grid with a constant value (e.g., a horizontal

plane or a constant density). The resulting grid is based on the index grid and is

compatible with the current 3D model.

Grid Editing submenu

These routines allow you to create or modify the values in an existing grid.

Menu item "Grid Editing | Path Edit” – NGAGEDIT.GX

The NGAGEDIT GX allows you to change the value of a grid along a digitized

path. The user must specify the amount of change, the width at which the change

rolls off to zero, and the roll-off method. For roll-off method, you may choose

from one of six tool shapes, shown below, or select a "constant" value to fill the

area.

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Gaussian cosine tent

left wedge right wedge square

You may use a polygon file to clip the effective area of the Path Edit function.

Clipping options are "none," "existing polygon," or "digitize polygon."

We recommend that you have the grid displayed in the current map prior to

initiating this procedure.

NGAGEDIT allows a single undo level; simply re-run the GX and select the

"Undo" button.

Menu item "Grid Editing | Polygon Area Edit” – NGAPOLYSM.GX

The NGAPOLYSM GX replaces the area inside an existing polygon by

calculating a 0- to 3rd

-order polynomial surface, a minimum curvature surface or

by applying an expression. The polynomial surface may be based on all points or

edge points only. Selecting “All” will utilize all points within the polygon. The

minimum curvature surface is calculated to fit to the area surrounding the

polygon. The expression can be any valid Geosoft expression involving up to six

grids or a Geosoft expression file.

Menu item "Grid Editing | Minimum Curvature Fill” – USGS_GRIDPLUG.GX

Fills dummy values in a grid by using polynomial or local median initialization

followed by minimum curvature iterations. This is the GX released by the USGS

in Phillips and others, 2003.

Menu item "Grid Editing | Digitize Polygon” – POLYDEF.GX

The standard Oasis POLYDEF GX allows you to digitize a polygon for use in the

"Path Edit" or "Polygon Area Edit" functions.

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Menu item "Grid Editing | Draw Polygon Location” – POLYDRAW.GX

The standard Oasis POLYDRAW GX draws an existing polygon file on the

current map.

Menu item "Grid Editing | Constraints from Wells” – NGAWCONS.GX

The NGAWCONS GX applies the standard edit tool shapes (see

NGAGEDIT.GX) to spot locations defined by a database (e.g., well locations) to

edit a “constraints” grid.

Menu item "Grid Editing | Constraints from Lines” – NGASCONS.GX

The NGASCONS GX applies the standard edit tool shapes (see NGAGEDIT.GX)

along a selected line in a database (e.g., seismic line shotpoints) to edit a

“constraints” grid.

-- end of Grid Editing submenu--

Display submenu

There are three customized routines for display of GMSYS-3D models in the

“Display” submenu. Four standard Oasis routines are also included without

modification in the GMSYS-3D | Display menu:

Menu item “Display | Create a Color Zone File for the Model” – GMS3ZONE.GX

The GMS3ZONE GX creates a color zone file encompassing all the structure

grids in the model.

Menu item “Display | Display Model as 3D stack” – GMS3DISPLAY.GX

The GMS3ZONE GX creates a 3D visualization of all structure grids plus an

optional observed data grid in the open model on a new map. If the map is

created and already open, this GX allows you to add additional grids to the 3D

visualization. Close an existing map before running the GX to recreate it.

Menu item “Display | Display Single Grid to 3D View” – GRIDIMG3D.GX

The GRIDIMG3D GX plots a single grid as a new drawing plane in a 3D View.

The grid may be placed in any orientation. Use this function to place a cross-

section (e.g., seismic image, GM-SYS profile, etc.) into your 3D model.

Menu item “Display | Display a Generic 3D Stack” – V3DIMG.GX

The standard Oasis GX, V3DIMG, creates a 3D display of a grid on the current

map, or on a new map, and opens the Geosoft 3D viewing tool. Additional

grids/surfaces can be added to the 3D display.

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Menu item “Display | Display Structure Grids as 2D maps” – GMS3SMAPS.GX

The GMS3SMAPS GX creates 2D displays of all structure grids in the open

model on separate maps in one step.

Menu item “Display | Display Mag Anomaly Grids as 2D maps” – GMS3MAGMAPS.GX

The GMS3MAGMAPS GX creates 2D displays of the calculated, observed, and

error magnetic anomaly grids from the open model on separate maps in one step.

The maps will be created at ~1/3 the size of the OASIS window and placed along

the left side of the OASIS window.

Menu item “Display | Display Grav Anomaly Grids as 2D maps” – GMS3GRVMAPS.GX

The GMS3GRVMAPS GX creates 2D displays of the calculated, observed, and

error gravity anomaly grids from the open model on separate maps in one step.

The maps will be created at ~1/3 the size of the OASIS window and placed along

the left side of the OASIS window.

Menu item “Display | Display a Single Grid” – GRIDIMG1.GX

The standard Oasis GX, GRIDIMG1, creates a 2D display of any grid on the

current map or on a new map.

Menu item “Display | Register Current Map” – GMS3REGMAP.GX

The GMS3REGMAP GX links a color method and map to one of the anomaly

grids in the GMSYS-3D model.

This GX is designed to be called from other GXs which presumably have altered

a grid and want the changes to be reflected in an associated map, but may be

called interactively as well.

Menu item “Display | Rezone Current Map” – GMS3REZONE.GX

The GMS3REZONE GX rezones a Map after an underlying grid has been altered.

This GX searches the given map for an Aggregate containing the given grid, and

creates a new ITR for the Aggregate based on the grid, color table and zoning

method.

This GX is designed to be called from other GXs which presumably have altered

a grid and want the changes to be reflected in an associated map, but may be

called interactively as well.

Menu item “Display | Show Error Grid Statistics” – GMS3ERREDB.GX

When used interactively, the GMS3ERREDB GX opens the statistics database

created following the calculation process. Note that if the database has been

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minimumised it will make it the active database, but will not restore it to the

window.

-- end of Display submenu--

2D Cross Sections submenu

The 2D cross section routines provide tools for creating and modifying cross

section slices through your model. They also provide a means of extracting and

editing GM-SYS Profile (2 D) models from your 3D model.

Menu item “2D Cross Sections | 2D Density Cross Section Slice” – GMS3CX.GX

The GMS3CX GX draws a vertical slice

through a 3D model with density or

susceptibility displayed as a color grid

on a map. Calculated and observed

anomaly profiles may be displayed over

the section. Cursor position is displayed

as model X, Y, and Z-coordinates at the bottom, right of the Oasis montaj

window.

Menu item “2D Cross Sections | Change Cross Section VE” – XYSCALE.GX

The standard Oasis GX, XYSCALE, allows user to change the X and Y scales of

a view on the current map.

Menu item “2D Cross Sections | Extract 2D GMSYS Model” – GMS3PROF.GX

The GMS3PROF GX loads the open 3D model horizon and data grids into the

GMS3PROF dialog to make extracting GM-SYS 2D models from the 3D model

more convenient. Currently the routine allows the extraction of 15 sub-surface

horizons plus topography.

Menu item “2D Cross Sections | Open Existing 2D GMSYS Model” – GMS.GX

The GMS GX prompts for an existing 2D GM-SYS model and launches GM-SYS

Profile with that model.

-- end of 2D Modeling submenu--

Grid Preparation

Proper grid preparation is necessary to get accurate results, extending to the edge

of your data.

Required grid preparations include:

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1. Upward (or downward) continuing the observed gravity (and/or magnetic)

grid to a constant elevation

2. Expanding all grids to “good FFT” numbers

3. Filling the expanded grids to be periodic

4. Prior to the final display, the grids should be masked back to their original

boundaries.

If “Input Grids are Pre-Expanded?” is set to "NO" in the “Forward Calculation"

dialogue box, items 2-4 above are done automatically in the calculation routine,

using parameters set in the “Setup Automatic FFT Preparation” menu item.

Otherwise, you must expand them manually, via the "Manual FFT Preparation"

submenu. For a more complete discussion of grid expansion see the section

Building Models on page 7.

Menu item "Setup Automatic FFT Preparation” –

The GMS3XPND GX is used to set the parameters for the GMSYS-3D’s internal

FFT grid preparation. Using the automatic preparation you are allowed to change

the size (percent expansion) and the shape of the expanded grid. Other fixed

parameters are listed in the help for GMS3XPND.

This GX is also available through the “Calculate” menu.

Manual FFT Preparation submenu

Menu item “Manual FFT Preparation | Make Grids Coincident” - NGAGRDMCH.GX

The NGAGRDMCH GX forces an input grid to match an index grid in size, cell

spacing, projection, X0, and Y0. If the grids do not match the input grid will be

regridded with the proper parameters.

Menu item “Manual FFT Preparation | Expand Grid…” -.GX

The standard Oasis GX, GRIDXPND, will expand a grid and fill it with dummies.

To be sure that you expand it to “good FFT” dimensions, use the menu item

“Minimum % to Expand Grid” rather than specifying “ --X”, “…--Y”.

Menu item “Manual FFT Preparation | Fill Grid…” - GRIDFILL.GX

The standard Oasis GX, GRIDFILL, will fill expanded grids to be periodic. Note

that the default parameters that appear in this dialog are different than the defaults

used in the “Automatic FFT Preparation” (GMS3XPND.GX) described above.

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Menu item “Manual FFT Preparation | NGA Fill Grid…” - NGAPFILL.GX

The NGAPFILL GX will fill holes (i.e. replace dummy values) in grids which

have been previously expanded periodically. Dummies will be replaced using a

function with minimum of variation in the filled area.

Additional information is available in the help file for NGAPFILL.

Menu item “Manual FFT Preparation | Mask Grid…” - GRIDBOOL.GX

The GRIDBOOL GX can be used to mask the resultant expanded grids back to

the size of the original grid. This standard Oasis routine has not been customized

(yet) for more convenient use with GMSYS-3D.

-- end of Manual FFT Preparation submenu--

Calculate submenu

The calculation routines allow forward and inverse calculations from the currently

defined model. The simplest inversion is the “structural” inversion which allows

grid points on a single geologic horizon (a structure grid) to move up or down to

minimize the misfit between the observed data and the calculated model response.

One structural grid, the “Inversion Surface”, must be selected to be “free” for the

inversion.

Inversions can also be performed allowing lateral changes in the density or

susceptibility within a given layer. Those routines require that the

density/susceptibility for that layer be defined by a grid, although the initial

starting grid may be a constant (uniform) grid. The edit model dialogs allow the

insertion of density or susceptibility grids; however the grid must be created prior

to entering the “Edit Model” dialog.

A database (<modelname>_stats.gdb) is created and statistics of the error grid are

added after each calculation. This database is automatically opened and appended

with each calculation.

An “inline” filter may optionally be applied to the calculated data grids. This

provides a means of removing high frequency calculation artefacts near the nyqist

frequency. Filter control files can be created using the FFT2CON GX from the

Magmap routines.

Menu item " Calculate | Setup Automatic FFT Preparation” –

The GMS3XPND GX is used to set the parameters for the GMSYS-3D’s internal

FFT grid preparation. Using the automatic preparation you are allowed to change

the size (percent expansion) and the shape of the expanded grid. Other fixed

parameters are listed in the help for GMS3XPND.

This option is also available in the Main menu.

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Menu item, “Calculate | Forward Gravity Calculation” – GMS3GRAVF.GX

The GMS3GRAVF GX performs a forward gravity calculation of the model

currently in memory. You may specify an “inline filter” that will be applied to

the calculated result. Filters can be created using the FFT2CON GX from the

MAGMAP Oasis module.

Menu item, “Calculate | Forward Gravity Calc. - Voxel” – NGAGLAYER.GX

The NGAGLAYER GX performs a forward gravity calculation of a Geosoft 3D

voxel grid. The result is calculated at a constant elevation and placed in a grid

that matches the X and Y dimensions of the Geosoft voxel. Note that the

calculation level can be within the voxel.

Menu item, “Calculate | Gravity Structural Inversion” – GMS3SI.GX

The GMS3SI GX performs structural inversion based on the gravity mis-fit on a

selected horizon. Inversion only adjusts the selected horizon. Inversion updates

the calculated response and error grids to reflect the structural changes.

Note: inversion modifies the grid in the model but offers several options for

saving each step in the inversion process.

Menu item, “Calculate | Single Density Inversion” – GMS3RHOI.GX

The GMS3RHOI GX calculates the constant density for a single layer that

minimizes the average error.

Menu item, “Calculate | Lateral Density Inversion” – GMS3DI.GX

The GMS3DI GX calculates the density distribution in a single-layer lateral

density grid that minimizes the average error. Note: inversion modifies the grid

in the model but offers several options for saving each step in the inversion

process.

Menu item, “Calculate | Forward Mag Calculation” – GMS3MAGF.GX

The GMS3MAGF GX performs a forward magnetic calculation of the model

currently in memory. You may specify an “inline filter” that will be applied to

the calculated result. Filters can be created using the FFT2CON GX from the

MAGMAP Oasis module.

Menu item, “Calculate | Magnetic Structural Inverse” – GMS3MAGINV.GX

The GMS3MAGINV GX performs structural inversion based on the magnetic

mis-fit on a selected horizon. Inversion only adjusts the selected horizon.

Inversion updates the calculated response and error grids to reflect the structural

changes. Note: inversion modifies the grid in the model but offers several options

for saving each step in the inversion process.

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Menu item, “Calculate | Magnetic Susceptibility Inversion” – GMS3SUSCINV.GX

The GMS3SUSCINV GX performs susceptibility inversion based on the

magnetic mis-fit on a selected horizon. Inversion updates the calculated response

and error grids to reflect the changes. GMS3SUSCINV ignores any existing

lateral susceptibility grid in its calculations.

Note: inversion modifies the grid in the model but offers several options for

saving each step in the inversion process.

Menu item, “Calculate | Magnetic 1-Step Suscept Inversion” – GMS3OSSI.GX

The GMS3OSSI GX uses an alternative approach to perform susceptibility

inversion. This GX applies the Oasis MAGMAP Apparent Susceptibility Filter

(SUSC) to arrive at the lateral susceptibility variation, rather then utilizing the

normal inversion routines of GMSYS-3D. This is a somewhat faster process than

the “normal” GMSYS-3D routines which are used by the GMS3USCINV GX

(Menu item, “Calculate | Magnetic Susceptibility Inversion”).

The apparent susceptibility filter is in fact a compound filter that performs a

reduction to the pole, downward continuation to the source depth, correction for

the geometric effect of a vertical square ended prism, and division by the total

magnetic field to yield susceptibility. Using this approach we assume that there is

no remanent magnetization and that the magnetic response is caused by a

collection of vertical, square-ended prisms of infinite depth extent.

GMS3OSSI applies the apparent susceptibility filter to the magnetic error grid

and adds the result to the existing lateral susceptibility distribution grid. This is a

faster process than the “normal” GMSYS-3D routines and can be run multiple

times to improve the fit.

For more information see the Geosoft Help for FFT2CON SUSC.

Note: inversion modifies the susceptibility grid in the model and does a simple

one-level backup, which can be undone with the undo command. Back up grids

manually before running multiple inversions if you want to retain the original

horizon.

Menu item “Calculate | Calculate Gravity Gradients” – GMS3GGRAD.GX

The GMS3GGRAD.GX calculates any one of the six standard gravity gradient

components. The user chooses the desired component from a dropdown list and

supplies the name of the observed, calculated, and error grids.

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Menu item “Calculate | Show Error Grid Statistics” – GMS3ERREDB.GX

The GMS3ERREDB GX writes a new entry in a database, recording the error

statistics of the gravity error grid. This GX will be called following recalculation

of the error grid, providing an ongoing record of changes during the modeling

process.

-- end of Calculate submenu--

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GMSYS-3D TOOLBAR

The GMSYS-3D toolbar provides an alternate means of accessing many of the

more commonly used procedures. All the procedures are available from the

GMSYS-3D menu and the discussion below refers you to the appropriate pages of

this User’s Guide. “Fly-over” help is available for each tools in you leave the

cursor over the tool for a few seconds.

New Model (page 10)

Open Model (page 11)

Check Model (page 11)

Edit Model (page 11)

Show Model Parameters (page 10)

New Constant Grid (page 12)

Edit Layer; shortcut into Edit Model (page 11)

Path Edit (page 12)

Polygon Area Edit (page 13)

Extract 2D Density Profile (page 16)

Build GM-SYS (Profile) Model (page 16)

Gravity Forward Calc (page 19)

Gravity Structural Inverse (page 19)

Magnetic Forward Calc (page 19)

Display Model as a 3D Stack (page 14)

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DEMO MODELS

Two demo models are included with this distribution: Salt and Death Valley.

These models are installed as subdirectories under Program Files\Geosoft\

Oasis montaj\ data\gmsys-3d. We recommend that you copy these subdirectories

into a separate “working directory” prior to opening and manipulating the models.

Thus, you will maintain the clean original copy.

Salt

Salt is a demonstration model that represents a simple offshore salt dome. The

model consists of six grids. The files are installed in the "salt_model"

subdirectory. You may also find it on the CD in the "Salt_demo" directory. The

Salt and Salt2 models are used in the GMSYS-3D Tutorial.

Grid Name Contents

sealevel.grd flat surface at elevation=0,

bathy.grd bathymetric surface,

top_salt.grd top of salt body,

bottom_salt.grd base of salt body,

basement.grd basement surface,

salt_obsgrav.grd observed gravity.

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Death Valley

Death Valley covers a small area in southern Nevada. The data were provided by

the USGS and were taken from OFR-00-189. The installation puts this model

into the “d_valley” subdirectory. In CD-based distributions, you may also find it

in the DeathValley_demo directory.

Just open the “dv_demo” workspace to see the 3D model. The grids in the initial

model are:

The model used a Bouguer density of 2.67, ZREF= 2315 meters, and half-space

bottom = -13,000 meters. The layer densities from top to bottom are 2.00, 2.10,

2.30, 2.42, and 2.65 gm/cc.

After calculating the response of the model, thereby refreshing dv_calc1.grd, we

performed a structural inversion on the basement horizon, using a “Regional

Offset” of 0.0 mGals. (This assumes that you used a “DC-shift” of 18 in the first

screen of the “Model Definition”.) Adjusting the Regional Offset moves the

average level of the layer being inverted up or down. The resulting basement grid

is in Basement_inv_1.grd, the resulting calculated response of the model is in

dv_calc2.grd, and the resulting error is in dv_err2.grd.

Note that inversion modifies the grid in the model and does a simple one-level

backup, which can be undone with the undo command. Back up grids manually

before running multiple inversions if you want to retain the original horizon.

Grid Name Contents

topo_m_1.grd Topography

c1_m_1.grd 1st subsurface horizon

c2_m_1.grd 2nd subsurface horizon

c3_m_1.grd 3rd subsurface horizon

Basement_m_1.grd Top of basement

gravity_m_1s.grd Observed Bouguer gravity

dv_calc1.grd Calculated model response

dv_err1.grd Calculated error

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REFERENCES

Blakely, R.J., 1995, Potential Theory in Gravity & Magnetic Applications: Cambridge

University Press, pp.441.

Blakely, R.J., Langenheim, V. E., and Ponce, D.A., 2000, Summary of Geophysical

Investigations of the Death Valley Regional Water-Flow Modeling Project,

Nevada and California. U.S.Geological Survey Open-File Report 00-189

Parker, R.L., 1972, The rapid calculation of potential anomalies: Geophysical Journal of

the Royal Astronomical Society, v.42:315-334.

Phillips, J.D., Duval, J.S., and Saltus, R.W., 2003, Geosoft eXecutables (GX’s)

developed by the U.S. Geological Survey, version 1.0, with a viewgraph tutorial

on GX development. U.S.Geological Survey Open-File Report 03-010

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Page A-1

APPENDIX A MODEL FILE (*.g3d) FORMAT

As GMSYS-3D is developed, new model parameters are added and some become

obsolete. In order to “update” a model’s parameters to the current specification,

you may run GMSPREEN GX immediately after opening a model with a new

release.

Model defining parameters for Release 1.2 include:

NOTES:

1. # represents an integer number

2. [GOBS|GCALC|MISFIT|#] means a number of one of GOBS, etc

3. Indexed parameters like DDRHO[#] are zero based. However, The zero

element as it shows up in the Parameter block is DDRHO, not DDRHO[0]. In

the GX, you can use either form.

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NGA_3MODEL NGA_3MODEL CONVERT ............. Conversion factor; input unit to Km NGA_3MODEL GDCSHIFT ............ Constant to add to calculated gravity response NGA_3MODEL DEN_0 ................... Constant Layer Density for background (Air) NGA_3MODEL DX ......................... Grid Spacing in X NGA_3MODEL DY ......................... Grid Spacing in Y NGA_3MODEL GRAVBG ............... Background Gravity (calculated) NGA_3MODEL HUNITS . ................ Units (horizontal) NGA_3MODEL KX ......................... Grid Orientation NGA_3MODEL M2H ....................... Conversion factor to km (horizontal) NGA_3MODEL M2V ....................... Conversion factor to km (vertical) NGA_3MODEL NAME .................... Model File name NGA_3MODEL NLAYERS ............. Number of Layers NGA_3MODEL NUMDENS ............ Number of sublayers in DD relationship NGA_3MODEL NX ......................... Number of grid columns NGA_3MODEL NY ......................... Number of grid rows NGA_3MODEL PREEXPANDED ... 0 if not expanded; 1 if expanded NGA_3MODEL RECALC ................ 0 if not needed; 1 if needed NGA_3MODEL ROT ....................... Grid Rotation NGA_3MODEL VUNITS ................. Units (vertical; should be same as horizontal) NGA_3MODEL X0 .......................... Grid Origin NGA_3MODEL Y0 .......................... Grid Origin NGA_3MODEL ZBOTTOM ............. Elevation of bottom of halfspace (Z+ up) NGA_3MODEL GZREF .................. Elevation of Gravity Obs/Calc. NGA_3MODEL DDOFF_[#] ............ Offset for DD sublayer # NGA_3MODEL DDRHO_[#] .......... Density for DD sublayer # NGA_3MODEL DEN_# .................. Constant density for layer # NGA_3MODEL DEN_APP_# ........ "Apparent" density for layer # ( last VDD

sublayer density ) NGA_3MODEL DEN_#[#] ............... ( obsolete ) NGA_3MODEL DENSRCNAME_# Source of density info for layer # (text) NGA_3MODEL DENSRC_# .......... Source of density info for layer # (list alias

integer) NGA_3MODEL SUSCSRC_# ....... Source of susc. info for layer # NGA_3MODEL MZREF .................. Elevation of Magnetics Obs/Calc. NGA_3MODEL MDCSHIFT ............ Constant to add to calculated Mag response NGA_3MODEL EFIELD .................. Earth's Field Strength (cgs) NGA_3MODEL EINC ...................... Earth's Field Inclination NGA_3MODEL EDEC .................... Earth's Field Declination NGA_3MODEL BSUSC_# ............. Susceptibility of Layer # (cgs) NGA_3MODEL BRMAG_# ............ Magnitude of Remanent Mag. (cgs) NGA_3MODEL BINC_# .................. Inclination of Remanent Mag. NGA_3MODEL BDEC_# ................ Declination of Remanent Mag. NGA_3MODEL DCSHIFT ............... ( obsolete ) NGA_3MODEL DENDIFF ............... ( obsolete ) NGA_3MODEL IDENSITY .............. ( obsolete ) NGA_3MODEL NSURF .................. (obsolete) Number of Layers NGA_3MODEL ZREF ..................... ( obsolete ) NGA_3MODEL DENGRID_# ......... ( obsolete ) NGA_3MODEL DENGRID# ........... ( a bug )

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Page A-3

NGA_3MODEL NDEN_# ............... ( obsolete ) NGA_3MODEL THK_#[#] ............... ( obsolete )

NGA_3MODEL NGA_3MODEL EDITDA ................ Decimal Year of last gms3mdef.gx run NGA_3MODEL EDITHR ................ Decimal Hour of last gms3mdef.gx run

NGA_E3GRID – filenames: NGA_E3GRID DDREFSUR .......... Reference surface for DD relationship NGA_E3GRID GOBS .................... Observed Gravity NGA_E3GRID GCALC .................. Calculated Gravity NGA_E3GRID GERR .................... Error in Gravity NGA_E3GRID MOBS .................... Observed Magnetics NGA_E3GRID MCALC .................. Calculated Magnetics NGA_E3GRID MERR .................... Error in Magnetics NGA_E3GRID IDX ........................ Index grid NGA_E3GRID MISFIT .................. (repl by GERR) NGA_E3GRID SURF_# ................ Surface grid for layer # NGA_E3GRID DENGRID_# .......... Lateral density grid for layer # NGA_E3GRID SUSCGRID_# ....... Lateral susc. grid for layer # NGA_E3GRID DENSITY ............... ( obsolete ) NGA_E3GRID IGOBS ................... ( obsolete ) NGA_E3GRID PARTIAL ............... ( to be obsolete ) NGA_E3GRID Z1 .......................... ( to be obsolete ) NGA_E3GRID Z2 .......................... ( to be obsolete )

NGA_3DISPLAY – display parameters NGA_3DISPLAY GWSDIR ............... The Geosoft Workspace directory NGA_3DISPLAY 3DGMETHOD ....... Color method for gravity anomaly grids in

3DMAPNAME NGA_3DISPLAY 3DGTBL ................ Color table for gravity anomaly grids in

3DMAPNAME NGA_3DISPLAY 3DMAPNAME ....... Map containing 3D model NGA_3DISPLAY 3DVXG .................. VE used in 3DMAPNAME NGA_3DISPLAY 3DZONE ................ Color zone file for structure grids in

3DMAPNAME NGA_3DISPLAY CTBL_ANOM_DEF Default color table for 2D anomaly maps NGA_3DISPLAY CTBL_ERR_DEF .. Default color table for 2D error maps NGA_3DISPLAY CTBL_STRUCT_DEF Default color table for 2D structure maps NGA_3DISPLAY ZONE_ANOM_DEF Default color zone file for 2D anomaly maps NGA_3DISPLAY ZONE_ERR_DEF . Default color zone file for 2D error maps NGA_3DISPLAY ZONE_STRUCT_DEF Default color zone file for 2D structure

maps NGA_3DISPLAY CTBL_[GOBS|GCALC|GERR] Color tables used for gravity

anomaly maps NGA_3DISPLAY MAP_[GOBS|GCALC|GERR] Maps containing gravity

anomalies NGA_3DISPLAY ZONE_[GOBS|GCALC|GERR] Color zone file for gravity

anomaly maps

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NGA_3DISPLAY CTBL_[MOBS|MCALC|MERR] Color tables used for magnetic anomaly maps

NGA_3DISPLAY MAP_[MOBS|MCALC|MERR] Maps containing magnetic anomalies

NGA_3DISPLAY ZONE_[MOBS|MCALC|MERR] Color zone file for magnetic anomaly maps

NGA_3DISPLAY CTBL_# ................ Color tables used for structure maps ( not implimented )

NGA_3DISPLAY MAP_# ................. Maps containing structure grids " NGA_3DISPLAY ZONE_# ............... Color zone file for structure maps "

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APPENDIX B NEW FEATURES IN VERSION 1.3

Release 1.3 adds:

Capability to write a GMSYS-3D model to a Geosoft voxel.

Capability to calculate the gravity response of a Geosoft density voxel.

Time-to-depth conversion: build a model in time, then use velocities to convert to

a depth model.

“Seismic” tab for time models.

“Velocity” tab for layer parameters.

DDTab – rewritten

ASCII file import is more robust

fixed several bugs in the dialog.

Backing up inversion target grid.

Resample model to half-size and write to a new model name.

Include up to 15 horizons from 3D model to GM-SYS Profile models.

Integrated GX help with Oasis montaj.

“Auto” DC-shift button added for Gravity and Magnetic calculations.

Better UNICODE handling

3D Model Explorer; displays information about the current model in a modeless

dialog.

Upward continue the observed grid to a draped surface.

In addition there are a number of improvements to existing features:

The Density/Depth dialog has been completely rewritten. ASCII file import is

more robust and several bugs were fixed in the dialog.

DDTab parameters weren’t correctly written to the model file.

Fixed loop that caused reasonable density values to cause errors.

Error grid name is not required to run calculations.

Improved “Undo” functionality for inversion.

Improved “redraw” code so maps are only redrawn when changed.

Dialogs don’t lose information when you switch tabs.

Building a constraints grid from a well database doesn’t fail when the first well is

off the map.

New grids should always have their projection set to match the model.

Improved checking to prevent calculations starting when required grids were missing.