Contents

Help is available on the following topics :-

Introduction

Beginning a Model

Setting up the Field Data File

Model Editing Options

Inversion Options

Display Options

System Options

Register your Mag2dc Now!

Legal Business

For Help on Help, type F1


Introduction

Mag2dc calculates the magnetic anomaly (induced and/or remanent) over one or more 2.5 dimensional bodies, and runs on any IBM PC or compatible capable of running Microsoft Windows. The model is entered and altered using an onscreen cursor operated by the mouse. Output can be produced on any device supported by Windows..

Mag2dc is written using Borland Delphi, and uses a Talwani type algorithm to calculate the anomaly. The contribution to the anomaly from each of the bodies is stored separately, so that a change to one body does not require the whole model to be recalculated. There may be up to 16 bodies in the model, each of which may have up to 50 corners. There may be up to 650 observed magnetic stations. Susceptibilities are measured in c.g.s . or SI units and the anomaly is measured in nT. Distances may be measured in Km or m.

As well as forward modelling, Mag2dc allows the use of inversion, wherein mathematical techniques are used to modify the shape (and other attributes) of the model to improve the fit of the calculated curves to the observed data. Whilst being a useful tool, it should be remembered that the blind usage of inversion without reference to the available geological information may result in an incorrect interpretation.

Installation consists of simply creating a subdirectory for Mag2dc and copying the files supplied on the distribution disk into it. It can then be added to any suitable group in your Windows environment in the normal way. The use of a maths coprocessor is reccomended. If there is one installed in the computer, then it will be used automatically.


3) BEGINNING A MODEL
Beginning a Model

When 'Start a New Model' is selected from the menu, a dialog window appears allowing you to enter the initial parameters that describe the model that you wish to create. The information required is :-

Profile bearing ; in degrees positive clockwise from true North.

Reference height ; ie. flight height for aeromagnetic data - positive upwards.

Maximum depth ; the initial maximum depth displayed in the model. It may be changed later.

Geomagnetic field intensity, in nT

Geomagnetic field inclination  ;  measured in degrees negative upwards

Geomagnetic declination  ;   measured in degrees positive clockwise from true North

Susceptibility of the initial body in the model in CGS units.

Strike length  ; the initial strike length of

Station spacing ; only used if there is no field data present in the model.

Number of points ; ''          ''          ''          ''          ''           ''           ''

Units of measure ; Km or m. , SI or CGS.

Remanence  is not input to the initial body at this time, but may be added later.  When all of the parameters have been entered, press the ok button to proceed.

If the checkbox indicating the presence of observed field data was checked, then a file dialog will now appear, enabling the filename to be input. it is possible to move around the disk to different subdirectories, or even to another disk, to get the file. The file format is described below.

The data will be read in from disk and displayed in a spreadsheet format. The number of comment lines at the top of the file can be adjusted. Any 'bad' data points will be marked with a row of asterisks ***** on the left of the display. If these values are not corrected then they will be ignored. If the field data profile does not lie on a straight line, the the x and y coordinates describing the profile shape may be read in from the data file also. See below for a description of the format. Indicate which columns of the file the data and positional information lie in using the spinbuttons.

If required, either a user-input value or the data average can be removed from the data as it is read in. Select the appropriate radiobutton.

The body shape itself must then be entered using the mouse. Use the left button to enter each corner in turn, and the right button to close the body. The corners must be entered in a clockwise manner. The coordinates and number of each corner will be displayed as they are entered.

If  'Load an Old Model' was selected, then the file dialog will appear, and all that is needed is to select the model. It will then be calculated and displayed onscreen, ready for further editing.


Setting up the Field Data File

 If there is observed data to be modelled, then this must have been stored as an ASCII file on disk prior to running Mag2dc. This file may be created with any editor, such as the Windows Notepad etc. The data must be in the following format :

The

Comment

Lines

  X1          Mag1        	Mag1'	Mag1''	Mag1'''

  X2          Mag2    	Mag2'	Mag2''	Mag2'''

  :               :                  :	   :	   :

  XN          MagN       MagN'	MagN''	MagN'''

The comment lines may be left blank, or may contain information on the survey etc. Following this are up to five fields of data. One must contain the station locations, which need not be at a fixed spacing, and the other the observed magnetic values.  None of the data fields need lie in fixed columns or be justified in any fashion. It is not necessary to know how many points are in the file prior to reading it in. The maximum number of points is 650. The file  Mag.dta is provided as an example.

If the profile did not lie on a straight line, and the checkbox indicating this fact was marked, then one of the columns in the the data file must be the Y coordinate, where X refers to E-W and Y to N-S.

The information in a file of observed data will override any details entered in the rest of the initial entry screen Eg. if the menu screen is completed with 207 points and a station spacing of 0.52 Km, but the observed data file contains 47 points with variable spacing, then the latter information will be used.

Separate the x, y, and mag. values by spaces, not tabs or commas. Do not leave any blank lines at the end of the file after the last data point - the program will read until it reaches the end of the file, and if there is no data there then it may get unhappy.


Model Editing Options

Selecting a Body

Adding a Body to the Model

Reading in a file containing body coordinates

Key in a Body's Coordinates

Changing the Shape of a Body with the Mouse

Changing the Shape of a Body Numerically

Adding Corners to a Body

Deleting Corners from a Body

Linking Body Corners

Unlinking Body Corners

Deleting a Body from the Model

Hiding a Body

Moving a Body

Copying a Body

Turning a Body

Changing the Susceptibility, Depth, Width, or Strike Length of a Body

Altering the Remanence of a Body

Abstracting Part of the Profile

Removing a Regional Field

Smoothing the Observed Data

Changing the Model Global Parameters


Inversion Options

Inversion is a mathematical technique that will try and modify various parameters of a body in such a way as to try and improve that fit between the calculated and observed data. More technically, the program uses ridge-regression and singular value decomposition to minimise the error between the two curves in a least-squares  sense. The parameters that may be inverted have been split into two groups ; those that affect the whole body (Eg. susceptibility), and those that affect only one corner of a body. Up to 10 parameters of a body may be inverted simultaneously. Obviously, the less the number of parameters that are being inverted, the faster the inversion will proceed. The model may contain multiple bodies, but only one body may be inverted at one time.

Inversion must be used with care. It will attempt to produce a model that fits the data, but there is no guarantee that such a model is geologically reasonable. It is the job of the geophysicist to ensure that the model makes sense. When applying inversion, remember also to apply common sense. If inverting a single body, get the whole body inversion parameters reasonably correct before inverting body corner positions. If several bodies are to be inverted, get the whole body parameters of each correct before doing the corner positions of any. Whilst Mag2dc can invert 10 body parameters simultaneously, this is rarely a good idea. Rather select a set of two or three parameters, invert them, them switch to an alternate parameter set. If your starting model is too far out, then the inversion will struggle, and may well get stuck in a local minimum of the error surface.

Whole Body Inversion Parameters

Body Corner Inversion Parameters

Inverting the Model


Display Options

Displaying the Body Numbers

Displaying the Body Susceptibilities

Highlighting the Body Corners

Plan View of the Model

Displaying the Difference between the Calculated and the Observed Magnetic Curves

Displaying the Contributions from Individual Bodies Separately

Plot the Observed Data Points

Selecting a Particular Range of Depths and Positions to Display

Zooming in to Part of the Profile

Zoom fully Back Out

Add Borehole Data

Display a Map of the Profile

Setting the Grid Optionsl

Setting the Font Size

Changing the Colour Scheme


System Options

Saving the Model to Disk

Importing a Model from Grav2dc

Editing the Field Data

Printout the Model  to Printer, Disk or Clipboard

Create an HP-GL File

Create a DXF File

Setting Up the Printer

Producing the HardCopy

Expert Mode

Leaving Mag2dc


Importing a Model from Grav2dc

This option will read in a Grav2dc saved model. Densities will be converted to susceptibilities, and any field data will be discarded.


Selecting a Body

All editing operations Eg. deleting a body, are done on the currently selected body. This is drawn in a bolder outline than the other bodies. To select a body, place the cross-hair cursor somewhere inside it and press the left mouse button. The model should then be redrawn accordingly.


Adding a Body to the Model

When this option is selected, a dialog window will appear allowing the entry of the new body's susceptibility and strike length. The remanent  parameters may optionally be added later if desired. The body shape itself must then be entered using the mouse. Use the left button to enter each corner in turn, and the right button to close the body. The corners must be entered in a clockwise manner. The coordinates and number of each corner will be displayed as they are entered.


Key in a Body's Coordinates

         As described in 'Adding a Body to the Model', all bodies will lie onscreen {since it is impossible to move the cursor offscreen} : however to avoid edgeeffects it is often useful to have a body extending offscreen, perhaps to 'infinity'. In this case the numerical values of the coordinates are entered, and the cursor keys are not used. Pressing the spinbutton will cycle through the corners of the body (which must be entered in a clockwise manner). When all of the corner positions have been entered press the OK button to see the updated model.


Reading in a File Containing Body Coordinates

     If the coordinates for the initial model are already known Eg. if they have been digitised from a seismic section, then this information may be read directly into the program. The file must have the following format :-

Comment Line

******************

* No. Bodies   	* 2

* Suscept. 1    	* 0.002

* No. Corners  	* 4

*      1       	* 10.0   10.0

*      2            	* 40.0   10.0

*      3            	* 40.0   20.0

*      4            	* 11.0   20.0

* Suscept. 2    	* 0.004

* No. Corners      	* 3

*      1            	*   100.0   20.0

*      2            	*   121.0    20.0

*      3            	*   110.0    30.0

******************

The coordinates need not lie in any fixed columns, but columns 1-15 are reserved for the explanatory comments. The file Model.bod is provided as an example. If a mistake is made in the file format then this should be detected, and an appropriate message will be displayed.


Changing the Shape of a Body (Alt-C)
Changing the Shape of a Body with the Mouse (Ctrl-C)

To select a corner of the currently selected body to be edited, click on it with the left mouse button. If it was selected correctly, then it will be marked with a large dot, and the corner number and coordinates will be displayed. Mark the new position by clicking again with the left button, and the body will be recalculated. This procedure may repeated as often as desired, with the same or different corners. When the editing is finished, press the right button.

It isn't possible to add a new corner to a body without reentering it. Be careful that you don't alter the corners of a body to such an extent that they  become ordered anticlockwise. If you are unable to find the corner you require, check that you are working with the correct body.


Changing the Body Shape Numerically

It is possible to enter the coordinates of the new position of a body corner directly, rather than indicating the location with the mouse. Selecting this option will call up a dialog box that shows the numerical position of the first corner (in the order that they were entered) of the currently selected body. To change the position, simply enter the new horizontal position or depth of the corner. To see the effect of the change upon the anomaly, press the Calculate button. To move to the next corner, press the Next button. In a similar manner, the Previous button will move to the prior corner in the sequence. Press the OK button when all changes are completed.


Linking Body Corners Together

Often it is desirable to have two bodies exactly abutting onto one another. This is possible by using the Grid option, but it can also be done by linking the two corners together. After selecting Link from the menu, mark the first corner to be linked (on the current body) by clicking on it with the left mouse button. It will be redrawn with a red dot to indicate it was successfully marked. Then click on the corner (of another body) that it is to be linked to. The first corner selected will then move to the position of the second one. Once all the desired links have been established, press the right mouse button to leave the option.

In addition, any changes that are made to the position of either of the corners by the options 'Change a corner with the mouse' , 'Change a corner numerically' , 'Move a Body', or 'Turn a Body' will be reflected in both corners ie they will remain linked together. One body may be linked to several other bodies at the same time, but only two corners may be joined together at any one intersection.  If an attempt is made to link a corner to a second corner which is already linked to a third body, then the old link will be broken when the new link is established.

Linking does not function with inversion. If an attempt is made to invert any parameter of a body that has linked corners, the links will automatically be broken.

Hidden bodies may not be linked, but a body may be first linked then made hidden later. Corners may be unlinked using the Unlink Body Corners  option.


Unlink Body Corners

To break a link between two body corners, use this option. Use the left mouse button to select the linked corners. Press the right mouse button when all desired corners have been unlinked.


Adding Corners to a Body

To add a corner to a body, first select the corner prior to the new corner (in a clockwise sense), then click on the position of the new corner. The model will then be updated.


Deleting Corners from a Body

After selecting this option, click on the body corners that use wish to delete, using the left mouse button. They will be marked with a red asterisk *. Click on the corner again to deselect it. Press the right mouse button to redraw the model and recalculate the body. Note that all bodies must have at least three corners.


Deleting a Body from the Model(Alt-D)
Deleting a Body from the Model(Ctrl-D)

This option will delete the currently selected body from the model. A window will appear first, asking for confirmation before the body is removed.


Hiding a Body

Sometimes it is useful to see what effect a particular body has to the model. If it is deleted, then it cannot be re-instated without being re-entered ; if the body is complex then this is very time consuming. A body may therefore be temporarily removed, or hidden, from the model using this option. The body will be displayed with a white fill with a dotted line type, and it's contribution to the model will not be included. To re-instate it, simply toggle the hide option for that body.


Moving a Body (Alt-M)
Moving a Body (Ctrl-M)

After this option is chosen a dialog window will appear. The distance that the body is to be moved may be entered directly by typing in the distances in the horizontal and vertical planes and pressing the calculate button, or by pressing the adjacent spinbuttons. These will add/subtract the distance shown in the adjacent box to the body's position and recalculate the model. The distance by which the body is moved at each button press may be changed by entering a new value into the box.

The distances may also be estimated graphically. Click on the checkbox, then press the OK button. Then click and hold down the left mouse button with the cursor over the body, and drag the body to its new position. When the mouse button is released the model will be recalculated and redrawn.


Copying a Body

This feature reduces the amount of work needed to enter a series of similar bodies. It works in a similar fashion to the above option for moving a body, except that when the movement vector has been entered a copy of the original body will be created. The copy will have the susceptibility , remanence , and strike length  of the original body.


Turning a Body

Any body may be rotated about it's centre by using this option. This is done by pressing the adjacent spinbuttons, which will cause the body to be rotated clockwise/anticlockwise by the angle shown in the adjacent box. This value may be changed by simply replacing it with the desired value.

Beware of rotating bodies when the scales in the horizontal and vertical directions are very different, since the results may be unexpected ! (If the rotation of a body would cause any part of it to move above the ground surface, then these corners are clipped).


Changing the Susceptibility, Remanence, or Strike Length of a Body
Changing the Susceptibility, Depth, Width, or Strike Length of a Body

Either double-clicking on a body or selecting the 'Change Body Properties' option from the main menu calls up a dialog window that shows the current values of the aforementioned parameters. Enter the new values and press the Calculate button, and the model will be updated. Pressing the adjaceny spinbuttons will move you forwards or backwards through the different bodies in the model, although any hidden bodies will be skipped. Pressing the spinbuttons near the parameters will increment or decrement them by the amount shown in the nearby entry field. This amount is initially set to 10% of the current value, but it may be changed at any time.The depth shown is the minimum depth of the body, whilst the width shown is the maximum width.

The invert buttons displayed will invert the adjacent parameters for the number of iterations displayed. The limits on inversion set up in the main inversion menu option will be used to control the inversion.


Changing the Remanent Parameters of a Body

The remanence of a body is described by three parameters: the Intensity, which is measured in nT and is equivalent to Susceptibility*Geomagnetic intensity for the induced anomaly : the inclination  of the remanent field, and the last parameter is an angle giving the direction of the remanence in the horizontal plane.

Entering the values of the remanent parameters and then either pressing the Ok or Calculate buttons will cause the model to be updated. Alternatively, pressing the spinbuttons next to each parameter will add or subtract the value in the adjacent box to it, and cause the model to be updated. Pressing the Invert button will invert the parameters that have been selected using the checkboxes for the number of iterations shown at the top of the dialog box.


Abstracting Part of the Profile

Sometimes it is desired to work with only a portion of the dataset. Part of the profile can therefore be abstracted for detailed study. When this option is selected, the profile will appear in a window, along with two vertical markers. The section of the profile between the markers is the part that is to be worked with. Use the left mouse button to move the left marker, and the right button to move the right marker. The exact positions of each marker is displayed near the bottom of the dialog. When the markers are correctly positioned, select the Exit option from the menu on the window.


Removing a Regional Field

If the dataset has not already had the regional field removed, then it can be done using Mag2dc. The profile will be initially displayed in a window, and the regional is shown as a straight line through the data.

There are three possible models for the regional ; a constant, a linear, and a cubic spline. The value of the constant may be  adjusted using the upper spinbuttons. Alternatively, a best fir linear can be applied to the data. This is calculated using the L1 norm rather than the more usual L2 norm, since this gives less weight to any spikes present in the data. The gradient and intercept of the line can be adjusted using the spinbuttons.

The cubic spline option allows the user to draw his own shaped regional field, of arbitrary shape. The profile is split into seven regions by vertical dotted lines. Clicking with the mouse to the right of each division will set the value of the field at the position of the dotted line.


Smoothing the Observed Data

The observed magnetic data may be smoothed if it is unduely noisy, by the application of a 3,5, or 7 point moving average filter. The filter may be applied repeatedly to enhance the smoothing effect. Beware of applying it too much however, since this will have the effect of broadening anomalies, and hence making the causative bodies appear deeper than they actually are.

The weights of the filter may also be adjusted Eg instead of a 3 point moving average filter having weights of (1,1,1), (1,2,1) could be used.  Make sure that 'None' is selected if you wish to close the dialog box without applying any filtering to the data. Example weighted filters can be chosen from those dipslayed.


Changing the Model Global Parameters

Once the model has been entered, it is not too late to change a whole range of parameters. Changing anything from this menu will cause the recalculation of the whole model, since all of the bodies will be affected.  The parameters which may be altered are:

Intensity of the Earth's field (nT)

Declination    ''    ''    '' 	(degrees positive from true North)

Inclination    ''    ''    '' 	(positive downwards)

Profile bearing	(degrees positive from true North)

Reference height	(positive upwards)

Distance units	Km or m.

Susceptibility units	SI or CGS

Use a value of 1-2m for the reference height for ground mag. data (depending on the height of your technician!) or the flight height for aeromagnetic data.


Displaying the Body Numbers

To find out which body is which, select this option and the body numbers will be displayed approximately in the centre of each body.


Displaying the Body Susceptibilities

To find out what the susceptibilities  of the bodies are, select this option and they will be displayed approximately in the centre of each body.


Highlighting the Body Corners

If several body corners lie on or near a straight line then it can be hard to see exactly where the corners are. Selecting this option will cause all the body corners to be highlighted, enabling them to be easily found. To return the model display to normal, select this option once more.


Plan View of the Model

To view the different strike lengths of the bodies in the model, select this option and the model will be displayed as seen from above. The deeper bodies will be displayed in darker colours, to give a sense of depth to the plot. Obviously, no editing of body corners may be done with this view displayed, but changes to susceptibilities and strike lengths are still possible. Toggle the option again to return to the normal editing mode.


Displaying the Difference between the Calculated and the Observed Magnetic Curves

Select this option to display a graph of the difference between the calculated and observed anomalies.  The zero line is drawn horizontally across the screen, with the difference graph across it. Where the curve is above the zero line the calculated curve values are higher than the observed values.


Displaying the Contributions from Individual Bodies Separately

This option enables the contribution from each body in the model to be displayed separately, rather than the total anomaly. If there are many bodies then the resulting display will be somewhat complex. Toggle the option again to return to the normal display.


Plot the Observed Data Points

Normally the observed data is displayed as a dashed green line. If required the data may be plotted as a series of green stars, one for each observed data point.


Selecting a Particular Range of Depths to Display
Selecting a Particular Range of Depths and Positions to Display

Both the minimum and maximum depths and horizontal positions to be displayed can be set, enabling a zoom feature. Note that even though a body isn't visible, the anomaly is still calculated and displayed.


Zooming in to Part of the Profile (Alt-Z)
Zooming in to Part of the Profile (Ctrl-Z)

         It is possible to zoom in to part of the model to obtain a more detailed view.  Mark any corner of the desired view by clicking with the left button, then with the button still held down, move the cursor to the opposite corner. A 'rubber-band' box will be displayed, showing the area which will be zoomed into. Release the button to enable the zoom to take place.


Zoom fully Back Out

This undoes the effects of any previous zooms, and redisplays the whole of the model.


Add Borehole Data

If there is some borehole information to control the model, then this may also be displayed. An ASCII file with the following format must be created :-

Ranger 1	: Label for the whole hole

Test Borehole Data	: Comment line

for Mag2dc for Windows	: Comment line

Shale	: Rock type

3000.0 0.0	: Position on the profile and depth of the hole

3000.0 20.0

3100.0 40.0

Conglomerate	: Next rock type

3150.0 60.0	: etc.

3150.0 70.0

Sandstone

3200.0 80.0

3210.0 100.0

Up to three boreholes may be displayed, each containing up to 50 coordinate pairs describing the hole position. There may be a maximum of 9 changes in rock type per hole (ie. 10 rock types per hole). The file test.hol is provided on disk as an example of the format.


Setting the Grid Interval
Setting the Grid Options

A user-specified grid may be overlain on the model, and body corner positions may be made to snap to it. The frequency at which the grid is plotted may be set, as well as the amount of bolder points in the display. The grid interval may be set independantly in the horizontal and vertical directions. If a model has been read in from the DOS version of Mag2dc, then the Snap values from that model will be used to set the grid interval here.

If desired, the corners of all bodies may be forced to lie on the grid. This makes it easier to create bodies with exactly vertical, horizontal, or parallel sides. Once this option is activated then all the corners of new bodies entered will lie on grid points. Existing bodies will not be affected. Body corner positions will also snap to the grid during editing, but inversion will not be affected.


Setting the Font Size

If a very high resolution screen is being used, the default font used for labelling the graphs can be hard to read. This option allows the font size to be adjusted to whatever size is desired.


Setting a Colour Scheme

By default all the bodies in the model will be coloured an exciting grey colour. Alternatively, the shade of grey used can be made to correspond to the susceptibilities of the body, with a light shade indicating a high value (relative to the other bodies), and a dark shade indicating a low value. Colours can also be used, with red indicating a high susceptibility value, blue a low value, and green an intermediate value.


Saving the Model to Disk (Alt+S)
Saving the Model to Disk (Ctrl+S)

To save the model when a session of work has been completed, select this option, and a file dialog window will appear. Any name or extension may be used, but an extension of .mod is reccomended. The saved model file is fully compatible with the DOS version of Mag2dc. Note that this file will be in binary format, and it is not possible to edit it outside of Mag2dc.


Entering or Editing the Field Data
Editing the Field Data

Selecting this option will display the field values in a spreadsheet format, enabling them to be edited as desired.


Printout the Model  to Printer, Disk or Clipboard

To get a record of the bodies in the model and of the numerical values of the points making up the magnetic curves, select this option and the information will be written out on the either the printer or to an ASCII disk file as desired. This file may be edited and read into commercial drafting packages for use in the preparation of reports etc. Note that this option does NOT save a model for future recall as an old model . Transferring the data to the Windows clipboard enables easy transfer of the information into almost any Windows application. Note that only 250 data points may be transferred to the clipboard (due to memory restrictions), and if the profile contains more points than this then it will be suitably degraded.


Create an HP-GL File

The HP-GL plot file option will create a file containing a series of Hewlett-Packard plotter commands describing the model. This file may be imported as a figure directly into many wordprocessors, such as WordPerfect or Word, enabling reports to be prepared more easily.


Create a DXF File

DXF (Drawing eXchange Format)  files are used by many CAD programs (such as AutoCad). If a DXF file of a model is exported from Mag2dc then it can be imported into a CAD program as part of the production of a report.


Setting Up the Printer

All printers can be configured in some fashion or another, with the exact attributes depending on which printer is being used. Common options are to set the printing resolution and the fonts. Selecting this option will bring up a window showing the settings for the currently installed Windows printer. If the printer/plotter is changed, then Windows print manager must be updated before correct printing can be achieved. Drivers for a large range of printers are supplied with Windows, and printer manufacturers generally supply drivers with their hardware.


Producing the HardCopy

Once the printer/plotter has been correctly set up (as described previously), then printing can take place.  Information about the first three bodies in the model, as well as on the Geomagnetic field parameters used, may be printed out below the model if desired. If automatic scaling is used (the default), then the plot will fill the paper (of whatever size used). Alternatively, the width and height of the plot may be specified directly (in inches or cm.). A scale plotting option is also available. Click on the appropriate radio button, then enter the desired horizontal and vertical scales Eg. 5000 means 1:5000 etc. If you want to enter a larger scale than there is room for in the dialog, use exponential notation Eg.  2.0e8 etc.


Expert Mode

Most of the editing options described previously will include an information window that indicates which keys must be pressed Eg. in Add a Body, the user will be informed that he must use the left button to enter body corners, and the right button to complete the body. Once the user has become familiar with Mag2dc then these information windows are superfluous however, and merely slow down the modelling process. To prevent their display, enable Expert Mode by clicking on this menu option. It will then be marked with a tick. Click on it again to revert back to the Normal mode.


Leaving Mag2dc (Alt-X)

Choose this option to end the program. A window will appear asking for confirmation, in case it was selected accidentally.


Whole Body Inversion Parameters

These parameters are the depth, the horizontal position, the susceptibility, and the angle of rotation of the body. Limits may be set for each parameter, to prevent it being altered to values known to be false.


Body Corner Inversion Parameters

The position of each body corner may also be inverted individually. To mark a corner for inversion, click on it with the left button of the mouse. A double-headed horizontal arrow will be displayed on the corner. This indicates that the corner will be inverted in the horizontal plane only. Clicking on it a second time will display a vertical arrow, indicating inversion in the vertical plane only. A third click will enable both horizontal and vertical  inversion, and a fourth click will switch off inversion for that corner. When all the corners have been marked as desired, press the right button to exit. Note that selecting a corner for both horizontal and vertical inversion uses up two of the ten inversion parameters.


Inverting the Model

Once all the parameters to be inverted have been set, select this menu option to start the inversion. A dialog box will display the parameters chosen, so that they can be checked before proceeding. The number of iterations can be set here ; 10-20 should be fine for a run with one parameter set. The error in the fit between the observed and calculated curves will be displayed throughout the inversion. To halt the inversion before all the iterations have been completed, click the left mouse button, and it will stop at the end of the current iteration.


2.5 Dimensional Models

The profile direction is orthogonal to the strike of the bodies, and they extend to a limited distance along strike in each direction. In Mag2dc the body exends to an equal distance on each side of the profile.


Susceptibility

     SI susceptibility = 4 PI * CGS susceptibility


Declination

The angle between magnetic North and true North.


Inclination

The angle which the magnetic lines of force make with the horizontal.


Remanence

     	The magnetisation remaining in a rock in the absence of an external magnetic field.


Least-Squares
Least-Squares Fit

The fit of the model to the data that minimises the sum of the squares of the distances from the model to the data. Also known as L2 norm.


Register Mag2dc Now !

When you register you get the following benefits :-

1. When the next upgrade is released, it will be sent to you absolutely free !

2. Support - if you have any queries then you can contact me using the information given below.

3. Information on new geophysical products will be sent to you, if you want it.

To register, send the registration form in the manual to :-

G.R.J.Cooper

C/O Geophysics Department

University of the Witwatersrand

Johannesburg 2050

South Africa

Telephone 11-716-3159, Fax 11-339-7367

e-mail  : 006GRC@cosmos.wits.ac.za

Users outside the Southern African region : please put your license number (as displayed in the About dialog) in your message.


Legalities

Professional Geophysical Software and G.R.J.Cooper shall not in any case be liable for incidental, consequential, or other damages arising from any claim, even if PGS or its agents have been advised on the possibility of such damages.