MAGPRISM_MANU
Contents
1. MAGPRISM Version 1 1 2003 Markku Pirttij rvi Introduction The objective of geophysical magnetic field method is to obtain indirect information about the subsurface structures from the measurements made above Earth s surface The most important petrophysical parameter affecting the formation of a magnetic field anomaly is the magnetic susceptibility which defines the ability of a material to become magnetized under an external magnetic field In geophysical applications the external magnetic field is Earth s geomagnetic field Some minerals especially the magnetite have considerably higher susceptibility than most other rock forming minerals Therefore field measurements that are made in the vicinity of a target that contains highly magnetized material reveal anomalous magnetic field The susceptibility position orientation and dimensions of the target as well as the intensity and the direction inclination and declination of earth s geomagnetic field define the spatial characteristics of the magnetic anomaly Although the magnetic field data cannot be interpreted uniquely the information about the susceptibility and its distribution can be used in geological and structural interpretation Another important factor affecting the geophysical magnetic field response is the remanent or remnant magnetization especially the thermal remanent magnetization which was fixed to the rock material when it crystallized from melted2. 4 Dip angle from horizontal plane degrees 3 X position easting of the prism m Y position northing of the prism m Z position depth of burial of the top of the prism m Strike angle direction of the elongated side of the prism degrees Susceptibility value dimensionless in SI units OANANNN The following three text fields define earth s geomagnetic field 1 Total intensity of extenal magnetic field nT 2 Inclination from horizontal plane of the external field degrees 3 Declination from geographic north of the external field degrees When remanent magnetization is taken into account the following three text fields define 1 K6ningsberg s ratio ratio between the remanent and induced magnetic field M M 2 Inclination of the remanent magnetization from horizontal plane degrees 3 Declination of the remanent magnetization from geographic north degrees The last five text fields define the system parameters of the measurement profile The X coordinate easting of the profile start position m The Y coordinate northing of the profile start position m The X coordinate easting of the profile end position m The Y coordinate northing of the profile end position m The spatial spacing between the profile points m ABWN The three slide controls below the graph window can be used to change the point of view for the 3 D model The view can be r
3. dynamic memory allocation and the analytical solution allows very fast computation even on slow computers The MAGPRISM program has a simple graphical user interface GUD that can be used to change the parameter values to handle file input and output and to visualize the magnetic field response and the model The user interface and the data visualization are based on the DISLIN graphics library The program requires following files MAGPRISM EXE the executable file DISLIN DLL dynamic link library for the graphics routines The distribution file MAGPRISM ZIP contains also a short description file README TXT this user s manual MAGPRISM_MANU PDF and an example input file EXAMPLE INP To install the program create a new directory and copy or unzip e g Pkunzip Winzip the distribution files there To be able to start the program from a shortcut that locates in a different directory one should move or copy the DISLIN DLL file into the WINDOWS SYSTEM or SYSTEM32 folder or somewhere where a path always exists Getting started On startup the program reads the parameters of the model parameters from the MPRISM INP file and the graph parameters from the MPRISM DIS file If these files do not exist default parameters are used and the files are created automatically The program then computes the magnetic field response and builds up the user interface shown in the Appendix The response is plotted in the graph area along with a 3 D v
4. magma The remanent magnetization usually has different direction than the induced magnetization because the direction of geomagnetic field has changed and the position and orientation of the rock material has changed in geological tectonic processes The remanent magnetization should be taken into account because it can affect the interpretations based on the normal induced magnetization X position Y position Z position Thickness Susceptibility Strike length Strike angle Depth extent Dip angle Figure 1 The parameters of a dipping prism model 1 The MAGPRISM program computes the static magnetic field on a profile above an isolated dipping magnetized prism like body The parameters of the prism model are shown in Figure 1 In addition to the susceptibility the Q ratio and the inclination and declination of the remanent magnetization of the prism as well as the demagnetization effect of a highly magnetized target can be taken into account Only one body and one profile can be used The forward computation is based on the analytical solution and the original algorithm of S E Hjelt 1972 The MAGPRISM program is intended primarily for educational purposes Installing the program The MAGPRISM program requires a PC with 32 bit Windows 9x NT4 2000 XP operating system and a graphics display capable for at least 1024x768 resolution Memory requirements and processor speed and are not critical since the program uses
5. PRISM DAT format Results from MAGPRISM the magnetic field of a prism Copy of the input file MPRISM INP 50 00 400 00 200 00 60 00 0 00 0 00 20 00 10 00 0 05000 52000 0 75 00 5 00 0 00 0 00 0 00 0 00 250 00 0 00 250 00 10 00 I Description Thickness 50 00 Strike length 400 00 Height 200 00 Dip angle 60 00 X position 0 00 Y position 0 00 Z position depth 20 00 Strike angle 10 00 Susceptibility 0 05000 Normal field 52000 00 Field inclination 75 00 Field declination 5 00 K ningsbergs ratio 0 00 Reman inclination 0 00 Reman declination 0 00 X start position 0 00 Y start position 250 00 X end position 0 00 Y end position 250 00 Point spacing 10 00 Demagn coeffs 0 000 0 000 0 000 X pos Y pos Dist Total Total anom Z comp X comp Y comp Horiz comp 0 0 250 0 0 0 0 5201E 05 0 1770E 02 0 3224E 01 0 5711E 02 0 4576E 01 0 56354E 02 0 0 240 0 10 0 0 5202E 05 0 2095E 02 0 5361E 01 0 6185E 02 0 4714E 01 0 60970E 02 s et Note that the output file contains a copy of the input file the format of which should become clear from the example above Since there should rarely be any need to edit model files manually a more detailed description of model files is not shown here The only exception is the single parameter on the last line of the model file that can be used to disable 0 or enable 1 the GUI Thus setting this param
6. d to define the corner where the legend text is positioned Values 1 4 put the legend in SW SE NE or NW corner of the page outside the graph Values 5 8 put the legend in the SW SE NE or NW corner inside the graph The default values are 1 1 1 e The 3 rd line defines the x horizontal and y vertical distance of the origin of the main graph in pixels from the bottom left corner of the page and the length of the x and y axis relative to the size of the remaining origin shifted width and height of the plot area The size of the total plot area is 2970x2100 pixels landscape A4 e The first parameter on the 4 th line defines the size in pixels of the square area reserved for the 3 D model The position of the model area is always next to the lower right corner of the response graph The remaining three parameters define horizontal and vertical viewing angles and a perspective viewing distance for the 3 D model view e The 5 th line should be left empty e The following lines define various text items of the graph max 70 chars These are e The first line defines the main title of the graph The following 6 lines define the texts for possible response components The next line is the title of the model description text The next 2 lines define the axis titles of the graph The last 3 lines define the axis title texts of the 3 D model view References A Aharoni 1998 Demagnetizing factors for rectangular ferromagnetic
7. deg 0 Declination deg EECEREEGELEGEGE Konings ratio 200 300 Distance m Rem incl deg 0 0 Rem decl deg 0 0 Total field anomaly X start m 0 0 Horiz rotation Vert rotation Rel distance ETE mo K a ie ma 2 Y start m 250 0 40 3 m x end m 0 0 Magnetized prism by M Pirttij rvi 2003 5 E Hjelt 1972 Y end m 250 0 Stepping m 10 0
8. e the model prism 4 Input file format The MAGPRISM program is intended for forward modeling only More appropriate data interpretation can be made using the MAGINV program However external data total magnetic field only can be read in for comparison using the menuitem Read data in File menu Thus interpretation of field data is possible on trial and error basis The example below illustrates the format of the simple column formatted data file 21 3 250 07 52000 200 0 52010 150 0 52040 a etC The first line defines the number of data points n and the column number i from which the data is to be read from The next n lines then define the x and y coordinates easting and northing of the data points and the successive columns contain the data values In the example above only one data column is available The data file should not contain any empty lines Note that the data can be irregularly spaced and therefore need not to be from a straight profile However the MAGPRISM program displays the data only in profile format Note the input data must represent the total magnetic field However the total field component does not need to include the total field component of the geomagnetic field The menuitem Total Anom field which is available only when data has been read in can be used to include and to exclude Earth s field from the computation Output file format The following text illustrates the output file M
9. eter to zero forces the program to just perform the forward computation and to write the results without any additional hassle Graph options Several graph parameters can be changed by editing the MPRISM DIS file Note that the format of the MPRISM DIS file must be preserved If the format of the file should become invalid one should delete the file and a new one with default parameter values will be generated automatically the next time the program is started The file format is shown below 40 32 32 32 32 1 I 1 350 380 0 45 0 70 1100 130 25 8 Magnetic field of a dipping prism Total field anomaly Z component X component Y component Horiz field anomaly Total field Model parameters Distance m Response nT X East m Y North m Z Depth m e The 1 st line defines five character heights The first one is used for the main title and the graph axis titles the second height is used for the axis labels the third height is used for the plot legend text the fourth height is used for the model description text and the last height is used for the axis labels in the 3 D model view e The 2 nd line defines parameters that modify the graph appearance The first one can be used to include 1 or exclude 0 the model information text to from the top right corner of the page The second one can be used to include 1 or to exclude 0 the model view to from the bottom right corner of the page The third parameter is use
10. etization Total Anom field to include exclude the external magnetic field from total field The Exit menu has only one item which is used to confirm the exit operation On exit the latest model and results are automatically saved in MPRISM INP and MPRISM OUT files replacing the original ones Errors that are encountered before the GUI starts up can be found from the MPRISM ERR file When operating in GUI mode run time errors arising from illegal parameter values are displayed on the screen Using program controls The Change component push button at the top of the control pane on the left side of the MAGPRISM window can be used to change the field component that is shown in the graph The magnetic field components are shown in the following order Total field anomaly the magnetic field of the prism without earth s magnetic field Z component vertical field anomaly X component horizontal field in NS direction anomaly Y component horizontal field in EW direction anomaly Horizontal field anomaly Total field earth s magnetic field added All anomalous field components total z x y horis NYDN BWN Re The Update parameters button is used to read and update the model parameters and to perform a new forward computation The following nine text fields define the parameters of the prism model see Fig 1 these are 1 Thickness m 2 Length along strike m 3 Depth extent vertical height m
11. iew of the model and a description of the model parameters All model parameters can be altered using the controls on the left side of the MAGPRISM program window As shown in Appendix the main window of the MAGPRISM application contains two menus The File menu contains the following nine options Open Model to open an existing model file Read Data to read in data for comparison total field only Save Model to save the model into a file Save Data to save the results description response into a file Read disp to read in new graph parameters from a DIS file Save Graph as PS to save the graph in Adobe s Postscript format Save Graphas EPS to save the graph in Adobe s Encapsulated Postscript format Save Graphas PDF to save the graph in Adobe s Acrobat PDF format Save Graph as WMF to save the graph in Windows metafile format Selecting these menu options brings up a standard Windows file selection dialog that can be used to provide the name of the file for open save operation Model and data including other information files are saved in standard ASCII text format The graphs are saved in landscape A4 size cf Appendix The Edit menu contains four items Comp Meas to put current data as measured data for comparison Remove measured to discard all information about measured data Demagnet on off to include exclude the demagnetization effect Remanence on off to include exclude information about remanent magn
12. o provide any support for the program If you find the computed results erroneous or if you have suggestions for improvements please inform me Terms of use and disclaimer You can use the MAGPRISM program free of charge If you find the program useful please send me a postcard The program is provided as is The author and the University of Oulu disclaim all warranties expressed or implied with regard to this software In no event shall the author or the University of Oulu be liable for any indirect or consequential damages or any damages whatsoever resulting from loss of use data or profits arising out of or in connection with the use or performance of this software Contact information Markku Pirttijarvi Division of Geophysics GSM 358 40 5548959 Department of Geosciences Tel 358 8 5531409 PO Box 3000 Fax 358 8 5531484 FIN 90014 University of Oulu URL http www gf oulu fi mpi Finland E mail markku pirttijarvi oulu fi Appendix E Magnetic prism BE El x Fie Edit Exit _ Change component Magnetic field of a dipping prism Total field anomaly Thickness m 50 0 Model parameters um Thick 50 0 Length 400 0 Height 200 0 Length m 400 0 Xpos 0 0 pos 0 0 Z pos 20 0 Height m 200 0 Dip 60 0 Strike 0 0 Dip angle deg 60 0 Suse 0 0500 M Fld 52000 M Incl 75 0 M Decl 5 0 pas m 0 a e a Y pos m a Depth m a Strike deg 0 0 Suscept SI Normal field nT Inclination
13. otated horizontally and vertically and the relative distance of the viewing point can be changed as if zooming in and out Notes on model parameters All distances and dimension are defined in meters m and all angles are defined in degrees For historical reasons the geophysical unit of the magnetic field is nano Tesla nT which equals to 10 Vs m As shown in Fig 1 the xyz position of the prism is defined by the center of the top of the prism In addition the top and bottom surfaces of the prism are horizontal Although the negative z axis points downwards in the 3 D model view the z position depth of burial is given a positive value The dip angle is taken from the horizontal plane and the depth extent means the vertical height of the prism not along the dip Thus the dip can not be equal to 0 or 180 degrees because the length of the prism along the dip would become infinite The strike angle is taken counter clockwise from the positive x axis For example if strike is 90 degrees the prism is oriented along the y axis The definition of the declination angle may vary from some other modeling programs For example in Finland the declination is about 6 degrees since the magnetic north locates counter clockwise from geographical north The value of inclination is positive on the northern hemisphere and negative on the southern hemisphere The remanent magnetization and the susceptibility is considered to be constant insid
14. prisms J Appl Physics 83 3432 3434 S E Hjelt 1972 Magnetostatic anomalies of dipping prisms Geoexploration 10 239 254 R I Joseph and E Schl man 1965 Demagnetizing field in non ellipsoidal bodies J Appl Phys 36 1579 1593 Additional information I started to develop the MAGPRISM program at the University of Oulu in May 2002 when I began to work in a 3 D crustal model project funded by the Academy of Finland The forward computation is based on the MPRISM algorithm of Sven Erik Hjelt see the reference The algorithm for the demagnetizing factors was originally derived from and Joseph 1967 by J Lerssi missing exact reference The equations for the demagnetizing factors can be found also from Aharoni 1998 More information about geophysical magnetic field method can be found from any textbook on applied geophysics The MAGPRISM program is written in Fortran 90 style and compiled with Compaq Visual Fortran 6 6 The graphical user interface is based on the DISLIN graphics library version 8 by Helmut Michels The program distribution includes the DISLIN DLL file that is required for the GUI The WWW homepage of DISLIN is lt http www dislin de gt Since the DISLIN graphics library is independent form the operating system the MAGPRISM program could be compiled on other operating systems Solaris Linux without any major modifications At the moment however the source code is not made available and I do not intend t
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