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2LAYINV

1. More information about the theory and applications of the VLF R method can be found from McNeill and Labson 1992 The 2LAYINV program is written in Fortran 90 style using Compaq Visual Fortran 6 6 The graphical user interface is based on the DISLIN graphics library version 9 0 by Helmut Michels The program distribution includes the required DISDLL DLL file The official WWW homepage of DISLIN is at http www dislin de Since the DISLIN graphics library is available for other operating systems Solaris Linux the 2LAYINV program could be compiled and run there without any major modifications I do not intend to provide any support for the program However if you find the computed results erroneous or if you have suggestions for improvements please inform me References Kunetz G 1972 Processing and interpretation of magnetotelluric soundings Geophysics 37 1005 1021 McNeill J D and Labson V F 1992 Geological mapping using VLF radio fields In M N Nabighian Ed Electromagnetic methods in applied geophysics Volume 2 Application SEG Orellana E 1974 Prospeccion geoelectrica por campos variables Paraninfo Madrid Pirttij rvi M 2003 Numerical modeling and inversion of geophysical electromagnetic measurements using a thin plate model PhD thesis Acta Univ Oul A403 University of Oulu Terms of use and disclaimer You may use the 2LAYINV program free of charge If you find the program useful please send m
2. from or for the inversion Fixed parameters are indicated by a small cross symbol above the center of the corresponding element in the pseudosection The editing mode is similar to layer thickness editing cursor becomes crosshair left mouse button selects points and right mouse button stops the editing mode However no interpolation is used and therefore each model block must be selected manually If editing is made without any selections left mouse clicks then the particular parameter Rhol Thic Rho2 is set free in every point The three Edit Roughness buttons Rhol Thic Rho2 are used to manually create discontinuities into the model The editing mode is similar to fix free status editing with the exception that the discontinuities locate between two model blocks instead of above them Discontinuities are indicated by black vertical resistivity or horizontal thickness lines between the blocks in the pseudosection If editing is made without any selections left mouse clicks then the discontinuities are removed from the particular parameter Rhol Thic Rho2 in every point The three Reset to mean buttons Rhol Thic Rho2 are used to reset the resistivity or thickness of the overburden layer or the resistivity of the basement to the mean parameter value However if discontinuities have been set up in the model then the mean value is computed between them see Appendix C This option may help the user to create rough categorical int
3. often in geophysical interpretations there is no unique inverse model The practice however will create masters File formats Before starting up the program make sure that your input data files DAT are formatted properly The format of the input VLF R data file is shown below Synthetic VLF R data 30 45 0 20 000 2 349 5144636 22225633 373753 40 000 3 731 16 339 2159 510 38 755 0 60 000 5 763 18 146 2079 178 39 902 0 80 000 8 158 20 152 1981 082 41 240 0 580 000 1 003 10 426 2370 703 35 563 0 600 000 1 490 9 340 2394 223 34 925 0 Lines 2 and 4 are used for comments and can be left empty The first line defines a header text max 40 characters which is used as a second title in the graph If the beginning of the header line is empty the default title in the 2LAYINV DIS file is used instead Usually the header line should define the name of the measurement site and possibly the name of the VLF station The 3 rd line defines the number data points on the profile NOP and the column indices ICO1 and ICO2 for the components of the VLF R data apparent resistivity and phase and the column index ICO3 of the data weights Data weights are used to increase wgh gt 1 or decrease wgh lt 1 the importance of individual data points If the data file does not include data weights which is normally the case the corresponding column index should be set to zero ICO3 0 The remaining NOP lines define the profile coordinate X
4. Freq text field at the top of the control pane on the left side of the main window is used to define the freguency of the VLF station used in the measurements The default value is 16000 Hz which corresponds to GBR Examples of other European VLF stations are DHO38 23400 Hz JXN JXZ 16400 Hz and FUO 15100 Hz The RI mean text field shows the mean resisitivity of the overburden layer The resistivity of the overburden is reset to this value when the 2 Layer push button below it is pressed The 2 Layer button performs direct two layer inversion using the well known exact inverse solution for the thickness of the overburden and the resisitivity of the basement when the resisitivity of the overburden is fixed to R mean This option does not utilize any lateral constraining and the inverse solution is exact for each point as long as the response suits two layer model e g phase 45 deg The Update button is used to validate the changes made to the text fields and to perform a forward computation The following six text fields define various inversion parameters Lagsca defines the so called Lagrange multiplier L which determines whether the inversion tries to minimize the data error instead of the model roughness If L gt 1 then the model will become very smooth and the fit will be poor If L lt 1 then the fit will be good but the model will become rugged Typically the values range between 0 1 and 10 Note that both the data and the
5. SE 10000 et gt v 1000 E z CS z Bes S 100 Se a a a lt 10 200 100 0 100 200 Distance m 0 10 m E z 20 B B 305 a 3 40 me 50 200 100 0 100 200 Distance m 1 00 2 00 3 00 4 00 logi0 Resistivity Ohmm 12
6. the resisitivity and thickness of each layer was reset Note that the mean values are restricted between discontinuities Appendix A W 2 layer inversion EB S File 24ayer Exit Freg 23400 0 Freg 23400 0 Hz Bhoap m Ri mean 297 45 RMSd 0 15 VLF R measurement Rhoap c EEA RMS m 0 69 Phase m 2 Layer Ervastinranta DHO38 Phase c Update Lagsca 1 0 R1 wght 1 0 TH wght 1 0 R2 waht 1 0 F length 2 Iter 10 Optimize Edit Thic Axis limits R min F max Distance m T max Edit Fix Free Rhol Thic Rho2 Edit Roughness Rho U sme Distance m Rho2 Reset to mean Rho Z Thie d 2 00 par 3 00 logl0 Resistivity Ohmm Rho2 90 70 50 o o o o 2 Ka gt E o bei gt d x a o EN 5 2 a a a lt 39p seyd ke Depth m w yjdo Constrained 2 layer inversion of VLF R data v 1 0a c MTP 2006 11 Appendix B Freg 23400 0 Hz o Rhoap m RMS d 0 10 VLF R measurement Rhoap c RMSm 0 90 A Phase m Ervastinranta DHO38 ves Phase e E 10000 o be ds Ki 1000 z z 2 E s 5 100 SS a a a Ka 10 200 100 0 100 200 Distance m 0 10 s U 20 3 S 30 5 a 3 40 ER 50 200 100 0 100 200 Distance m 1 00 2 00 3 00 4 00 logiO Resistivity Ohmm Appendix C Freg 23400 0 Hz o Rhoapm RMS d 3242 50 VLF R measurement Rhoap c RMSm 0 90 Phase m Ervastinranta DHO3 Phase c
7. the system path control panel system environment Starting up After starting the program it reads graph parameters from the 2LAYINV DIS file If the file cannot be found a new one with default parameter values is created automatically The program then displays the standard Windows Open file dialog so that the user can select the input data file containing the measured VLF R data If the file format is incorrect the program halts and error messages may be read from 2LAYINV ERR file The program then creates an initial two layer model based on the data and computes the synthetic two layer response Finally the program builds up the GUI window and creates the graph of the measured and computed VLF R response and the two layer resistivity model as shown in the Appendices If the user cancels the open file operation the graph area will be disabled Menus The main window of the 2LAYINV application has three menus The File menu has items Read VLF R data open and read in measured VLF R data from a DAT file Save VLF R data save the measured and computed VLF R data into a OUT file Save 2 Layer model save the current 2 layer model into a LAY file Read 2 Layer model read a previously saved 2 layer model from a LAY file Read disp read new graph parameters from a DIS file Save Graph as PS save the graph in Adobe s Postscript format Save Graph as EPS save the graph in Adobe s Encapsulated Postscript format Save Graph as P
8. 2LAYINV Laterally constrained two layer inversion of VLF R measurements User s guide Version 1 0a Markku Pirttijarvi Depth m ur ydq Distance m 1 00 2 00 3 00 4 00 1og10 Resistivity Ohmm OY GF M 2006 1 University of Oulu Department of Physical Sciences Division of Geophysics Introduction The 2LAYINV program is used to interpret geophysical electromagnetic VLF R data apparent resistivity and phase measured along a single profile at a single frequency The inversion is made separately for each data point using a one dimensional two layer earth model As such the program suits well the interpretation of the thickness and resistivity of relatively thin resistive soil and till layers and the resistivity variations of the basement rocks Note that the inversion model is still only one dimensional and that the interpretation results actually yield a pseudosection of the 2 D resistivity distribution Starting from an initial model linearized inversion with adaptive damping is used to optimize the thickness and the resistivity of the overburden layer and the resistivity of the basement so that the model minimizes the error between the measured and the computed VLF R data Laterally constrained inversion is obtained by minimizing the roughness of the model that is the variation of the model parameters between neighboring points together with the data error As a result a smoothly varying Occam inversion model is ob
9. DF save the graph in Adobe s PDF format Save Graph as WMF save the graph in Windows metafile format Save Graph as GIF save the graph in GIF compressed format Selecting any of these options brings up a typical Open Save file selection dialog that can be used to provide the name of the file for open save operation Data files are stored in text format The graphs are saved as they appear on the screen in landscape A4 size The 2 ayer menu has options Weights in out enable and disable data weights Labels in out show or hide the resisitivity labels in pseudosection Color scale contains submenu items to change the current color scale Data weights are used to increase or decrease the importance of individual data points in the inversion The weights are read from the input data file together with the measured VLF R data See the chapter on file formats for more information If weights do not exist the corresponding menu item will be inactive Total of five color scales can be used in pseudosections The reverse rainbow scale is the default red conductive blue resistive The Exit menu has only one item which is used to confirm the exit operation On exit the current results are automatically saved into 2LAYINV OUT file is that contains the interpreted two layer model Errors that are encountered before the GUI starts up are reported in the 2LAYINV ERR file Inside GUI mode run time errors are displayed on the screen Controls The
10. d when the Optimize button is pressed default Ier 10 The Optimize button performs a given number of inverse iterations The inversion is based on linearized inversion scheme The sensitivity matrix is constructed numerically using forward differences The linear system is solved for the parameter steps using singular value decomposition SVD together with adaptive and automatic damping The Edit Thic button is used to give manually varying thickness for the overburden layer using the computer mouse After pressing the Edit Thic button the mouse cursor changes into a crosshair cursor above the graph The user then starts to pick up depth values by pressing the left mouse button The editing mode is ended after pressing the right mouse key The program then interpolates the overburden thickness values between the first and the last depth point and makes a forward computation The maximum amount of selected points is egual to the original number of data points Note that the points must be picked in an ascending order 1 e from the beginning of the profile towards the end The R min R max and T max text fields in the middle of the control pane define the minimum and maximum value of the resistivity scale and the maximum depth value in the pseudosections Note that on the actual color scale the 10 base logarithm of the resistivity values is shown The three Edit Fix Free buttons Rhol Thic Rho2 are used to manually fix or free the parameters
11. e 95 confidence limits of the parameters min max values Note that in this example the remaining columns minR1 maxTh minTh maxR2 minR2 are omitted for clarity Results from 2LAYINV the interpreted 2 layer model Data file Data RMS error 0 251280E 02 Mod RMS error 0 137378E 03 Weights are not used Computational parameters Lagrange scale 0 10 Rho 1 scale 1 00 Thickness scale 1 00 Rho 2 scale 1 00 Filter length 3 Roughness 1 Number of data points N 21 Dist Rho 1 Thic Rho 2 Fxl Fx2 Fx3 Bol R92 Rg3 minR1 maxRl 250 00 591 629 18 280 27820 L L TL D 0 Q 586 964 225 00 579 886 18 340 27 689 L 1 L 0 0 0 575 548 The measured and the computed VLF R data and the data weights can be saved into a column formatted text file default suffix OUT The data can be used for example to prepare response graphs using other plotting programs An example of the file format is given below Results from 2LAYINV VLF R data Number of data points N 21 Dist m Rhoa c Ohmm Phase c deg Rhoa m Phase m Weight 250 00 110 00 67 00 110 57 66 40 1 00 225 00 110 00 67 00 111 13 66 11 1 00 Graph options Several graph parameters see Appendix can be changed editing the 2LAYINV DIS file Thus the graphs can be translated into another language for example Note that the format of the 2LAYINV DIS file must be preserved If the format of the file becomes invalid one should delete t
12. e a postcard If you use the 2LAYINV program and wish to publish the results please provide a notice 2Layinv University of Oulu without the guotes Since I have not had time to prepare a proper paper on this subject please use this manual as a reference e g Pirttij rvi M 2006 2LAYINV Laterally constrained two layer inversion of VLF R measurements User s guide University of Oulu Division of Geophysics The program is provided as is The author M P and the University of Oulu disclaim all watranties either 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 Tel 358 8 5531409 Department of Geosciences Fax 358 8 5531484 PO Box 3000 FIN 90014 University of Oulu URL http www gf oulu fi mpi Finland E mail markku pirttijarvi oulu fi 10 Appendices Appendix A shows a screen dump of the 2Layinv program The interpretation was made using default inversion parameters no fixed points no discontinuities Appendix B demonstrates the use of manually edited roughness lines to create a discontinuous model resistivity contact and overburden thickness variation Appendix C shows the same model after
13. erpretation model rapidly Interpretation procedure After the data has been read in it is necessary to check and provide correct value for the measurement frequency default 16000 Hz This can be checked from the upper right corner of the graph and changed by editing the corresponding text field and pressing the Update button The inversion can now be started by pressing the Optimize button The default number of iterations is often enough to get a good fit but usually the convergence is rather slow and the optimization must be made multiple times before stable solution is found Sometimes it may be advisable to increase the maximum number of iterations Important To see whether a stable solution has been found please pay attention to the values of the data RMS and the model RMS in the upper right corner of the graph as well as to any visible changes in the pseudosections The default values will usually create a rather smooth model To enforce better data fit one should reduce the L value 0 1 0 5 This will usually speed up the convergence as well If the data has been measured densely or if the it has low S N ratio then the L value can be increased 2 10 One should pay attention to the resulting model and see how different initial models affect the final inversion model The model can be perturbed by pressing the 2 Layer button which computes the exact inverse solution and sets the layer resistivity to the current mean value or by p
14. ft empty e The following lines define various text items of the graph max 40 characters Main titles of the VLF R data graph The second title line of all graphs used if the data file does not include a title Main title of the two layer interpretation graph Title of the x axis defined in the data file usually the distance along profile Two y axis names of the VLF R data graph Four legend names of the VLF R data graph measured and computed VLR R data The label of the color scale defining resistivity values log10 prefix added automatically The vertical axis name of the resistivity pseudosection 0 0 0 0 0 0 0 0 Additional information The forward computation of the VLF R response of two layered earth is based on the same traditional algorithm as in the MT and AMT modelling and inversion programs MTINV and AMTINV The original reference is Kunetz 1972 However in 2LAYINV the algorithm has been optimized for two layer earth model The optimization is based on a linearized inversion method where singular value decomposition and adaptive damping method is used The method has been described in my PhD thesis The constrained Occam inversion algorithm is based on the unpublished results of a 3 D block model inversion code of gravity data The direct two layer inversion is based on an old traditional code edited by several persons at the University of Oulu J Mursu K Komminaho The original reference is Orellana 1974
15. he file and a new one with default parameter values will be generated automatically the next time the program is started The file format and default values are shown below 32 32 32 18 0 300 300 0 82 0 72 10 000 50000 10 0 90 0 20 0 50 0 20 0 VLF R measurement Test data 2 Layer model Distance m Apparent resistivity Ohmm Phase deg Rhoap m Phase m Rhoap c Phase c Resistivity Ohmm Depth m e The 1 st line defines four character heights The first one is used for the main title and the graph axis title the second is used for the axis labels the third is used for the plot legend and the fourth is used for the resistivity labels of the pseudosection The fifth parameter 0 is reserved for future use e The 2 nd line defines first the x horizontal and y vertical position of the origin of the main graph in pixels The third and the fourth parameter define the length of the x and y axis relative to the size of the total width and height of the plot area eg 0 5 50 of the width or height which is equal to 2970x2100 pixels landscape A4 Note that in vertical direction the length defines the relative height of both the response plot and the pseudosection e The 3 rd line defines the minimum and maximum value of the logarithmic resistivity axes the minimum and maximum values and the tick mark step of the phase axis and the maximum value and the tick mark step of the depth axis e The fourth line should be le
16. meter and the measured data values There is no theoretical limit for the number of data points because dynamic memory allocation is used However since only a single profile can be used the simultaneous processing of large datasets should be avoided The profile coordinates must be in continuously ascending or descending order If they are in descending order the whole profile is reversed automatically Furthermore if the distance between the first two profile points X2 X lt 0 5 the profile coordinates are converted from kilometers to meters multiplied by 1000 Note also that the data file can contain several data columns two of which are read for processing This means that the same data file can contain other data components as in the example above Manual editing of the column indices is currently required to choose the correct columns for the interpretation The parameters of the two layer model are saved into a column formatted text file default suffix LAY The header of the file defines the parameters used in the inversion and the normalized root mean square RMS values of the data and model error For historical reasons the character is used in the results file to make comment lines for the Gnuplot plotting program Along with the model parameters Rhol Thic and Rho2 the file includes the fix free status O fixed 1 free the roughness parameters O smooth 1 left 2 right and 3 totally discontinuous as well as th
17. model parameters are scaled using the maximum data variation and maximum parameter variation resistivity scale limits and depth limits Therefore the default value L 1 provides reasonably good compromise between data fit and model smoothness However the Lagrange scaler is the most important parameter in the interpretation because it greatly affects the final model Rl wgh Th wgh and R2 wgh define separate Lagrange scaling for each layer parameter If any of the values is greater than the other two then that parameter will become smoother than the others Vice versa if some parameter weight is smaller than the others then it will be allowed to vary more If any of the weights are egual to zero then the corresponding parameter will be fixed in every point Because the parameters are scaled as well the default values 1 are guite a good compromise Often it is useful to increase the weight of the overburden layer if it can be considered to represent similar kind of material F length defines the filter length used to compute the parameter roughness default k 2 The roughness is defined as the difference of the parameter p from the mean of the surrounding points pi Sum p j i k i k 2k where Ae is Kronecker delta function The longer the filter length the smoother the resulting Occam model will be and the less sensitive the model is to data noise The Iter field is defines the number of successive iterations to be performe
18. ressing some of the Reset buttons which will place the mean parameter value into every model block Without any discontinuities the constrained inversion will create a quite smooth model After few basic inversions one should have a pretty good idea where there might be lateral conductivity or thickness variations Using the Edit roughness buttons one can create discontinuities manually After this a few more optimization runs should be made to see how the model changes Alternatively after editing discontinuities one might first disturb the model using the Reset buttons and start the inversion from a new initial model If the user has a priori information for example about the thickness of the overburden at some point then one can use the Edit thic button to set the thickness and the Edit fix free button to fix that element The inversion will then keep that parameter fixed during the inversion and the constraining will try to adjust the neighboring elements to fit the fixed value to minimize the model roughness Alternatively one can save the current model into a LAY file and manually edit the parameter and fix free values After reading in the edited model file it will allow to use fixed resistivity values as well that is currently not possible interactively The user should always test different models and different values of the various inversion parameters particularly the global Lagrange multiplier and the separate parameter weights As
19. tained The roughness and or the fix free status the parameters can be set manually to allow discontinuities and to incorporate a priori data in the model The 2LAYINV program requires a PC with 32 bit Windows 9x NT4 2000 XP operating system and a graphics display of at least 1024x800 resolution Memory requirements and processor speed and are not critical since the program uses dynamic memory allocation and the inverse computation is fast to perform even on slow computers The 2LAYINV program includes simple graphical user interface GUI that allows changing some inversion parameters handling file input and output and visualization of the VLF R data and the two layer model section The user interface and the data visualization are based on the DISLIN graphics library Setting up The distribution file 2LAYINV ZIP can be downloaded free of charge from author s web site at the University of Oulu Finland http www gf oulu fi mpi The program requires two files 2LAYINV EXE the executable file DISDLL DLL dynamic link library for the DISLIN graphics The distribution file contains also a description file README TXT this users manual 2LAYINV MANU PDF and an example data file EXAMPLE DAT To install the program unzip Pkzip Winzip the distribution file into a new folder To be able to start the program from a shortcut that locates in a different directory move or copy the DISDLL DLL file into the WINDOWS SYSTEM folder or define

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