Microlevelling using Bidirectional Gridding
Contents
1. A Geosoft Technical Note G osOr Microlevelling using Bi directional Gridding Introduction When working with complex datasets such as airborne magnetics or radiometrics a key data processing objective is to eliminate subtle levelling problems which were not removed during regular data processing such as lag corrections tie line levelling base level corrections etc One definition of microlevelling might be filtering a gridded dataset to reduce or remove non geological effects caused by long wavelength noise along survey lines Such noise manifests itself as apparent data shifts from one survey line to the next often creating very streaky looking images Normally microlevelling is applied only after other corrections such as diurnal removal and tie line levelling have already been applied to the data See the Geosoft Levelling System Tutorial and User Manual for details on these types of operations There are a number of methods for microlevelling datasets collected along survey lines The method described in this Technical Note uses the Oasis montaj Bi directional line gridding method You can apply the procedure to a gridded dataset alone however for optimum results you will require a database of the original located data The success of this method is largely data dependant however it works in the following manner A grid of levelling errors is derived from a magnetic grid this data is then subtracted from the or2. Bi directional line gridding and then click Dialog controls The Bi directional gridding of line data dialog will be displayed 20 Select the Channel to grid maglev and specify the output grid maglev grd You can leave the rest of the parameters to the default values click OK to create the final microlevelled grid You can use this grid for further processing or interpretation Since the low pass filtering in the Re grid a grid dialog is dependant on the storage configuration of the grid i e whether that data is stored by columns or rows it may be necessary to check that the data will be filtered across the original lines However if the above processing steps are followed in order there will be no need to change this parameter since the default orientations give the correct result Steps to Perform Bigrid Microlevelling from a Grid 1 Open the grid g1 grd in your current workspace 2 Onthe Grid menu select Gridding Re grid a grid The Regrid a grid dialog will be displayed 3 Select the Input grid g1 grd and specify the output grid g2 grd In the Low pass filter wavelength box set the low pass filter parameter to 4 10 times the line spacing You can leave the rest of the parameters to the default values click OK to create the second grid The grid g2 grd is a grid of the levelling errors plus a regional field 4 On the Grid menu select Gridding Re grid a grid The Regrid a grid dialog will be displayed 5 Select
3. and have the same physical dimensions Regridding and filtering may cause the grids to differ slightly in physical size If this happens 1 On the Grid menu select X Utilities Boolean operations The Mask One Grid Against Another dialog is displayed 2 Select the Input Grid 1 g2 grd the Input Grid 2 g4 grd and then specify the Output Grid g4a grd 3 Using the dropdown lists select the following parameters Boolean Logic Option AND Size of output Grid Same s Grid 1 and Grid Values used in Overlap areas Grid 2 Only 4 This procedure forces the size of grid g4a grd to be the same size as grid g2 grd Grid g4a grd can be used in the subtraction step see step 12 above rather than the grid g4 grd 10 The grid g5 grd is a levelling error grid The resultant leveling error grid should be analyzed to verify that no geological information would be removed from the data in the next step 11 On the Grid menu select Expressions Subtract grids The Subtract one grid from another dialog is displayed 12 Select the First input grid g1 grd and select Second input grid to subtract from first g5 grd and then specify Output difference grid maglev grd Click OK to create the final grid The grid maglev grd is the final microlevelled grid You can use this grid for further processing or interpretation North America Geosoft Inc 85 Richmond St W 8th Floor Toronto Ont Canada M5H 2C9 Tel 416 3
4. the nput grid g2 grd and specify the output grid g3 grd In the Low pass filter wavelength box use the same low pass parameter value used in step 3 You can leave the rest of the parameters to the default values click OK to create the third grid The grid g3 grd is a grid of the regional field plus some high frequency noise 6 On the Grid menu select Gridding Filters 3x3 convolution The Grid Filters dialog will be displayed 7 Select the Input Grid File g3 grd and specify the New resultant Grid g4 grd In the OPTION 1 Select a Predefined Filter box select Hanning and in the Number of passes to apply box specify 3 5 times You can leave the rest of the parameters to the default values click OK to create the fourth grid The grid g4 grd is a grid of the regional field 8 On the Grid menu select Expressions Subtract grids The Subtract one grid from another dialog is displayed 9 Select the First input grid g2 grd and select Second input grid to subtract from first g4 grd and then specify Output difference grid g5 grd Click OK to create the fifth grid Note If you receive an error with the following message Unable to proceed because the size of the two grids were not the same One grid is 404 404 elements in size while another grid is 404 405 elements please read the following In order to perform any mathematical expressions on grids the grid nodes must be georeferenced in the same place
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6. ameters to their default values click OK to create the second grid The grid g2 grd is a grid of the levelling errors plus a regional field On the Grid menu select Gridding Re grid a grid The Regrid a grid dialog will be displayed Select the nput grid g2 grd and specify the output grid g3 grd In the Low pass filter wavelength box use the same low pass parameter value used in step 6 You can leave the rest of the parameters to the default values click OK to create the third grid The grid g3 grd is a grid of the regional field plus some high frequency noise On the Grid menu select Gridding Filters 3x3 convolution The Grid Filters dialog will be displayed Select the nput Grid File g3 grd and specify the New resultant Grid g4 grd In the OPTION 1 Select a Predefined Filter box select Hanning and in the Number of passes to apply box specify 3 5 times You can leave the rest of the parameters to the default values click OK to create the fourth grid The grid g4 grd is a grid of the regional field On the Grid menu select Expressions Subtract grids The Subtract one grid from another dialog is displayed Select the First input grid g2 grd and select Second input grid to subtract from first g4 grd and then specify Output difference grid g5 grd Click OK to create the fifth grid The grid g5 grd is a leveling error grid The resultant leveling error grid should be analyzed to verify that no geol
7. iginal located data if available or from the grid if not available The Bi directional microlevelling method derives the error grid by assuming that the gridded data consists of geology a regional field and levelling errors These are separated out at various stages by low pass filtering during the gridding process Overview of Bi directional Microlevelling As an example given a magnetic grid surveyed with a nominal east west survey line spacing of 200 metres and has a line to line levelling problem Mag_data gdb Note that if your survey lines are not E W or N S you will need to rotate the data using the Coordinates Rotate menu option Initially we will create a low pass filtered grid that contains the levelling errors plus a regional field g2 grd We will then re grid the g2 grd using the same low pass filtering parameters creating a grid that contains the regional field plus some high frequency noise g3 grd A Hanning 3x3 convolution smoothing filter will then be applied to the g3 grd to remove the high frequency noise and create grid g4 grd We will the subtract grid g4 grd from grid g2 grd creating a levelled error grid g5 grd We will then extract the resultant levelling error grid values g5 grd into the original database using the Grid Utilities Sample a grid menu item We will then create a microlevelled maglev channel in the database Mag_data gdb by applying a math formula that subtracts the lev channel fro
8. m the mag channel The result is a levelled magnetic data channel in your database In the final step we will grid the maglev channel to create the final microlevelled grid You can use this grid for further processing or interpretation Note If the original located database is not available you can subtract the levelling error grid g5 grd from the original grid g1 grd to produce a final micro levelled grid See Steps to Perform Bigrid Microlevelling from a Grid Steps to Perform Bi directional Microlevelling from a Database 1 Open the database Mag_Data gdb in your current workspace 2 On the Grid menu select Gridding Bi directional line gridding Dialog controls The Bi directional gridding of line data dialog will be displayed 10 11 12 Select the Channel to grid mag specify the output grid g1 grd and specify the Cell size to be 1 4 of the mean line spacing You can leave the rest of the parameters to the default values click OK to create the first grid On the Grid menu select Gridding Bi directional line gridding Dialog controls The Bi directional gridding of line data dialog will be again displayed Select the Channel to grid mag and this time specify the output grid as g2 grd and click the Advanced button The Advanced Gridding options dialog will be displayed In the Low pass filter wavelength box set the low pass filter parameter to 4 10 times the line spacing You can leave the rest of the par
9. n a new channel in the database 14 Select X reference channel X and the Y reference channel Y from your current database In the Grid sampled channel box specify the channel as lev and in the Grid file box select the Grid file g5 grd Click OK to extract the grid values into the database 15 Examine the lev channel in profile view to determine how much geology remains in the error level grid You can remove this signal using either a low pass or non linear filter X Utility Filters menu items In these final steps you will create a microlevelled maglev channel by applying a math formula that subtracts the lev channel from the mag channel The result is a levelled magnetic data channel in your database You can then grid the levelled channel to create a final microlevelled grid 16 To create a new channel click the right mouse button on the header cell the top cell of a blank spreadsheet column On the popup menu click New 17 Specify the channel name as maglev Click the OK button The new channel is created 18 On the X Utility menu click Expressions Expression The Apply Formula dialog is displayed In the Expression box type maglev mag lev and click the OK button The equation is applied to the maglev channel and the values displayed in the spreadsheet column 19 You can now grid the microlevelled data channel maglev using Bi directional line gridding On the Grid menu click on Gridding then click
10. ogical information would be removed from the data when we subtract the leveling error data from the original mag data in the database Note If you receive an error with the following message Unable to proceed because the size of the two grids were not the same One grid is 404 404 elements in size while another grid is 404 405 elements please read the following In order to perform any mathematical expressions on grids the grid nodes must be georeferenced in the same place and have the same physical dimensions Regridding and filtering may cause the grids to differ slightly in physical size If this happens 1 On the Grid menu select X Utilities Boolean operations The Mask One Grid Against Another dialog is displayed 2 Select the Input Grid 1 g2 grd the Input Grid 2 g4 grd and then specify the Output Grid g4a grd 3 Using the dropdown lists select the following parameters Boolean Logic Option AND Size of output Grid Same s Grid 1 and Grid Values used in Overlap areas Grid 2 Only 4 This procedure forces the size of grid g4a grd to be the same size as grid g2 grd Grid g4a grd can be used in the subtraction step see step 12 above rather than the grid g4 grd 13 On the Grid menu click Utilities Sample a grid The Sample from a Grid dialog box is displayed This dialog uses the X and Y values in your current database Mag_Data gdb to sample a selected grid The grid values are then placed i
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