Decorrugation (T32) - Intrepid Geophysics
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1. Back gt Decorrugation practical methods We require a set of filters that removes all errors but at the same time excludes as many real anomalies as possible To avoid removing real anomalies we extract residual errors with the longest possible wavelength along the acquisition lines the shortest wavelength perpendicular to the lines and the smallest dynamic range that still produces a well levelled grid In practice you will need to use some trial and error until you achieve a satisfactory result Decorrugation is our recommended method of removing Corrugations Like any other similar method it is far from perfect and you must use it under your own close supervision Some other filtering techniques such as spectral domain filters may work just as well No matter which method you use you can still use the resulting grid with Microlevelling to level your original line data Note that you must still perform conventional tie line levelling The Decorrugation technique can only correct small residual levelling errors The data must be reasonably level prior to the gridding process If you have real anomalies that may be removed by Decorrugation you should use a subsection mask to specify parts of the grid that should not be filtered You should truncate residuals corrections that are obviously too large to be valid Two types of filter are available Each has imperfections See Potential limitations of Decorrugation You can sele2. INTREPID displays the current options in a separate report window Using task specification files You can store sets of file specifications and parameter settings for Decorrugation in task specification job files gt gt To create a task specification file with the Decorrugation tool 1 Specify all files and parameters 2 If possible execute the task choose Apply to ensure that it will work 3 Choose Save Options from the File menu Specify a task specification file INTREPID will add the extension job INTREPID will create the file with the settings current at the time of the Save Options operation For full instructions on creating and editing task specification files see INTREPID task specification job files RO6 gt gt To use a task specification file in an interactive Decorrugation session Load the task specification job file File menu Load Options modify any settings as required then choose Apply gt gt To use a task specification file for a batch mode Decorrugation task Type the command decor exe with the switch batch followed by the name of the task specification file For example if you had a task specification file called surv329 job in the current directory you would use the command decor exe batch surv329 job Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 14 Library Help Top lt 4 Back gt Task specification
3. Library Help Top Apply Show zoca In Zacam Aut zoca Area You can enlarge and reduce the display zoom in and out and view different parts of it pan gt gt To zoom in and out enlarge reduce To zoom in enlarge the display choose Zoom In at the right edge of the Decorrugation window Each time you choose this button INTREPID will enlarge the display by one step To zoom out reduce the display choose Zoom Out at the right edge of the Decorrugation window Each time you choose this button INTREPID will reduce the display by one step 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 9 Library Help Top lt 4 Back gt gt gt To zoom in on a selected area of the display Choose Zoom Area then hold down the left mouse button and drag diagonally corner to corner across the area that you wish to enlarge INTREPID will enlarge the selected region to fill the display area of the window gt gt To pan the display view different parts The Pan zoom indicator at the right of the Decorrugation window in the centre consists of a small square within a larger square The large square represents the whole display and the small square the part visible on the screen When you drag the small square to a different part of the large square INTREPID shows the corresponding part of the display If you have zoomed out to view the whole display the small square may occupy
4. INTREPID User Manual Decorrugation T32 1 Library Help Top 4 Back gt Decorrugation T32 Top After you have corrected tie line levelled and gridded your survey data some residual line to line errors may still be visible These may appear as streaks or corrugations in your grid display They are sometimes referred to as levelling artifacts or levelling busts We describe them in detail in the section following INTREPID s Decorrugation tool removes these features from the grid and Microlevelling See Microlevelling T33 applies the grid corrections back to your line dataset This method is based on a paper by Brian Minty Simple Micro Levelling for Aeromagnetic Data B R S Minty Exploration Geophysics 1991 22 591 592 Decorrugation theory Sources of corrugations Library Help Top Corrugations levelling artifacts in your gridded data are caused by line to line levelling errors in your line data These are generally caused by conditions encountered during acquisition of airborne survey data and or errors in conventional tie line levelling SeeLine correction and tie levelling T30 For example Levelling is required to remove the effects of time variations in the Earth s magnetic field There may not be enough crossover points in the dataset to adequately model and correct for this variation The measured position of a crossover point of an acquisition and tie line may not be accurate
5. INTREPID spatial and time domain filters and transformations R18 for general information about the filters and their parameters Setting Decorrugation filter parameters To set the parameters choose Parameters from the Filters menu High Pass Type gt Low Pass Type gt Naudy Naudy Fuller INTREPID displays the Decorrugation parameters dialog box Filter Parameters Min Streak length 5000 0 Streak width 1600 0 Minimum adjustment Maximum adjustment 30 0 C Decorrugate along columns Decorrugate along rows for datasets with none NS or EW lines use rotate option in gridding OK Cancel Set the parameters according to your requirements then choose OK Specify wavelengths in metres Mean Streak Length The minimum length of an anomaly parallel to the acquisition line direction that INTREPID will recognise as a corrugation and remove Minimum streak length is usually twice the tie line spacing INTREPID will only treat anomalies longer than this as potential corrugations INTREPID uses this parameter in the low pass filter It corresponds to the window size parameter for the Fuller and or Naudy filter used in the process See Fuller filter in INTREPID spatial and time domain filters and transformations R13 and Naudy filter in INTREPID spatial and time domain filters and transformations R13 for further information Library Help Top 2012 Intrepid Geophysics 4 Bac
6. If an error arises here it may introduce an error covering a region that extends one line spacing in each direction The dimensions of this region would therefore be twice the tie line separation by twice the acquisition line separation Since the introduction of Global Positioning Satellites the likelihood of this type of error has lessened considerably There may be diurnal fluctuations not accurately measured at the base station because they are localised and or have a steep gradient These could affect a number of measurements along an acquisition line and be only partly corrected for by tie line levelling e Aircraft ground clearance varitaions cause line to line errors and these variations are generally difficult to properly correct for Whilst these residual levelling errors are usually small when you use image processing and display enhancement techniques and compute derivatives of the data they become more obvious and reduce the quality of your images In airborne gamma ray spectrometric surveying the following conditions can cause line to line errors in the data There is a period of rain during a traverse When the aircraft flies through rain moisture in the air and ground attenuates the gamma rays thus yielding a lower count rate The aircraft flies across land where it has recently rained perhaps where a thunderstorm has passed In this case The increased moisture content of the soil attenuates the gamma rays th
7. cification files for more information 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 10 Library Help Top lt 4 Back gt Using Decorrugation for oblique line datasets If the purpose of using this tool is to decorrugate an oblique line dataset one with traverse lines that are not oriented North South or East West you must have produced an input grid with either its columns or rows parallel to the traverse lines of the line dataset being corrected i e a rotated grid The Gridding tool has facilities for this process See Producing a rotated grid in Old Gridding T22 for instructions Note If you correctly use a rotated grid as the input grid for Decorrugation the output grid will be ready to input to the Microlevelling tool without further adjustment Do not rotate the output corrections or levelled grid before using it with Microlevelling See Microlevelling T33 for further information Choosing the Decorrugation filter type Use High Pass Type from the Filter menu to define the type of filter to use perpendicular to the acquisition line direction Use Low Pass Type from the Filter menu to define the type of filter to use in the acquisition line direction File Grid Output Help Parameters High Pass Type gt Naudy Naudy Fuller Fuller smoothed Fuller Naudy Use the Naudy filter Naudy Fuller Use Naudy and Fuller filters
8. ct the type of filter and use them in tandem to minimise their deficiencies The filter types available are the Naudy non linear filter and the Fuller hanned band pass convolution filter 29 Potential limitations of Decorrugation Library Help Top The technique could well eliminate real anomalies if they have the qualifying characteristics for Decorrugation Ifthere are adjacent errors of similar magnitude Decorrugation will only detect the differences between these errors The two filter techniques each have their own problems The Naudy filter is not a true frequency filter It operates by detecting anomalies of wavelength shorter than the defined cutoff Such anomalies are then removed by extrapolating over them from data values that bound them We have found that The anomaly detecting algorithm is not foolproof and some anomalies may not be removed The extrapolation algorithm may introduce data of frequency beyond the intended cutoff 1 8 Henri Dreyer and Henri Naudy Non Linear Filtering Applied to Aeromagnetic Profiles Geophysical Prospecting 16 2 June 1968 2 D C Fraser B D Fuller and S H Ward Some Numerical Techniques for Application in Mining Exploration Geophysics vol XXXI NO 6 December 1966 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 6 Library Help Top 4 Back gt The Fuller hanned band pass convolution filter technique is
9. file notes and example Here is an example of a Decorrugation task specification file Process Begin Name decorrugate Parameters Begin HighPassFilter NAUDY LowPassFilter SMOOTHEDFULLER StreakLength 5000 00 StreakWidth 800 00 HighPassTolerance 01 MinimumCorrection 10 00 MaximumCorrection 10 00 GridBand 1 StrikeDirection 90 00 CorrectionGrid Yes Parameters End InputGrid disk1 grids sp596_mag OutputGrid disk1 grids p596_corr Process End Notes You can specify the tolerance for a high pass Naudy filter by adding the line HighPassTolerance to the parameters block as shown in the above example See Naudy filter in INTREPID spatial and time domain filters and transformations R13 for further information Library Help Top 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Decorrugation T32 15 Library Help Top lt 4 Back gt Frequently asked questions Q I am having trouble finding the right decorrugation filter combination Can I run it more than once Yes you can run Decorrugation multiple times with different filter settings If you output a levelled grid each time you can apply different filter settings to remove different shaped streaks from your data Library Help Top 2012 Intrepid Geophysics lt 4 Back gt
10. in When the process has finished you can zoom enlarge an area of the display and pan examine different regions while enlarged See Zooming and panning the grid display for details 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 8 Library Help Top 4 Back gt 9 Ifyou wish to record the specifications for this process in a job file in order to repeat a similar task later or for some other reason use Save Options from the file menu See Specifying input and output files for detailed instructions 10 If you wish to repeat the process repeat steps 3 8 varying the parameters and or data files as required 11 To exit from Decorrugation choose Quit from the File menu To view the current set of specifications choose Show from the Decorrugation window INTREPID displays them in a separate report window See Displaying options and using task specification files for details and an example of a set of specifications After using Decorrugation you can carry out a more detailed inspection of the filtered grid using an INTREPID visualisation tool See Visualisation T26 You can view Help information by choosing options from the Help menu See Help You can execute Decorrugation as a batch task using a task specification job file that you have previously prepared See Displaying options and using task specification files for details Zooming and panning the grid display
11. in tandem For the low pass filter INTREPID will apply the Naudy filter first followed by a Fuller filter This gives the best results but takes longer Fuller Use the Fuller filter Smoothed Fuller Use the Fuller filter INTREPID uses subsampling of the data for low pass filters In this case a secondary Fuller filter will remove any introduced high frequency noise We recommend that you use a Naudy filter for the high pass filter and a Smoothed Fuller filter for the low pass filter Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 11 Library Help Top lt 4 Back gt Filter parameters Window size wavelength This parameter corresponds to the length and width of the streaks you are removing See Setting Decorrugation filter parameters for instructions High Pass Tolerance First high pass filter In normal interactive mode the Naudy filter tolerance for the first high pass filter is set to 0 01 This is satisfactory for magnetic data but too high for such data as a vertical derivative grid If you use the Decorrugation tool with a task specification file in interactive or batch mode you can specify a different tolerance by adding the line HighPassTolerance to the parameters block See Displaying options and using task specification files for details See Fuller filter in INTREPID spatial and time domain filters and transformations R13 and Naudy filter in
12. k gt INTREPID User Manual Decorrugation T32 12 Library Help Top lt 4 Back gt Streak width The maximum width of an anomaly perpendicular to the acquisition line direction that INTREPID will recognise it as a corrugation and remove usually twice the acquisition line spacing INTREPID will only treat anomalies narrower than this as potential corrugations INTREPID uses this parameter in the high pass filter It corresponds to the window size parameter for the Fuller and or Naudy filter used in the process See Fuller filter in INTREPID spatial and time domain filters and transformations R13 and Naudy filter in INTREPID spatial and time domain filters and transformations R13 for further information Minimum adjustment The minimum value for any detected correction If INTREPID finds any correction less than this minimum value it will set the correction for this cell equal to the minimum value The minimum adjustment typically has the same absolute value as the maximum adjustment see below Maximum adjustment The maximum value for any detected correction If INTREPID finds any correction greater than this maximum value it will set the correction for this cell equal to the maximum value The maximum adjustment typically has the same absolute value as the minimum adjustment see above Decorrugate along Columns Rows Select the option corresponding to the direction of the traverse lines that you used to produce
13. kely for corrugations arising from causes listed in the previous section Shorter wavelengths in the acquisition line direction or longer wavelengths in the tie line direction would give a high risk of eliminating real anomalies A symmetrical waveform We make this assumption in order to keep the Decorrugation process simple A relatively small dynamic range Corrugations are by definition very small errors A large magnitude anomaly must be real or have some other cause besides those suggested for corrugations See Sources of corrugations Tie Lines A l Acquisition Lines Levelling error at this crossover Corrugation Corrugation arising from a tie line levelling error 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 3 Library Help Top lt 4 Back gt Decorrugation general technique Library Help Top To simplify the process one of the grid axes must be parallel to the acquisition line direction You can then remove the corrugations by applying one dimensional filters in turn to the rows and columns of the grid as follows 1 Call the original grid A Apply a high pass filter extract data with a short wavelength to grid A in the direction perpendicular to the acquisition line direction This removes all data except that with a wavelength perpendicular to the acquisition line direction of twice the acquisition line spacing or less St
14. less than perfect since the response of the filter at the cutoff wavelength is not abrupt For low pass filters it acts as a smoothing operator where the area under the profile remains constant We have found that lt Leakage occurs around anomalies of moderate amplitude that have wavelengths comparable to the length of the cutoff wavelength of the low pass filter When high amplitude short wavelength anomalies occur the magnitude of the filtered values about the anomaly are raised giving a streak effect exactly like the anomalies that are to be removed For very long wavelengths program efficiency requires that the number of convolution coefficients be restricted with the data effectively being subsampled This results in high frequency noise being introduced at a wavelength equal to the subsampled interval For both types of filters at the edge of the grid the data must be extrapolated to allow filtered values to be determined up to the boundary These extrapolation techniques do not always provide desirable filtered results The Decorrugation tool Library Help Top The Decorrugation tool is a two dimensional grid filtering program specifically designed to remove anomalies elongate in the line direction If required you can use the INTREPID subsection facility See Subsections of datasets T21 to specify parts of the grid windows that should not be filtered INTREPID will set corrections within the windows to zero and li
15. lowing steps as required 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 7 Library Help Top Library Help Top 4 Back gt 4 Specify the grid dataset to be processed Use Open Input Grid from the File menu See Specifying input and output files for detailed instructions INTREPID displays the dataset in the Decorrugation window a Intrepid decorrugate Tool V3 2beta9 File Filter Grid Output Help Apply Show Zoomin ZoomOut Zoom Area Specify the output grid dataset to be created with the results of the process Use Specify Output Grid from the File menu See Specifying input and output files for detailed instructions Specify the parameters and filter type for the Decorrugation using Parameters and Type from the Filters menu See Setting Decorrugation filter parameters and Choosing the Decorrugation filter type for details Specify whether you require the output file to be a levelled corrected grid or a grid dataset containing the corrections Choose the corresponding option from the Grid Output menu See Decorrugation output grid contents for more information When you have made specifications and settings according to your requirements choose Apply INTREPID will perform the corrections and display the new data in the Decorrugation window as it is produced a Intrepid decorrugate Tool V3 2beta9 File Filter Grid Output Help Apply Show Zoom
16. nearly extrapolate the values at the edge of the window to zero over an interval that you specify You can restrict the dynamic range of the residuals corrections within two bounds Since the residuals corrections will always have an average value of zero these bounds can be n where the value n is chosen to be close to the maximum size of the expected corrections In this manner you can truncate those residuals that are obviously too large to be valid corrections The output grid can be either a filtered levelled version of the input grid or a grid of the residuals corrections after filtering Normally you would produce a levelled output grid for inspection and visualisation processes but a grid of corrections for input to the Microlevelling tool See Microlevelling T33 gt gt To use Decorrugation with the INTREPID graphic user interface 1 Ensure that you have already levelled gridded and subsectioned your data as required 2 Choose Decorrugate from the Level menu in the Project Manager or use the command decor exe INTREPID displays the Decorrugation window 3 Ifyou have previously prepared file specifications and parameter settings for Decorrugation load the corresponding task specification file using Load Options from the File menu See Specifying input and output files for detailed instructions If all of the specifications are correct in this file go to step 8 If you wish to modify any settings carry out the fol
17. ore the result in grid B Levelled and gridded data Results of high pass filter Acquisition lines Apply a low pass filter extract data with a long wavelength to grid B in the acquisition line direction This removes all data except that with a wavelength in the acquisition line direction greater than twice the tie line spacing Store the result in grid C Grid C should now contain only the corrugations we wish to remove Levelled and gridded data Results of low pass filter Tie lines 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 4 Library Help Top lt 4 Back gt 3 Restrict the dynamic range of grid C by setting all values so that they fall between two limits defined by you Set any values beyond the limits to equal the limits themselves Grid C now represents residual errors corrections to be made Data with restricted Original data dynamic range 4 Subtract grid C from grid A to give the grid without Corrugations The following diagram illustrates the two filter steps in the decorrugation process Anomalies before identifying corrugations Anomalies after low pass filter lt gt Low pass filter Identified corrugations after high pass filter High pass filter Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 5 Library Help Top 4
18. the whole of the large square and may not be visible Specifying input and output files Library Help Top To use Decorrugation you will need to specify the grid dataset to be corrected and the grid dataset for saving the corrections Choose the options as required from the File menu Specify Output Grid Load Options Save Options Quit In each case INTREPID displays an Open or Save As dialog box Use the directory and file selector to locate the file you require See Specifying input and output files in Introduction to INTREPID R02 for information about specifying files Open Input Grid Use this option to specify the grid to be filtered INTREPID will open and display it in the Decorrugation window Specify Output Grid Use this option to specify the name for the grid dataset you are creating with this process Load Options If you wish to use an existing task specification file to specify the Decorrugation process use this option to specify the task specification file required INTREPID will load the file and use its contents to set all of the parameters for the Decorrugation process See Displaying options and using task specification files for more information Save Options If you wish to save the current Decorrugation file specifications and parameter settings as an task specification file use this option to specify the filename and save the file See Displaying options and using task spe
19. the grid See Using Decorrugation for oblique line datasets for important notes if the source traverse lines for the grid were not oriented North South or East West Decorrugation output grid contents Help Exit Library Help Top Use the Grid output menu to specify whether the output grid should contain the corrections to be made or the final corrected values File Filter Grid Output Help Corrections You would normally produce levelled grids while you are arriving at ideal parameter settings and filter choices You can visually inspect this filtered grid to ensure that unwanted streaks have been removed Once you have determined the optimal filters you would produce a grid containing only the corrections to be made This is the grid for input to the Microlevelling tool See Microlevelling T33 You can use the Help menu to display help text on the topics shown in the menu illustration below File Filter Grid Output Decorrugate Main Window gt gt File Filter gt gt Grid Output Batch Processing To exit from Decorrugation choose Quit from the file menu 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 13 Library Help Top lt 4 Back gt Displaying options and using task specification files Displaying options gt gt To display the current file specifications and parameter settings Choose Show in the Decorrugation window
20. us yielding a generally lower count rate 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Decorrugation T32 2 Library Help Top lt 4 Back gt There can be an increased surface concentration of radioactive daughter elements of airborne 7 7Ra These daughter products attach themselves to dust particles in the atmosphere Rain precipitates them onto the Earth s surface This phenomenon increases the count rates in the TC and U channels Because of the short half lives involved anomalous activity will only last about 6 hours The aircraft flies through a radon cloud Under still air conditions inversion layers over topographic depressions and lakes often trap radon close to the ground Radon clouds will increase the count rate primarily in the Uranium and Total Count channels These radon clouds disperse during the day as atmospheric mixing occurs When a survey is interrupted at the end of a working day radon clouds may have largely dispersed The following morning as the survey recommences at the same place radon clouds may have reappeared Types of error removed by Decorrugation Decorrugation can remove anomalies with the following characteristics Library Help Top Wavelength e Perpendicular to the acquisition line direction approximately twice the acquisition line spacing or less In acquisition line direction at least twice tie line spacing These are minimum wavelengths li
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