Other useful interpretation techniques (C05)
Contents
1. HL INTREPID spatial and time domain filters and transformations R13 General information about spatial and time domain filters for both line and grid datasets Hat INTREPID spectral domain operations reference R14 General information about spectral Fourier domain filters for both line and grid datasets gt te Line Filtering T31 Applying filters to line data in both the spectral and spatial domain EH te E b gt Spatial Convolution Grid Filters T34 Old spectral domain grid filters OldGridFFT T38 Applying convolution kernel filters to grid data in the spatial domain Applying spectral Fourier domain filters to grid datasets You may also need to refer to INTREPID Functions in INTREPID expressions and functions R12 and Spreadsheet Editor T15 Library Help Top 2012 Intrepid Geophysics 4 Back gt2. advantage is its ability to remove the dipolar effect of a magnetic anomaly INTREPID tool Line Filter Reference Taner Koehler and Sheriff 1979 Instantaneous frequency The instantaneous frequency product sharpens edges of anomalies enhancing peaks and troughs INTREPID tool Line Filter Reference Taner Koehler and Sheriff 1979 Notes Try these filters if you want to enhance deep low frequency structure Run the filter and grid the resulting field This grid can be useful as an image backdrop for depth method results such as Phillips Euler or Naudy Automatic Model 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 4 Library Help Top lt 4 Back gt Tips for using enhancement methods Enhancing regional magnetic structure Accentuating magnetic structure We recommend the following products for accentuating structure Bat First vertical derivative or a fractional vertical derivative of order close to 1 followed by Automatic Gain Control AGC For gridded data the spatial CNorm contrast normalisation filter has a similar effect to an AGC filter Z te Complex amplitude Enhancing structure in certain directions We recommend the following processes for accentuating structure in certain directions He Spatial convolution using for example the North_South or East_West kernel You can also configure your own kernels Contact our tech
3. phase shifted If you combine the two signals in complex space you obtain a signal which can be visualised as being in the shape of a spiral see below The complex wave C is the sum of the real and imaginary waves R and I C R I At any point on the curve c r i The complex amplitude a is the distance from the x axis to the point on the curve Note In some earlier editions of INTREPID spatial and time domain filters and transformations R13 the complex amplitude is erroneously called the analytic signal Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 3 Library Help Top Library Help Top lt 4 Back gt The instantaneous phase is the angle between the real and imaginary component i instantaneous phase atan P The instantaneous frequency is the rate of change of instantaneous phase INTREPID uses its diff1 function to calculate this See INTREPID Functions in INTREPID expressions and functions R12 for details Inst Frequency diff1 Inst Phase Complex Amplitude is distance from axis to curve I maginary Instantaneous Phase is the angle between the Real and Imaginary components Time distance along line This is equivalent to the angle between the wave and the Real axis in the complex plane Complex amplitude You can use complex amplitude to enhance edges of anomalies Its
4. INTREPID User Manual Other useful interpretation techniques C05 1 Library Help Top 4 Back gt Other useful interpretation techniques C05 Top l An important part of the interpretation process is identifying and analysing anomalies and regional structure in magnetic survey data INTREPID can make this task easier by enhancing the features you wish to examine This chapter summarises INTREPID enhancement methods The next chapter Presenting regional depth and structure data C06 shows you the best ways to display your interpretation data For an explanation of the dataset and domain symbols which appear frequently in this chapter see Symbols for INTREPID dataset types and domains in Using INTREPID Cookbooks R19 Enhancement methods Pass continuation and directional filters l Pass and continuation filters are traditional aids to interpretation They can remove ranges of wavelengths from the data enhancing the features at depths characterised by the remaining wavelengths Directional filters enhance features in certain directions grid datasets only INTREPID tools Line Filter Spectral Domain Grid filters Spatial Convolution filters Vertical gradient h gt Library Help Top The vertical gradient is another traditional interpretation aid It can sharpen and enhance shallow source anomalies and the edges of anomalies It can be used in combination with Automatic Gain Co
5. gs and laterite deposits Apparent shallow sources can also simply be noise We recommend the following processes Bat Upward continuation filter Bat Low pass filter Butterworth filter for grids E Spatial convolution upward continuation grid filters for example up_cont_1 and up_cont_2 These kernels perform upward continuation of one and two cell widths respectively Enhancing shallow sources We recommend that you perform these processes on the line data before gridding The gridding process tends to remove high frequency data which may have value The processes will work on grids however if required We recommend the following processes Hir e Vertical derivative of order 0 5 1 5 e AND High pass filter Butterworth filter for grids AND Downward continuation E Spatial convolution high pass kernels residual_1 and residual_2 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 7 Library Help Top 4 Back gt Enhancing very shallow sources strand line enhancement This method enhances strand lines up to 10 m below ground The main purpose of this process is to locate heavy mineral sands or laterite Due to the prevalence of noise at this level and other difficulties the process needs some expertise We recommend that you refer to the reference paper Mudge 1991 before trying it If you are un
6. nical support service for assistance if required EH He Directional spectral domain filters directional pass and directional cosine Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 5 Library Help Top lt 4 Back gt Enhancing edges of anomalies We recommend the following processes for showing the edges of anomalies Hat First vertical derivative or a fractional vertical derivative of order close to 1 followed by Automatic Gain Control AGC E gt Analytic signal ais Instantaneous frequency a e MagMage coherence map Bat Horizontal derivative Enhancing features at different depths Enhancing features deeper than 200 m We recommend the following processes Bat Low pass filter Butterworth filter for grids Set it to pass wavelengths corresponding to depths greater than 200 m E Spatial convolution upward continuation grid filters for example up_cont_1 and up_cont_2 These kernels perform upward continuation of one and two cell widths respectively Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 6 Library Help Top lt 4 Back gt Removing very near surface sources It is often desirable to remove sources which are very close to the surface 50 m below ground or shallower for example buildin
7. ntrol AGC Start by using the default settings for the AGC filter in the Line Filter tool Another useful INTREPID feature is the ability to calculate fractional vertical derivatives for example the 1 5 derivative INTREPID tools Line Filter Spectral Domain Grid filters Reference Gunn Maidment and Milligan 1997 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Other useful interpretation techniques C05 2 Library Help Top lt 4 Back gt Analytic signal The analytic signal can sharpen and enhance regional structure and the edges of anomalies in a grid It has the advantage particularly in low latitudes that it does not require reduction to the pole INTREPID tools Grid FFT tool gfilt exe Euler Deconvolution tool euler exe Reference Roest Veroef and Pilkington 1992 Magmage coherence map The MagMage coherence map detects discontinuities in a grid We have found it useful for mapping peaks and troughs of magnetic trends It has been able to identify z weak magnetic axes and troughs that often only show up in vertical gradient images INTREPID tool MagMage magnetic interpretation Under Interpretation Magnetics Reference Bahorich and Farmer 1995 Gunn et al 1997 Hilbert complex wave products Theory X The Hilbert transform is commonly used in seismology It obtains an imaginary component of data The imaginary component is similar to the real component in shape but is
8. sure about parameters contact our technical support service for advice Perform the following processing on line data not gridded data Method 1 Apply a light low pass filter to remove high frequency noise This is available in the Line Filter tool For noisy data a 7 point moving average filter would be suitable e Calculate the horizontal derivative with order in the range 1 4 depending on the data This is available in the Line Filter tool Method 2 Apply a light low pass filter to remove high frequency noise Calculate the nth difference for the data This is available using the diffn function in the Spreadsheet Editor Select an order for the diff function e g diff4 depending on the data Either apply an Automatic Gain Control AGC filter to the data using the Line Filter tool or take the square root of the data using the sqrt function in the Spreadsheet Editor See Illustrating linear low amplitude high frequency data strand line enhancement in Presenting regional depth and structure data C06 for details about displaying the results of the process 2012 Intrepid Geophysics lt 4 Back gt Library Help Top INTREPID User Manual Library Help Top Other useful interpretation techniques C05 8 4 Back gt Reference Manual sections relevant to this chapter While reading this chapter you may need to refer to the following chapters Chapter Topic
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