Gravity & Magnetic Interpretation
The Gravity and Magnetic Interpretation extension enables automatic location and depth determination for gridded magnetic and gravity data. Depth to Basement provides automated determination of position, dip, and intensity of magnetic source bodies. Isostatic Residual tools calculate Airy isostatic regional and residual gravity from a topographic grid.
The Grav/Mag Interpretation toolset automatically locates and determines depth for gridded magnetic and gravity data with Euler 3D Deconvolution processing routines. It includes the Keating Magnetic Correlation Coefficients tool for kimberlite exploration.
The Grav/Mag Interpretation toolset includes Euler 3D Deconvolution processing routines to automatically locate and determine depth for gridded magnetic and gravity data.
Euler 3D automates 3D geologic interpretation by delineating magnetic and gravimetric boundaries and calculating source depths.
It also includes the Keating Magnetic Correlation Coefficients function used in Kimberlite Exploration. This tool uses a simple pattern recognition technique to locate magnetic anomalies that resemble the response of a vertical cylinder model typical of Kimberlite pipes.
A Source Edge Detection (SED) tool is included for locating edges (e.g. geological contacts) or peaks from potential field data by analyzing the local gradients.
The Source Parameter Imaging (SPI) tool automatically calculates the depth of magnetic sources from a gridded magnetic dataset. The depths are displayed as a grid and are based on source parameters of the following source models : contacts (faults), thin sheets (dikes) or horizontal cylinders.
Use Grav/Mag Interpretation to:
- Locate and determine depth rapidly for large amounts of area data.
- Apply FFT and convolution grid enhancement and processing routines to calculate X and Y derivative grids.
- Display Total Field and Derivative grids for analysis.
- Analyze grids (perform Euler source inversion).
- Choose a structural index (any real value between 0.0 and 3.0).
- Display solutions in Oasis database and search large areas for similar targets.
- Perform and display solution statistics.
- Display/Plot database solutions.
- Window and plot solutions (based on location uncertainty and offset) to extract the solutions you consider relevant and remove erroneous solutions.
- Apply Keating Magnetic Correlation Coefficients tool for locating magnetic anomalies that resemble the response of modeled Kimberlite pipes.
- Apply Source Edge Detection tool for locating edges (i.e. geological contacts) or peaks from potential field data by analyzing the local gradients.
- Apply Source Parameter Imaging™ (SPI™) tool for quickly and easily calculating the depth of magnetic sources.
SPI and Source Parameter Imaging are trademarks of Geoterrex.
Depth to Basement
The Depth to Basement toolset provides an automated method for determining the position, dip and intensity of magnetic source bodies for a magnetic profile. The depths are determined using Werner Deconvolution, Analytic Signal and Extended Euler Deconvolution.
The Depth to Basement toolset provides an automated method for determining the position (i.e., distance along the profile and depth), dip (i.e., orientation) and intensity (e.g., susceptibility) of magnetic source bodies for a magnetic profile. With large, distinct density contrasts, it can also be used on gravity profiles to determine the position of gravity source bodies.
Three different depth to basement techniques are included: Werner Deconvolution, Analytic Signal and Extended Euler Deconvolution. Each Depth to Basement function utilizes a different accepted technique for determining the depth to the source. Each function has advantages in particular geologic situations. Applying several functions to the same geology greatly improves the reliability of results.
Solutions are saved in an Oasis montaj database (GDB), allowing you to immediately view the results in profile, edit the solutions, and plot the solutions on 2D and 3D maps. Additional functions also enable you to cluster solutions, export solutions to GM-SYS models, and generate starting GM-SYS models from data profiles.
Werner Deconvolution is an automated function for determining depth to source from profiles. It is based on the popular Werner Deconvolution technique (Werner, 1953; Ku & Sharp, 1983).
The user can control Werner's parameters in order to customize the application to each situation. The Werner Deconvolution GX will calculate the horizontal derivative or you may provide your own pre-calculated horizontal derivative for greater control. The adjustable "Residual cut-off" parameter enables the user to control the separation of "signal" from noise. The Werner Deconvolution GX assumes the source bodies are either dikes or contacts with infinite depth extent and uses a least-squares approach to solve for the source body parameters in a series of moving windows along the profile. The user specifies both the range of window sizes and the increments between window placements, thereby maximizing solution accuracy.
Analytic Signal is an automated function that enables you to determine Analytic Signal depth solutions from gravity and magnetic profiles. The Analytic Signal function is based on the U.S.G.S. program PDEPTH (Phillips, 1997), which is based on the Nabighian method published by Misac Nabighian. (1972, 1974).
The input profiles are interpolated to an even sample interval using the standard Oasis spline method before processing by the Analytic Signal GX. The sample interval is the total profile length divided by the number of points in the profile. Therefore, profiles with large gaps should be split into multiple lines.
The Analytic Signal technique first calculates the analytic signal of the input profile using a Hilbert Transform. Local peaks in the Analytic Signal profile are interpreted as corners of source bodies and the shape of the peak contains information about the depth to the corner. In the absence of high-frequency noise and aliasing in the data, horizontal locations from Analytic Signal are highly accurate.
For noisy input profiles, the results can be improved significantly by filtering the input anomaly and gradient data. The Analytic Signal GX uses a FFT technique to calculate the horizontal derivative if the user does not specify an input gradient channel.
Extended Euler Deconvolution
Extended Euler Deconvolution is an automated function for determining the depth to source from profiles. It is based on the the paper by Mushayandebvu and others, (2001).
The Extended Euler Deconvolution function calculates the horizontal and vertical derivative profiles or you may provide your own. If your input profiles are noisy, you can improve the performance of Extended Euler significantly by filtering or smoothing the profiles before running Extended Euler.
The number of solutions generated are controlled by four parameters. The "Min." and "Max." Depth parameters set the minimum and maximum depth cut-off values. The "Window Length" parameter sets the length of the Extended Euler operator, which is moved across the profile and used for each calculation. The "Max % error" parameter filters out solutions that differ in depth by more than this % when calculated by both Euler and Extended Euler calculations.
The Extended Euler calculation routine used in this tool was provided by GETECH™ and is based on the paper by Mushayandebvu and others, (2001). This approach calculates solutions using both the conventional Euler equation, Reid and others, (1990) and the "rotational constraint" equation from Extended Euler. Solving both equations jointly (Extended Euler) gives distance, depth, dip and susceptibility, assuming there is no remnants magnetization. Using conventional Euler gives a second estimate for distance and depth. If the relative difference in depth for the two estimates is less than the Max % error given by the user, the solution is retained; otherwise it is rejected.
The Isostatic Residual toolset calculates a depth to the Moho (the “root”) using the topographic grid, terrain density, Moho density contrast and depth of sea level compensation.
The Isostatic Residual toolset uses a modified version of the USGS algorithm to calculate the Airy isostatic regional and residual gravity from a topographic grid.
Isostatic Residual calculates a depth to the Moho (the "root") using the topographic grid, terrain density, Moho density contrast and depth of sea level compensation. It then calculates the 3-D gravity response of that root, at sea level, out to 166.7 km. The output must be combined with a solution beyond 166.7 km to make a complete Airy regional.
Use of Isostatic Residuals
The long wavelengths of the Bouguer gravity field correlate inversely with the long wavelengths of topography. Masses or "roots" at the base of the crust supporting the topography cause this correlation according to the theory of isostasy. These regional-scale anomalies are especially prominent in mountainous areas and near the edges of continents (i.e. continental shelves) and often obscure anomalies caused by upper crustal structures. Simpson et al., (1986) have shown this correlation analytically for the Conterminous U.S.; a qualitative comparison of the Bouguer anomaly and topography maps for other continents show this same correlation.
Polynomial fitting or wavelength-filtering techniques have been routinely used for the removal of topography-induced regional and the enhancement of gravity anomalies related to shallow geologic features. However, these techniques suffer from the fact that they eliminate all wavelengths longer than some threshold, whether of not they are related to topographic features In fact, long-wavelength anomalies due entirely to lateral variations in crustal density will be eliminated by these techniques. The isostatic correction is preferable because the isostatic regional accounts for the effect of the topographic "roots", thereby removing the observed correction between Bouguer values and topography. Isostatic residual gravity maps reveal more clearly than most gravity maps the density distributions within the upper crust that are of interest in most kinds of geologic and tectonic analyses.
The isostatic approach applies a consistent, 3-dimensional technique at virtually all scales for an entire continent including the continental margins. Additional geophysical data such as seismic refraction can be incorporated into the determination of the isostatic residual. The isostatic anomaly is based on an accepted geologic concept rather than a mathematical filter. The alternate methods can still be applied to the isostatic residual to remove regional trends not related to topographic loads.