montaj Extension Depth to Basement

The Depth to Basement extension 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 extension to Oasis montaj 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, the extension can also be used on gravity profiles to determine the position of gravity source bodies.

The extension includes three different depth to basement techniques: 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

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

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.

References

Mushayandebvu, M. F., van Driel, P., Reid, A.B., and Fairhead, JD., 2001, Magnetic source parameters of two dimensional structures using extended Euler deconvolution: Geophysics, vol.66, no.3, p814-823

Reid, A.B., Allsop, J.M., Grasner, H., Millet, A.J., and Somerton, I.W., 1990, Magnetic

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