Effects of Strike on Automatic Depth Estimation for 2D Magnetic Structures
Abstract
Many analysis methods have been developed to process densely sampled magnetic and/or gravity data to estimate source parameters. Werner deconvolution (Werner, 1953), analytic signal (Nabighian, 1972) and Euler deconvolution(Thompson, 1983) are among the most popular methods. They work either on profile data (Bastani and Pedersen, 2001) or on a regular grid (Thurston et al., 2002). All methods developed to estimate source parameters of the 2D magnetic structures work in the strike co-ordinate system. Werner deconvolution makes use of profile data to locate and compute the depth to the top and dip of thin sheets (dikes) with infinite strike and depth extent. Nabighian (1972) introduced the analytic signal to calculate the dip and depth to a set of 2D magnetic sources. The strike angle is assumed to be the same for all the magnetic anomalies along the profile. These methods use the horizontal and vertical derivatives of the total magnetic field to estimate the source parameters. While the vertical derivative is independent of strike direction the horizontal derivative is proportional to the sine of the angle between the profile and strike directions: the profile angle. Bastani and Pedersen (2001) used the analytic signal of the total magnetic field anomaly along a profile to estimate the dip, depth, width and strike of dikes. They introduced a method to estimate the strike of various anomalies at selected points along profiles by searching for coherent signals in neighboring profiles. Here we have used the same method to estimate strike of 2D anomalies. In order to illustrate the importance of strike angle on the estimated source parameters we have constructed synthetic data from a model that comprises a set of thin dikes with the same physical characteristics but with different strikes. We then applied 2D Werner deconvolution, 2D analytic signal (by Bastani and Pedersen), 2D and 3D Euler deconvolution to the data set. As expected the depth estimates are highly biased and varies with the inverse of the sine of the profile angle. Especially for small profile angles this effect becomes significant. With 3D Euler deconvolution depths estimates were not affected by strike variations, but large deviations were observed at the grid points between the profiles and anomalies crossings. Therefore it is necessary to use the strike information prior to gridding. We do this by estimating artificial data points by linear interpolation along lines connecting adjacent profiles. In this way the so-called bull's-eyes effects are reduced and the quality of the interpolation is improved. As a result the error introduced on the 3D depth estimates between adjacent profiles was considerably reduced. References: Bastani, M., and Pedersen, L. B., 2001. Automatic interpretation of magnetic dike parameters using the analytical signal technique. Geophysics, 66, No. 2, P. 551 -561. Nabighian, M. N., 1972. The Analytic Signal of Two dimensional Magnetic Bodies with Polygonal Cross-section: Its Properties and Use For Automated Anomaly Interpretation. Geophysics,37, NO. 3, P. 507--517. Thompson, D. T., 1982, EULDPH: A new technique for making computer-assisted depth estimates from magnetic data, Geophysics, 47, 31-37. Thurston, J. B., Smith, R. S., and Guillonz, J.-C., 2002. A multimodel method for depth estimation from magnetic data. Geophysics, 67, No. 2, P. 555 -561. Werner, S., 1953, Interpretation of magnetic anomalies at sheet like bodies: SGU. Serv C. Årsbok 43, no. 6.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMNS22A..08B
- Keywords:
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- 0910 Data processing