POTENTIAL FIELD METHODS - Homepage Usask
POTENTIAL FIELD METHODS - Homepage Usask
POTENTIAL FIELD METHODS - Homepage Usask
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GEOL 481.3<br />
<strong>POTENTIAL</strong> <strong>FIELD</strong> <strong>METHODS</strong><br />
OUTLINE<br />
<strong>POTENTIAL</strong> <strong>FIELD</strong> <strong>METHODS</strong><br />
GEOL 481.3<br />
“Many of us secretly dream of six months without gravity”<br />
Allan Fotheringham<br />
COURSE DESCRIPTION<br />
email: jim.merriam@usask.ca<br />
This course will examine the processing and interpretation of potential field data.<br />
Labs will focus on the processing of gridded potential field data, that is, interpolation to a<br />
grid, regional residual separation, continuation, vertical and horizontal derivatives, Werner<br />
and Euler deconvolution and depth estimates. Forward Modeling of selected profiles will<br />
be performed with GMSYS.<br />
The text is:<br />
MARKING SCHEME<br />
Mid Term Exam 25%<br />
Labs 25%<br />
Final Exam 50%<br />
Potential Theory in Gravity and Magnetics Applications by Richard Blakeley.<br />
Also useful are:<br />
An Introduction to Applied and Environmental Geophysics by John Reynolds;<br />
Interpretation of Gravity and Magnetic Anomalies for Non Specialists, these are notes<br />
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for a short course given by the Canadian Geophysical Union.<br />
Interpretation Theory in Applied Geophysics by Grant and West;<br />
Gravity and Magnetics in Oil Prospecting by Nettleton;<br />
Time Sequence Analysis in Geophysics by Kanasewich.<br />
Other literature (eg SEG extended abstracts and SAGEEP papers) will be referred to<br />
as needed.<br />
Labs and handouts are available in pdf format on PAWS<br />
SUMMARY<br />
WEEK 1 Uses of potential fields (gravity and magnetics) in geophysics. Definition of a Potential<br />
Field. The fundamental ambiguity of potential fields -its origin, and the limitations it<br />
puts on interpretation. A review of the drift correction, latitude, free air, and Bouguer<br />
corrections and how they relate to survey design. Leveling in aeromag.<br />
WEEK 2 Simple concepts of survey design, station spacing vs. target depth. Flight line and tie<br />
line spacing in aeromag. The terrain density, and the terrain correction. Methods of<br />
determining the terrain density: Nettleton, co-variance, scatterplot and others.<br />
WEEK 3 Introduction to processing. Gravity vs. magnetics in terms of targets, processing,<br />
interpretation. 1-D Fourier transforms and convolutions<br />
WEEK 4 2-D Fourier Transforms and convolutions. The use of 2-D transforms in potential<br />
fields.<br />
WEEK 5 Regional-residual separation, and anomaly separation. Graphical, spectral factorization,<br />
polynomial fitting and other techniques are discussed.<br />
WEEK 6 Gauss’ Theorem and the derivation of the excess mass calculation. Limitations on the<br />
excess mass, corrections for under-sampled flanks.<br />
WEEK 7 Magnetic potentials. The fundamental differences between gravity and magnetics as<br />
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potential fields.<br />
WEEK 8 Downward continuation in integral form and in FFT form. Upward continuation.<br />
Limitations on the depths or heights to which data can be continued.<br />
WEEK 9 The equivalent stratum and its use. The region of validity of equivalent strata.<br />
WEEK 10 Survey parameters and survey design.<br />
WEEK 11 Second vertical derivative, uses, examples, problems. Source depth estimates for gravity<br />
and mag. Locating the center of mass in gravity.<br />
WEEK 12 First vertical derivative and horizontal derivatives, strike filters, trend filters. Reduction<br />
to pole.<br />
WEEK 13 New methods in potential field processing and interpretation: Werner deconvolution,<br />
Euler deconvolution, fractals and potential fields. Trends in exploration, high resolution<br />
magnetics and vector magnetics, the analytic signal.<br />
LABS<br />
All Labs must be completed within one week.<br />
LAB 1 Drift corrections on mag and gravity data. Examination of repeats. All gravity (with<br />
elevations) and mag data should be on disk in a suitable format before the first LAB.<br />
LAB 2 Latitude, free air and Bouguer corrections. Profiles of all three should be presented<br />
at the end of the labs.<br />
LAB 3 Gridding and Contouring.<br />
LAB 4 Exercise on Fourier transforms I and II<br />
LAB 5 Terrain Density determination with Nettleton, scatterplots, covariance etc.<br />
LAB 6 Terrain corrections for gravity profiles.<br />
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LAB 7 Regional residual separation.<br />
LAB 8 Depth estimates.<br />
LAB 9 Gravity and Mag modeling with GM-SYS.<br />
LAB 10 DIPOLE and DIPOLE2 on MATLAB. Their use in simple modeling.<br />
LAB 11 Leveling in aeromag surveys.<br />
REFERENCES<br />
Bell, Robin, 1998. Gravity Gradiometry. Scientific American. June 98, p74.<br />
Chapin, David. 1996. The theory of the Bouguer gravity anomaly. The Leading Edge,<br />
May 1996 361-363.<br />
Chapin, David, 1998. Gravity instruments: past, present and future. The Leading<br />
Edge, 17, 100-112.<br />
* Cordell, L., and V.J.S. Grauch, 1982. Reconciliation of the discrete and integral<br />
Fourier transforms. Geophys. 47, No2 237-243.<br />
Debeglia, N. and J. Corpel 1998 Automatic 3-D interpretation of potential field data<br />
using analytic signal derivatives. Geophys. 62 No 1 87-96.<br />
* Ebner E. Developments in potential field methods and their application to petroleum<br />
exploration in western Canada. A four part series in the Recorder, Jan, March, June,<br />
and Sept. 1995.<br />
Fairhead, J.D., M Green, Denizar Blitzkow, 2003, The use of GPS in gravity surveys.<br />
The Leading Edge, 22, No 10, p945-959<br />
Grauch, V.J.S., and Campbell, 1984. Does draping aeromagnetic data reduce terrain<br />
induced effects? Geophysics 49, 75-80.<br />
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LaCoste, L. 1991. A new calibration method for gravity meters, Geophys. 56, #5<br />
701-704.<br />
LaCoste, L. 1991. Gravity meter calibration at LaCoste and Romberg, Geophysics,<br />
56, # 5 705-711.<br />
* LeFehr, T.R., Standardization in gravity reduction, Geophysics, 56, 1170-1178.<br />
LeFehr, T.R., 1991. An exact solution for the gravity curvature, Geophysics, 56, #8<br />
1179-1184.<br />
Fedi, M. T. Quarta, and A. DeSantis, Inherent power law behaviour of magnetic field<br />
power spectra from a Spector and Grant ensemble, Geophys, 63 No 4 1143-1150.<br />
Ferris, C., 1987. Gravity anomaly resolution at the Garber field. Geophys 52, No 11<br />
1570 1579.<br />
Grant, F.S., 1972. Review of data processing and interpretation methods in gravity<br />
and magnetics Geophysics, 37, 647-661.<br />
Grauch V.J.S., 1993. Limitations on digital filtering of the DNAG magnetic data set<br />
for the conterminous US. Geophys. 58, # 9 1281-1296.<br />
Gupta, V.K. and Ramain N. 1980. Some aspects of regional residual separation of<br />
gravity anomalies in a precambrian terrain. Geophysics, 45 1412-1426.<br />
Hammer, S., 1982. Critique of terrain corrections for gravity stations. Geophys. 47,<br />
839-840.<br />
Hammer, S., 1939. Terrain corrections for gravimeter stations. Geophys. 4, 184-194.<br />
Hinze, W.J., 2003. Bouguer Reduction Density, why 2.67? Geophysics 56,No 5 1559-<br />
1560.<br />
Jackson, B.H., 1987. A case for upward continuation as a standard separation filter<br />
for potential field maps, Geophysics, 52, No 8 1138-1148.<br />
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Negi J.G., 1967. Convergence and divergence in downward continuation. Geophysics,<br />
32, 867-871.<br />
Nettleton, L.L., 1954. Regionals, residuals and structures Geophys 19, 1-22.<br />
Nowell, D.A.G., 1999. Gravity Terrain Corrections - An Overview, J. Appl. Geophys.<br />
42, 117-134.<br />
Pilkington, M., and W.R. Roest, 1992. Draping aeromagnetic data in areas of rugged<br />
topography. J. Appl. Geophys. 29, 135-142.<br />
Pilkington, M. and W. Roest, 1998. Removing varying directional trends from aeromagnetic<br />
data. Geophys. 63 No 2 446-453.<br />
Ander, M.E. 1999. LaCoste&Romberg gravity meter: System Analysis and Instrumental<br />
Errors, Geophys, 64, N0 6 1708-1719.<br />
Rauth,M., and T. Strohmer, 1998. Smooth approximation of potential fields from<br />
noisy scattered data. Geophys. 63. No 1 85-94.<br />
* Rymer, H. 1989. A contribution to precision microgravity data analysis using Lacoste<br />
Romberg gravity meters. Geophys. Jour. 89,311-322.<br />
Xia, J., D.R. Sprowl, and D. Adkins-Heljeson, 1993. Correction of topographic distortions<br />
in potential-field data: A fast and accurate approach, Geophysics, 58, #4<br />
515-523.<br />
Also, the Geological Survey of Canada, publishes summaries of papers by its staff on<br />
the WWW. The Regional Geophysics Section<br />
http://www.gdcinfo.agg.nrcan.gc.ca<br />
has a list of several topical summaries.<br />
* THESE ARE VERY IMPORTANT AND YOU SHOULD READ THEM.<br />
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