1. Curvature Depth Analysis of Gridded Aeromagnetic Data J. Phillips, R. Saltus, and D. Daniels U.S. Geological Survey EGS XXVII General Assembly Nice, April 21-26, 2002 U.S. Department of the Interior U.S. Geological Survey

2. Objectives Establish a relationship between the curvature of special functions and magnetic source depth, and use it to develop a new depth analysis method. Correct shallow depth solutions to a known minimum depth surface by increasing their structural index. Apply the new method to real data examples.

3. Special Functions: F(x) and F(x,y) Horizontal Gradient Magnitude Local Wavenumber Squared Analytic Signal Amplitude:

4. Special Functions for Magnetic Profiles For a source at (x 0,z 0 ), F(x) will peak over the source and have the functional form:

5. Curvature of Special Functions Curvature Definition At Peak of Special Function Curvature Depth:

6. Profile Example

7. Curvature Depths for Gridded Magnetic Data Where K(x,y) is the “most negative curvature” (Roberts, 2001):

8. Structural Index Values Contact Thick Dike Thin Sheet 0.0 0.5 1.0 Ribbon Pipe Finite Pipe Dipole 1.5 2.0 2.5 3.0

9. Correcting shallow depths by increasing the structural index z TRUE (SI=s) z AS (SI=0.0)

10. Wisconsin Aeromagnetic Data Aeromagnetic Map Precambrian Surface

11. Wisconsin Curvature Depth Results Depth SI

12. National Petroleum Reserve Alaska (NPRA) Aeromagnetic Data Aeromagnetic MapSeismic Basement

13. NPRA Curvature Depth Results Curvature Depths Structural Indices

14. Intra-Sedimentary Magnetic Sources Located on aeromagnetic mapMinimum elevation above seismic basement

15. Seismic Intrusions

16. Flow Chart Mag.grdPLUGGRIDMag.plgAS4Mag.a32 ADDGRD (m) Maga32.mskCURVDEPMaga32.depCURVCurv.pst Obsurf.grdADDGRD (+) Maga32d.asl Basement.grdCURVSIAscor.out Assi.out GRIDSAMP (1) Samp1.pst GRIDSAMP (6) Samp2.pst x y p1 p2 p3 p4 p5 p6 x y z -- str -- -- SI

17. Conclusions Local curvature can be used to transform special functions F(x,y) of magnetic fields into depth functions z(x,y). Shallow depth estimates can be corrected to a known minimum depth surface by increasing the structural index. Real data examples show the utility of curvature depth estimates.