Project Title: Magnetic Induction Effects on Magnetic Observations
Mendenhall Fellow: Paul A Bedrosian, (303) 236-4834, firstname.lastname@example.org
Duty Station: Denver, CO
Start Date: March 28, 2005
Education: Ph.D., 2002, Physics, University of Washington, Seattle
Research Advisors: Jeff Phillips, (303) 236-1206, email@example.com; Vic Labson, (303) 236-1312, firstname.lastname@example.org; Tom Hildebrand, (650) 329-5303, email@example.com; Jeff Love, (303) 273-8540, firstname.lastname@example.org; Louise Pellerin, (650) 329-5016, email@example.com; Tiku Ravat (Southern Illinois University), (618) 453-7352, firstname.lastname@example.org
Project Summary: Ground-based and airborne magnetic data permit a range of geologic and hydrologic investigations; compiling the wealth of existing data further allows us to examine lithospheric structure and evolution. An example shown in Figure 1 is the recently compiled Magnetic Anomaly Map of North America ( http://pubs.usgs.gov/sm/mag_map/index.html), in which accreted terrains, subduction zones, and orogenic belts can be readily identified. Such data may further be used to constrain the composition and thermal structure of the lower crust and upper mantle. In standard practice, magnetic observations are corrected for secondary magnetic fields which are induced in the conductive earth by time-varying magnetospheric and ionospheric fields. These secondary fields are typically removed from the measured field using a magnetic base-station. Commonly overlooked, however, are spatial variations in the inducing fields and the induction effects of a heterogeneous, conductive earth.
Figure 1. Magnetic Anomaly Map of North America constructed from over 1000 airborne and ship borne surveys spanning the past 50 years. Long-wavelength data will be better constrained by the upcoming High altitude Magnetic Survey.
The first effect is negligible for small (~100km or less) surveys but becomes problematic for large-scale surveys such as the proposed High Altitude Magnetic Survey (http://geopubs.wr.usgs.gov/open-file/of02-366/) which will acquire aeromagnetic data over the entire United States. Ignoring spatial variations in the inducing fields may result in errors of up to ~80nT in the measured fields, several times larger than the stated error limit. To correct such data, an array of base stations is required, the spacing and placement of which depend on the coherency, polarization, and frequency content of the inducing fields. The earth's heterogeneous conductivity structure also factors into base-station placement, as regions of high conductivity contrast (e.g., coastlines, mountain ranges, volcanic arcs) will require more closely-spaced base stations. The aim of this project is to determine an optimum base-station layout for North American magnetic surveys and, more generally, to identify and develop the means with which to accurately remove the induction effect from magnetic survey data. Research is based on a three-fold approach:
- Analyzing induced field structure from existing magnetometer array data.
A number of magnetospheric and ionospheric processes lead to secondary magnetic fields with varying degrees of polarization, coherency, and frequency content. These range from pulsations with periods of a few seconds to diurnal (daily) variations and their higher harmonics. Arrays of vector magnetometer data provide a means to examine source effects over the relevant time and length scales. An example is shown in Figure 2, where magnetic field variations show a striking similarity at two locations separated by over 300km. The largest discrepancies are observed in the vertical (Z) component of the magnetic field and are associated with a strong conductivity contrast that lies between the two locations.
Figure 2. Synchronous magnetic field variations at two sites separated by 320 km. Discrepancies in the vertical field component reflect the inductive response to heterogeneous earth conductivity.
- Forward-modeling the secondary magnetic fields from earth conductivity models.
Three-dimensional conductivity models can be used to estimate secondary field contributions. Existing magnetotelluric and geomagnetic depth sounding data, together with crustal and sediment thickness models, are being compiled to produce a conductivity model for North America. In areas where measured conductivity data is sparse or absent, conductivities will be derived from available heat flow and seismic data. Once assembled, this model can be driven by either base-station data or geomagnetic indices derived from magnetic observatory data to estimate the secondary induced fields.
- Verification of secondary magnetic fields in a region of high conductivity contrast.
The prominent conductivity contrasts associated with coastlines give rise to strong induced fields observed hundreds of kilometers inland. In spring 2006, we will examine the scale and distribution of such fields across an array of vector magnetometers stretching several hundred kilometers inland from the west coast. This field survey will serve as a ground-truth with which to compare various approaches to secondary field estimation.
URL: http:// geology.usgs.gov /postdoc/profiles/bedrosian/index.html
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Last modified: 16:08:25 Thu 13 Dec 2012