Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Magnetic Methods- closing comments

Northwest Geophysical Associates (2000)

Cutler Maine “Goliath” antenna system consists of foot high antennas covering a 2 square mile peninsula.

McNeill & Latimer Cutler, Maine – signal intensity map

Top loaded antenna system in Hawaii – foot towers. The guys are energized to into a ground screen

Valley Span antenna consists of 6 cables strung across a canyon in the Cascades north of Seattle.

McNeill & Latimer Antenna array for the Seattle, Jim Creek, site

The VLF signal can be quite variable through time at a given location. Drift correction of survey observations are necessary.

Significant variation occurs over shorter time frames

Preston County Coal Mine Refuse Area

VLF Profiles across the refuse area

Terrain Conductivity VLF response

Andrews (2003)

In practice, it is common to use only one of the diagnostic positions, for example, the half-max (X 1/2 ) distance for an anomaly to quickly estimate depth if the object were a sphere or buried vertical cylinder…. Burger limits his discussion to half-maximum relationships. Breiner, 1973

Recall that the maximum sample spacing might be chosen to equal the expected half-max distances. Or X 1/2 =Z/2 For the Horizontal Cylinder or Sphere

The sample rate you use will depend on the minimum depth of the objects you wish to find. Which anomaly is potentially associated with the deepest source? Which has the highest magnetic susceptibility?

+ - N S

Sign Conventions Vectors that point down are positive. Vectors that point south are negative.

The analytic signal Symmetrical anomalies Original data

A paper thin 50 foot long cluster of drums with area equal to two drums? Drums in the bedrock?