The magnetometer measures the vertical magnetic intensity of subsurface materials. Higher values can generally be attributed to rocks of relatively high magnetism. Lower values can indicate materials of low magnetism or discontinuous zones, such as fractures, faults, channels, or contacts, where the magnetic materials have been leached out.

Care must be taken in the location of the traverses, because buried pipes, power lines, and metal objects can adversely affect the readings and cause abnormal magnetic peaks. The magnetometer helps locate areas of fracturing or channeling in the underlying formations, which may form favorable geologic conditions for ground-water transmission or storage.

The gamma-ray spectrometer is used to measure the natural radiation from near-surface earth materials, primarily uranium, potassium, and thorium. This technique is useful in mineral exploration for radioactive materials as well as in geologic mapping and fault location, where contrasting, weakly radioactive rocks are buried beneath the soil cover.

The very low frequency electromagnetic (VLF/EM) survey measures secondary fields generated by conducting bodies in the subsurface, when subjected to a primary electromagnetic signal initiated from a fixed military transmitter. Readings are taken at closely spaced intervals and the vertical in-phase and vertical out-of-phase, or quadrature, components of the secondary fields are measured. Anomalous conditions, generally indicated by a "cross-over" may be related to subsurface contacts or faulting.

ELECTROTELLURIC SOUNDINGS

Electrotelluric soundings measure the relative conductivity throughout a specific depth interval and allow an accurate evaluation of ground-water production potential to depths of up to 2,500 feet. This is particularly useful in desert environments, where deep wells are often necessary. In the past, traditional resistivity soundings were only able to penetrate to depths of 600 feet before the vertical accuracy became suspect. Now an analysis of potential deep aquifers can be made before attempting expensive exploratory drilling.

The electrotelluric survey measures the electrical fields generated by telluric currents flowing in the subsurface formations of differing lithology. These currents are induced by the ionosphere as pulses, with duration depending on the depth of penetration. A surface receiver transforms the electrotelluric radiation into an audible signal.

VLF/ELECTROMAGNETIC SOUNDINGS

Very low frequency electromagnetic (VLF/EM) resistivity soundings measure the ratio and phase angle between the horizontal magnetic field and the horizontal electric field components of the plane wave initiated from a fixed military transmitter. Apparent resistivity of one or two earth layers can be determined, with the effective depth of penetration dependent on both the electrical resistivity and the frequency of the radio wave received. The equipment is both lightweight and quick to set up, making these soundings useful in areas of limited accessibility and as a check on suspected resistivity conditions.

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