Geophysics: The Tools of the Trade (Magnetic)

by Dr. Peter Hood | northernminer.com

Exploration activity in Canada last year fell markedly from the 1989 level, which was about 70% or so down from the record 1987 and 1988 levels.

Taxation changes have greatly affected the ability of the junior mining companies to raise money, resulting in a generally depressed level of grassroots activity.

Companies offering ground geophysical surveys have been hit particularly hard. But, rather than sitting idle, these companies have been busy developing newer techniques as you’ll discover in the following pages…

The opportunity for prospectors and exploration personnel to become familiar with the use of ground geophysical equipment under actual field conditions is rather infrequent. In order to provide such a training opportunity, the Haileybury School of Mines in Haileybury, Ont., organized a 4-day field school in Kirkland Lake in June, 1990. Introductory lectures were presented each morning, and the rest of the day was spent using the various geophysical instruments on a test range about seven kilometers south of Kirkland Lake. More than a dozen instrument and contracting companies supported the field school by demonstrating their equipment and presenting discussions that included the compilation of the day’s survey results.

In this opportunity we will reproduce the presentation about magnetic equipment.

Magnetic

Bartington Instruments of Oxford, England, makes a magnetic susceptibility system with a measurement range of 1-9999 x 106 cgs units. The MS-2 gives digital readings in about one second on a large LCD. The MS-2 has a range of coil sensors that produce an alternating one-oersted magnetic field at the sample. The change in frequency due to the change in inductance of the coil is measured. The MS2B laboratory sensor can accept one-inch cubes and cores and can operate at two frequencies. The MS2D field sensor is used for in situ volume susceptibility measurements in the top 2-metre subsurface.

Gem Systems has introduced an option to the 0.1 nT GSM-19 Overhauser magnetometer-gradiometer series that allows the operator to record magnetic and/or gradient field data every 0.5 seconds as the operator walks along the survey line. The GSM-19 has undergone various software improvements, such as increased cycling time of five or 10 readings per second; decimal station spacing; and 60 Hz or 50 Hz filters. Additionally, the memory capaciy has been standardized to 128 kilobytes.

This latter improvement became necessary to accommodate the volume of data that can be accumulated by the GSM-19 in the “walking” option. A second “hip-chain” option will automatically assign grid co-ordinates to readings measured and stored by the walking option. This provides tighter ground control to the magnetic data collected using the walking mag option. The GSM-19 can also use other magnetometers, such as the Omni Series and GSM-10/18, as diurnal base stations.

Geoscan Research of Bradford, U.K., is manufacturing three microprocessor-controlled fluxgate gradiometers having 0.5-metre separation of the sensors. The FM9 basic version has a 5 nT resolution and a range of +/- 20,000 nT. The FM18 has a 0.5 nT data storage resolution and a 4,000-reading memory. The FM36 has a 0.05 nT data storage resolution and a 16,000 reading memory. Both the FM18 and FM36 keep track of the survey position. The ST1 sample trigger permits rapid sampling of up to eight readings per metre for detailed archeological work.

Geotech has produced two new versions of its proton precession magnetometer. The Model M100S is a 1-nT ground magnetometer that can be converted to a base station. The data for the M100S can be recorded by connecting it to Geotech’s miniature hand-held data acquisition computer.

Geotech has also developed a 0.1-nT proton magnetometer, the M1000 that incorporates auto-tuning and can be used with a wide range of sensors. Versions are available for airborne, portable marine and base station use. The ground memory magnetometer version will interface to portable GPS and VLFEM receivers.

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See a lot more about magnetometer, magnetometers, or ground magnetometer, by visiting our website: http://www.heritagegeophysics.com.

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