MAGNETOMETERS

Airborne, ground and borehole magnetometer surveys are not what they used to be. A revolution of sorts came in the early 1980s. “Smart” microprocessor-based magnetometers made it possible to store, reduce and present mag data automatically in the field, freeing the operator from manually writing values in a field notebook and correcting and plotting in the office or camp.

At least six companies market magnetometers in Canada. A brief description of their wares follow. Various companies, including Aerodat, Dighem and Geosoft of Toronto offer software for plotting, manipulating, modelling and interpreting mag survey results. These companies are not included here, but may be included in a future Specs article.

Barringer Research

Barringer Research of Rexdale, Ont., is marketing two new models of proton precession-type magnetometers. This company was involved in the initial development of proton precession magnetometry and has been making the instruments for more than 20 years.

The two models, called the M-234 and M-244, offer better performance, flexibility and price, compared with previous models, says sales manager David Martinak.

What makes the two unique is the menu-driven software. An operator can easily enter or change the operating characteristics in a straightforward manner. Characteristics include: cycle rate, time and date, starting value for automatic tuning, line number, manual/external selection, recorder format, outputs, and liquid crystal display (LCD) viewing angle.

The M-234 is a field unit. It includes a high-speed, thermal printer and a large LCD. The unit is dc-powered, rugged, waterproof and lightweight.

The M-244 is a 2-component, standard 19-inch rac-mounted unit. One component contains the magnetometer electronics, a large LCD and control keyboard. The second component contains a high-speed, thermal printer and a power supply.

A 512-kilobyte memory is optional.

Other standard features include high sensitivity (0.1 gamma at one sample every two seconds), wide selection of cycling rates, four scale selections of two overlapping scales, two recorder formats (standard and full analog), on-line single or dual trace selection, and RS232 analog and audio outputs.

Barringer also markets a small, lightweight proton magnetometer called the GM-122. It has a sensitivity of one gamma with a 3- or 6-second sampling rate.

EG&G Geometrics

California-based EG&G Geometrics is big in the high-priced, U.S. military magnetometer market. The company recently signed a $4.5- million contract to supply cesium magnetometers to the military and it has orders for a helium magnetometer that has yet to be built. It is probably safe to say that the price of a helium unit would be beyond the range of any prospector. (Besides, a resolution of one gamma, which is attainable by a simple proton or cesium magnetometer, is often all that prospectors require.) The company outsold all magnetometer manufacturers in Canada in 1988 by selling 170 units, according to sales manager Ross Johnson.

Exploranium

Exploranium of Bolton, Ont. is no longer marketing, in Canada, magnetometers manufactured by Geometrics of California. Instead, Geometrics is marketing Exploranium’s instrumentation in the U.S.

Gem Systems

Gem Systems of Richmond Hill, Ont. has been marketing proton, Overhauser-effect magnetometers for several years. Overhauser-effect technology offers several advantages, says president Ivan Hrvoic.

First and foremost is exceptionally high resolution. The GSM-19 model has noise standard deviation of gradient readings of less than 0.02 nt per metre at one reading per second (nt is the abbreviation for nanoTeisa, a unit of flux density).

Second, the Overhauser effect allows the operator to polarize (using an rf source) and measure at the same time. That is to say, the GSM-19 gradiometer can run up to five times per second, at a fraction of an nt of noise and 2-W power consumption.

Gem Systems’ GSM-11 has a steady, nondecaying proton precession signal. This is especially useful in airborne applications. The GSM-11 can measure five times per second at 0.1 nt noise envelope. The sensor weighs five kilograms.

Sander

Sander Geophysics has stopped using proton magnetometers in its aircraft, because the units are too heavy for the stingers. Cesium magnetometers are used instead.

Scintrex

Scintrex of Concord, Ont., which recently merged with the earth science division of EDA Instruments, markets 50 different magnetometers, the largest selection in the country. But only five of those are of interest to the average prospector.

The MP-2, developed in the 1970s, was an industry standard for proton procession magnetometers for many years. Its one-gamma sensitivity, quick reading time and rugged design make it ideal for field surveys.

The Scintrex MP-3 and eda omni are examples of microproprocessor- based magnetometers.

The latest innovation in field magnetometers is the integrated geophysical system (IGS). With this system, one central control console can process data from several sensors, including a magnetic sensor, enabling the explorationist to conduct several surveys simultaneously. The Scintrex IGS-2/MP-4 and the EDA OMNI PLUS are examples of magnetometers in integrated systems.

The company’s patented cesium magnetometer leads in the airborne magnetometer field worldwide.

Urtec Instruments

Urtec Instruments of Markham, Ont. (Bill Thuma 475-1133) markets five instruments for use in magnetic field surveying. They include:

* the FM-18 vertical fluxgate gradiometer;

* the JH-13 handheld vertical fluxgate magnetometer;

* the JH-8 magnetic susceptibility meter;

* the TH-15 magnetic susceptibility and conductivity meter; and

* and the MAGLOG borehole geophysical system.

The F-18 gradient magnetometer emphasizes near-surface anomalies and basically ignores the influences of deeper bodies, geophysicist William Thuma explains. The fluxgate is less apt to be affected by local noise sources than their total field cousins (the proton precession magnetometers). This makes them ideal for placer gold exploration where the gold is associated with magnetite and ilmanite.

Susceptibility instruments, such as the company’s JH-8 and TH-15 units, are very sensitive to small variations in magnetite. The JH-8 is a small, handheld, unit while the TH-15 is much larger (3.5 kg). The JH-8, with its analog output to a meter, is ideal for scanning core or outcrops whereas the digital TH-15 is intended for more accurate readings of core, crushed and hand samples. Analog readouts are more convenient for detecting small variations in magnetite, Thuma says, since they are about 10 times more sensitive and react instantly. (Digital models, on the other hand, take an incremental time to display an averaged reading.)

For borehole geophysics, the company’s MAGLOG system measures many variables simultaneously: three component magnetic field scalars to 0.25 nt, total magnetic field to 0.5 nt, magnetic susceptibility and electrical resistivity, as well as temperature and differential self potential to a depth of 1,000 metres. There is a growing demand to get as much information from each drillhole as possible, Thuma says.