A Three-Component Borehole Magnetometer Probe For Mineral Investigations And Geologic Research Available to Purchase

A small-diameter three-component fluxgate magnetometer probe with gyroscopic and inclinometer orientation has been developed by OWL Technical Associates to meet U. S. Geological Survey design and performance specifications for measurement of the direction and intensity of the Earth's magnetic field in vertical and inclined boreholes. The orthogonal fluxgate magnetometer elements have a measurement resolution of 10 nanoteslas (nT) and a range of +80,000 nT. The gyroscope has an effective resolution of one degree, and the orthogonal inclinometers, 0.1 degree. A temperature sensor is mounted near the magnetometer and inclinometer sensors to provide a means of correcting them for temperature effects. The analog outputs of all downhole sensors are sampled and digitized in the probe over a brief time interval (2 Ms), and are multiplexed for transmission to the surface via a standard seven-conductor well-logging cable. High-speed downhole sampling makes it feasible to obtain reliable reconnaissance logs at logging speeds up to 25 ft/min, although more accurate logs can be obtained by making static readings. A module in the logging truck receives the digitized information transmitted up the logging cable and formats it for input to a desk-top computer via a serial interface. The computer monitors the depth of the logging probe as it is lowered (or raised) in the borehole. When the probe reaches the bottom (or top) of each depth readout increment (usually one foot), the computer obtains the latest available probe information from the uphole module, combines it with depth and time information available in the computer, and then records the complete data set on magnetic tape. A data reduction program is used to play back the data stored on the acquisition tape, apply corrections for temperature variation and gyroscope drift, and to compute and plot the following information probe alignment and orientation in the drill hole, direction and intensity of the magnetic field, and orthogonal components of magnetic field intensity (vertical, north-south, and east-west, referenced to magnetic north at the ground surface). The magnetometer probe has been field tested in several holes drilled through volcanic rocks in Nevada. Results indicate that reversals of polarization can be detected, and some rock units in this area appear to be characterized by unique magnetic signatures. Test stand experiments indicate that the probe is also capable of detecting strongly magnetized pods of ore that are narrowly missed by a borehole.