James H. Scott, Robert L. Seeley, and Joseph J. Barth
U.S. Geological Survey, Denver, Colorado
ABSTRACT
An improved magnetic susceptibility well-logging system has been developed by redesign of a similar system developed by the Simplec Company in the early 1970’s. The downhole sensor is a solenoid, 30 cm long, wound on a high permeability core and connected in a Maxwell induction bridge. The coil, the bridge, and a 1OOO-HZ crystal-controlled oscillator that energizes the bridge are all located in the probe. The off-null signal from the bridge is transmitted up the logging cable as is a reference signal from the oscillator. A phase detector at the surface is used to determine the amplitude of the quadrature component of the bridge output signal. The quadrature amplitude is a nearly linear representation of downhole magnetic susceptibility in the range of susceptibility of most rocks that contain low amounts of iron minerals (O to 10,000 micro cgs units). The sensing coil is located in a thermally insulated chamber in which the temperature is held constant by a thermostatically-controlled electrical heater. The temperature set-point can be adjusted from
60° to 100°C so that the equipment operator can select a probe temperature that is somewhat higher than the temperature of the borehole. Preliminary tests indicate that the system drift is less than 10 micro cgs units per hour and that the electronic noise is less than one micro cgs unit, considerably lower than the geologic and drill-induced noise in most mineral-exploration boreholes. The high stability and sensitivity of the system will make it possible to detect subtle anomalies (less than 10 micro cgs units) that are commonly associated with mineralization in sedimentary rocks, as well as larger anomalies (1000 micro cgs units or more) that are sometimes associated with alteration zones and mineralization in igneous and metamorphic rocks. Exploration for roll-type uranium deposits is one of several potential applications of this magnetic susceptibility measurement system.