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Laboratory NMR apparatus used for medical diagnosis consists of (1) a magnet which provides a strong, steady magnetic field that is as uniform as possible and (2) a coil that produces an oscillating magnetic field perpendicular to the static field direction. A relatively small, compact sample, such as a person, is placed inside the coil. For borehole NMR, the sample - namely the earth - is not inside the measurement apparatus, but outside of it. It requires a leap of imagination to see how NMR measurements can be made "inside-out" on a sample external to the apparatus.

This design problem has been solved in a number of distinctly different ways. Our borehole NMR apparatus consists of two samarium cobalt magnets, each about one foot (30 centimeters) long, that project a magnetic field into the formation. An antenna located in a cylindrical well on the face of the apparatus generates the oscillating magnetic field, which is also projected into the rock formation. The two magnetic fields are substantially perpendicular within the volume of rock being examined, which extends about an inch (2.5 cm) into the formation. The antenna is used as both transmitter and receiver. A durable polymer wear plate covering the antenna is the only nonmetallic surface on the exterior of the apparatus. Metal cannot be used here because it would prevent the transmission and reception of the magnetic field.