A New Locating Towed System for Mapping Smal Buried or Exposed Sealines

Abstract

ABSTRACT Locating buried small diameter sealines (say < 10") with conventional geophysical methods has always been a problem for the off-shore oil industry. Standard towed survey tools, based on reflection seismics or proton magnetometers, suffer from their lack of resolution and sensitivity. The towed magnetometer, developed by the CEA, thanks to its very high sensitivity (0.01 nT) and measuring rate, allows small diameter sealines to be accurately detected. Moreover, when simultaneous injection in the sealine of an electric low frequency signal is performed, even cables or flexible pipes with little magnetization are detected. I. MAGNETOMETRY AND ITS APPLICATIONS Measurement of the Earth's magnetic field using airborne techniques is a standard military approach for submarine locating, and for mapping natural resources in geophysics. The measurement is based on the principle of proton or electron magnetic resonance which transforms the intensity of a magnetic field H to a proportional frequency F. The relation is given by Larmor's formula:(Available In Full Paper) The frequency detector then measures the total field. The measurement can be made by a moving sensor which is either towed behind an aircraft by a cable which reduces the effect of the magnetism of the carrier, a relatively simple solution, or embarked on the aircraft, which then calls for appropriate compensation techniques. A fixed magnetic observation station equipped with a total field magnetometer having a relative precision better than a millionth will record time-dependent fluctuations of the magnetic field. Diurnal variations, magnetic storms and pulses are related to the sun?s activity and amount to a thousandth of the Earth's almost constant internal field. For high sensitivity measurements, a differential technique is used. This eliminates the time dependent fluctuations which are identical at two geographically close points. The use of such a differential technique is standard in aeromagnetic mapping, but in marine applications, it is exceptional. In submarine archeology for example, the use of three magnetometers towed behind a boat can be used to obtain the differences in total field experienced by the sensors (figure 1). These differential measurements are insensitive to temporal fluctuations, and the interpretation of the anomalies detected can result in the discovery of ancient shipwrecks essentially made of wood. II. SPECIFICATIONS OF A HIGH SENSITIVITY MAGNETOMETER Magnetometers available commercially at present, are based on either proton or electron magnetic resonance and are known as proton magnetometers or optically pumped magnetometers. The magnetometers presented here have been developed by the French Atomic Energy Commission (C. E.A.) and are based on simultaneous proton and electron magnetic resonance (Abragam - Overhauser effect) (1).