High-Sensitivity Magnetic Profiling

Abstract

INTRODUCTION A high sensitivity rubidium 87 magnetometer, designed and built by the author, is used at sea to make magnetic profiles over subsurface structures of interest. The Texas A&M University Research Vessel GYRE was used to launch, tow and recover a nonmagnetic fiberglass skiff that carried the magnetometer. To avoid magnetic field disturbances of the GYRE, the skiff with the magnetometer was towed 600 ft behind the ship. Loran C, and sometimes SATNAV, position data were used to determine the ship location. Two recording depth finders using 3.5 kHz and 12 kHz respectively were used to profile the bottom. Time marks were plotted on the magnetic and sonar data in accordance with WWV time signals received on 10 MHz. (15 MHz and 5 MHz were also available if there happened to be poor radio transmission at 10 MHz). Magnetic data were recorded in digital form on a strip chart recorder, using the last two digits of the six digit resonance frequency of the Rb 87 atoms. THEORY OF MAGNETOMETER The magnetometer is an optically-pumped rubidium 87 magnetometer. The system was patterne1 after a rubidium clock built by Dr. Peter Bender of the U.S. National of Standards. It uses rubidium 87 rather the rubidium 85 because the spin of Rb 87 is 3/2 rather than 5/2 for the more plentiful isotope Rb 85. Thus by obtaining Rb 87Cl from Oak Ridge National laboratory and reacting it with solid Ca in a vacuum, we were able to obtain 500 ml pyrex spheres with some solid Rb 87 in them.(Equation Available In Full Paper) Approximately 1 mm (of mercury) pressure of neon gas is also introduced into the sphere to reduce collision broadening of the spectral line caused by rubidium atom spin-spin transfer. The rubidium atoms have their absorption lines split in the presence of a magnetic field (such as the earth's, that we wish to measure). This natural phenomenon is known as the Zeeman effect. The splitting of the energy levels is given by the Breit-Rabi 2 equation.(Equation Available In Full Paper) In this way we measure the absolute value of the earth is field H in terms of frequency. This is the optimum situation because frequency, and its inverse time, is the parameter we can measure best. By means of a servo oscillator that keeps the radio frequency oscillator locked onto resonance in the earth I s magnetic field of approximately 50,000 gamma, and a frequency multiplier of ten, we then can measure the earth I s magnetic field to 1 part in 3.5 million, because we measure the frequency to ± 1 count. Thus the sensitivity of the magnetometer is 50,000 gamma divided by 3,500,000 or a sensitivity of ±.01 gamma. This is 100 times more sensitive than the usual proton precession magnetometer which operates at a much lower frequency and is not a continuous instrument.