The Use Of A Giant Magneto-Resistance (Gmr) Based Magnetometer For Differentiation Of Subsurface Electrical And Non-Electrical Materials

Abstract

Ground penetrating radar (GPR) has been used with moderate success to locate electrical conduits buried within reinforced concrete slabs. However, when conduits are located very near walls or directly beneath rebar of similar size, GPR as a methodology of investigation alone has been inadequate. X-ray techniques are also not useful as they emit dangerous gamma radiation, and both X-ray and EM induction techniques penetrate only a few inches and cannot differentiate between rebar and conduits cased in steel or aluminum. Therefore, the ability of a magnetometer with a wide frequency response bandwidth is of interest for these situations. This experiment uses Smoke Creek Instrument's IMAG01 magnetometer to explore the frequency responses of several common construction materials such as pipes, conduits, and spikes. The material samples are placed beneath 0.15 m of a natural, decomposed granitic soil with a consistency of coarse sand similar to that used in concrete. The magnetometer is used to measure each situation and the results presented herein. Measurements are first made with no artificial external field applied, and then with a controlled source magnetic field. The IMAG01 is a single-axis magnetometer based on the giant magneto-resistance (GMR) effect and detects magnetic field strength of both static and electromagnetic fields. This version of the instrument has a frequency response of DC – 100 kHz with a sensitivity of approximately 0.02 nT. As such, it is expected to respond to the potential magnetic field of the ferrous material and the magnetic component of the 60 Hz electromagnetic field of electrically-active wire without the controlled source transmitter. With the transmitter, the instrument is expected to respond to the induced magnetic field of the metallic materials as well as the potential and 60 Hz fields of the previous situation. Of particular interest is the ability to differentiate between metallic, non-electrical materials and electrically-active conduits when placed in close proximity. The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide information that limited bandwidth instruments can not. A magnetic field receiver with a small physical size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic measurements over relatively small surface areas. This makes feasible 2D and 3D electromagnetic imaging, although for purposes of this experiment, only 1D measurements were made.