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The Use Of A Giant Magneto-Resistance (Gmr) Based Magnetometer For Differentiation Of Subsurface Electrical And Non-Electrical Materials
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems, Mar 2001, cp-192-00019
Abstract
Ground penetrating radar (GPR) has been used with moderate success to locate electrical<br>conduits buried within reinforced concrete slabs. However, when conduits are located very near walls or<br>directly beneath rebar of similar size, GPR as a methodology of investigation alone has been inadequate.<br>X-ray techniques are also not useful as they emit dangerous gamma radiation, and both X-ray and EM<br>induction techniques penetrate only a few inches and cannot differentiate between rebar and conduits<br>cased in steel or aluminum. Therefore, the ability of a magnetometer with a wide frequency response<br>bandwidth is of interest for these situations.<br>This experiment uses Smoke Creek Instrument's IMAG01 magnetometer to explore the<br>frequency responses of several common construction materials such as pipes, conduits, and spikes. The<br>material samples are placed beneath 0.15 m of a natural, decomposed granitic soil with a consistency of<br>coarse sand similar to that used in concrete. The magnetometer is used to measure each situation and the<br>results presented herein. Measurements are first made with no artificial external field applied, and then<br>with a controlled source magnetic field.<br>The IMAG01 is a single-axis magnetometer based on the giant magneto-resistance (GMR) effect<br>and detects magnetic field strength of both static and electromagnetic fields. This version of the<br>instrument has a frequency response of DC – 100 kHz with a sensitivity of approximately 0.02 nT. As<br>such, it is expected to respond to the potential magnetic field of the ferrous material and the magnetic<br>component of the 60 Hz electromagnetic field of electrically-active wire without the controlled source<br>transmitter. With the transmitter, the instrument is expected to respond to the induced magnetic field of<br>the metallic materials as well as the potential and 60 Hz fields of the previous situation. Of particular<br>interest is the ability to differentiate between metallic, non-electrical materials and electrically-active<br>conduits when placed in close proximity.<br>The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide<br>information that limited bandwidth instruments can not. A magnetic field receiver with a small physical<br>size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic<br>measurements over relatively small surface areas. This makes feasible 2D and 3D electromagnetic<br>imaging, although for purposes of this experiment, only 1D measurements were made.