A Multi–Frequency Electromagnetic Image System

Abstract

A digital, multi-frequency electromagnetic image system is developed based on the induction principle for subsurface imaging near a sewage pipe. This system, which features a broadband operation, can transmit sinusoidal in a frequency range of 1.25 KHz to 10 KHz. A microprocessor is used for all controls and computations for both the transmitter and receiver circuits. Synchronous sampling is used for digitization in order to compute the phase as well as the amplitude of the receiving signal. The system operates in the frequency domain and both the phase and amplitude information can be computed through a digital signal-processing algorithm. Test results indicate that the multi-frequency EM sensor is superior in both the missing manhole detection and general subsurface conductivity imaging to the conventional singlefrequency sensors. For loop antennas with a spacing of 0.5 m, the system has a phase resolution of about 1 • and an amplitude resolution of about 0.5%.