A Multi Frequency EM Image System

摘要

A digital, multi-frequency EM 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 1.25KHz to 10KHz. A micro controller 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 DSP 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 single-frequency sensors. For loop antennas with a spacing of 0.5m, the system has phase resolution about 1° and amplitude resolution about 0.5 percent.