The development and applications of a wide band electromagnetic sounding system using a pseudo-noise source

A very wide band (0.03 Hz to 15 kHz) electromagnetic (EM) system has been developed. It was used over the band 1 Hz to 10 kHz to determine the electrical structure of the earth's crust from depths of a few meters to over 40 km. A direct current of from 1 to 5 A was reversed through a long wire bipole transmitter in a pseudo-random binary sequence (PRBS). Depending upon the frequencies selected, a sensitive one-component flux gate magnetometer or an air-core coil was used to monitor temporal changes in the vertical component of the magnetic field at a recording site. The measured signal was crosscorrelated digitally in real time with an exact copy of the transmitted waveform in order to obtain a good signal-to-noise ratio at distances up to 5 times the length of the bipole. The output crosscorrelogram was deconvolved from the system input, the autocorrelogram of the transmitted waveform, using a Wiener least-squares filter to give the impulse response of the earth. This was then transformed into frequency domain to yield directly the phase and amplitude transfer function, which was inverted in a routine manner with a Marquardt algorithm to find layer conductivities and thicknesses.Two field experiments are described. The first demonstrates the viability of the technique to sound a simple layered section of interbedded shale and limestone. It was a relatively shallow sounding to a depth of about 500 m in the sedimentary basin of southern Ontario. The second experiment was designed to search for a conductive zone in the lower crust. A bipole over 20 km long carried a transmitter current of only 5 A, yet field data were obtained over the range 35 to 70 km. The primary magnetic fields never exceeded 10 mgamma and were often considerably smaller. The interpretation indicates a conductive layer of resistivity less than 270 Omega -m at a depth of about 20 km.