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

Abstract

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 Ω-m at a depth of about 20 km.