Depth of investigation in electromagnetic sounding methods

Abstract

The time or frequency at which the electromagnetic (EM) response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section; it is relatively independent of the type of source or receiver and their separation. The ability to make measurements at this time or frequency, however, depends on the sensitivity and accuracy of the instrumentation, the signal strength, and the ambient noise level. These factors affect different EM sounding systems in surprisingly different ways. For the magnetotelluric (MT) method, it is possible to detect a buried half‐space under about 1.5 skin depths of overburden. The maximum depth of investigation is virtually unbounded because of high signal strengths at low frequencies. Transient electromagnetic (TEM) soundings, on the other hand, have a limited depth of penetration, but are less affected by static shift errors. For TEM, a buried inhomogeneity can be detected under about one diffusion depth of overburden. For conventional near‐zone sounding in which induced voltage is measured (impulse response), the depth of investigation is proportional to the [Formula: see text] power of the source moment and ground resistivity. By contrast, if the receiver is a magnetometer (step response system), the depth of investigation is proportional to the [Formula: see text] power of source moment and is no longer a function of resistivity. Magnetic‐field measurements may, therefore, be superior for exploration in conductive areas such as sedimentary basins. Far‐zone, or long‐offset, TEM soundings are traditionally used for deep exploration. The depth of investigation for a voltage receiver is proportional to the [Formula: see text] power of source moment and resistivity and is inversely proportional to the source‐receiver separation. Magnetic‐field measurements are difficult to make at long offsets because instrumental accuracy limits the measurement of the very slow decay of the magnetic field. Frequency‐domain controlled‐source systems are ideally suited for sounding at the very shallow depths needed for engineering, archaeological, and groundwater applications because of the relative ease of extending the measurements to arbitrarily high frequencies, and also because geometric soundings can be made at low induction numbers.