Electromagnetics for Mapping Shallow Geologic Structure

Overview

Electromagnetic geophysical techniques are appropriate for a broad range of shallow environmental projects: locating lateral discontinuities such as landfill boundaries and changes in soil composition, or in the search for buried objects such metallic utilities, underground storage tanks, or buried concrete structures.

There are two main classifications of electromagnetic geophysical techniques, based on the induced field and the parameters being recorded: frequency-domain EM induction and time-domain EM induction. In both classes, a transmitter loop is used to generate a strong, controlled, alternating current that propagates into the subsurface and induces eddy currents in the earth materials.

For lateral variation in conductivity, frequency-domain EM may be used. For vertical variation in conductivity, geologic structure or lithology, time-domain EM may be preferable. The terrain conductivity (analogous to apparent resistivity) is measured. This is an average conductivity for nearby geologic materials.

As the EM energy encounters different subsurface materials, a secondary EM field is generated by the eddy currents. This secondary field is recorded by a receiver loop. A data logger records the components of the secondary field that are in-phase with the transmitted energy, and the portion that is 90-degrees out-of-phase (the quadrature component). Under normal conditions, the in-phase component is affected by the presence of buried metallic objects, while the quadrature component is directly related to the conductivity of the surrounding terrain.

Geology

Earth resistivity (inverse conductivity) is effected by (in approximate order of importance) the amount of pore water (including fractures), total dissolved solids in the pore water, resistivity of rock matrix, amount of clay minerals present, shape and connectivity of pores, and temperature.

Logistics

Requires low interference; must stay away from: grounded fences or utilities, power lines or electric fences, tanks, used car lots, curbs with rebar.

Crew size is usually one or two persons-the specific instrument determines the number. However, another person can often help with gridding or other tasks which make the crew more efficient. A grassy field is ideal but thick brush and steep slopes simply require more effort.

Deliverables

Plan maps of station locations, profiles for two dimensional investigations, profiles of soundings for one-dimensional soundings, inversions of the data are done but the ratio of observations to parameters is usually low.