Overview of TEM
The transient electromagnetic or TEM technique is a commonly used surface based geophysical method which provides resistivity information about the subsurface.
A transmitter (usually a loop of wire on the ground) is driven by a time varying current. The change in current, and resulting EM field, establishes an image current within the earth equal in magnitude, but opposite in sign to that of the transmitter. This image current then interacts with conductive materials, setting up secondary magnetic fields that are measured at the receiver site. The depth of exploration attained can vary from 10’s of meters to over 1000 meters, depending upon transmitter loop size, available power from the transmitter, and ambient noise levels.
TEM systems have been used in exploration for geothermal sources, mapping structure and lithology, searching for sources of groundwater and groundwater contamination, and for engineering applications. TEM systems have also been used to identify buried metallic objects such as buried utilities, abandoned wells, UST, and UXOs.
Field logistic vary depending upon the target, but all configurations are non- intrusive and low-impact. For large (deep sounding) scale TEM surveys a typical field crew consists of 3 or 4 people, with one pick-up truck at the transmitter site and one at the receiver site. At the receiver site, the equipment can be carried by backpack, and no off- road driving is necessary. Shallow surveys utilizing a fast-turn off system requires 1-3 people and all the equipment is backpack portable.
Depending upon the target, the set-up of the system varies greatly from large loops on the ground, to cart mounted systems, to boat towed arrays. For loops that are not self contained in a cart mount system or towed array, the transmitter consists of a thin, insulated surface wire laid out by walking along the ground, (vehicle access along the length of the transmitter is not necessary).
The equipment consists of a transmitter and receiver which can be contained in a single box (with an external power source for the transmitter) or used with separate transmitter and receiver enclosures.
The TEM method is based on transmitting a time domain, square-wave signal into a large ungrounded loop. At some point in time, the loop current is interrupted as fast as possible thereby causing a rapid change in the magnetic field generated by the transmitter. The rapidly changing magnetic field induces eddy-currents to flow in nearby conductors producing small secondary magnetic fields that are generally measured by observing induced voltages in receiver loops.
Induced currents in poor conductors (moderate resistivity) decay quickly, currents in good conductors (very low resistivity) decay slowly, and very poor conductors (highly resistive silicified dikes, for example), will not sustain any measurable induced currents.
Smooth-model inversion is a robust method for converting TEM measurements to profiles of resistivity versus depth. The result of the TEM smooth-model inversion is a set of estimated resistivities which vary smoothly with depth. Lateral variation is determined by inverting successive stations along a survey line. Results for a complete line are presented in cross section form by contouring model resistivities.