Time Domain Electromagnetic Methods
Time-domain electromagnetic (TDEM) surveys are conducted to map changes in resistivity or its inverse, conductivity, with depth. This method is, in effect, an EM equivalent of the resistivity sounding method. TDEM soundings can be made at
stations along a profile to yield two-dimensional models of the resistivity structure of the subsurface.
TDEM data are generally modeled using computer inversion techniques, and
output is a model of resistivity as a function of depth. These techniques can be
used to explore depths ranging from about 30 feet to over 5,000 feet,
depending on methodology used.
- Common applications of TDEM methods include:
- Mapping geologic structure
- Mapping large fracture zones
- Imaging deep conductive contaminant plumes such as oil field brines and acid-mine drainage
- Characterizing salt-water intrusion
- Determining depth to groundwater and groundwater resources
- Mapping subsurface stratigraphy
- Characterizing mineral resources
Electrical Resistivity Method
The electrical resistivity method involves measuring the apparent resistivity of soils and rock as a function of depth or position. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. The most common electrical methods used in hydrogeologic and environmental investigations are vertical electrical soundings (resistivity soundings) and resistivity profiling.
During a resistivity survey, current is injected into the earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. Common arrays include the dipole-dipole array, pole-pole array, Schlumberger array, and the Wenner array. The apparent resistivity is the bulk average resistivity of all soils and rock influencing the current. It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array used and electrode spacing.
In a resistivity sounding, the distance between the current electrodes and the potential electrodes is systematically increased, thereby yielding information on subsurface resistivity from successively greater depths. The variation of resistivity with depth is modeled using forward and inverse modeling computer software.
In resistivity profiling, the electrode spacing is fixed and measurements are taken at successive intervals along a profile. Data are generally presented as profiles or contour maps and interpreted qualitatively.
When information on both the horizontal and vertical extent of a subsurface feature is desired, it is common to combine the sounding and profiling techniques. The recent advent of automated data acquisition systems has made it possible to very efficiently gather 2-D resistivity data. With these systems it is possible to lay out a large portion of the line, connect the electrodes to the data acquisition system using multi-core cable or intelligent nodes, and have the resistivity system automatically gather all of the measurements using preprogrammed arrays. The resistivity data is then downloaded to a computer and modeled using 2-D forward and inverse modeling software.
