The seismic method measures the response of seismic (sound) waves that are input into the earth and then refract along or reflect off subsurface soil and rock boundaries. The seismic source is usually a sledgehammer blow to a metal plate on the ground, a larger weight drop, or an explosion. The earth response is measured by sensors called geophones, which measure ground motion. Two basic methods of seismic exploration are used refraction and reflection.
The seismic refraction method measures head waves that are refracted along geologic formations below the earth's surface. Refractions generally occur along the top of the water table and the uppermost bedrock formation. A plot of the arrival time of the first seismic wave to each geophone gives information about the depth and location of these geologic horizons. This information is plotted in a cross section that shows the depth to the water table and to the first bedrock layer.
The reflection method measures the time necessary for a sound impulse to travel from the source, bounce off a geologic boundary, and return to the surface at a geophone. The reflection from a geologic horizon is similar to an echo off a cliff face.
Electromagnetic Induction Method
The electromagnetic induction (EM) method measures the response of an induced alternating current. A current is induced into the ground by a transmitting coil. A receiving coil is placed a short distance away to measure the induced earth current. The size of the induced current depends on the geologic material (lithology) beneath the transmitter and receiver. By mapping changes in the induced current, it is possible to map out changes in lithology, in order to determine the potential presence of an aquifer.
The EM method is also very sensitive to metal. Thus, the location of buried metal objects, such as drums or pipes, can be mapped with this technique.
Electrical Resistivity Method
The resistivity imaging method uses standard arrays developed for electrical resistivity sounding and profiling techniques and modifies them to create two dimensional resistivity profiles. A line of electrodes is placed at equal intervals along the desired profile. Four electrodes are used at one time. Two inject current into the ground and two read the electrical potential between them. The resistivity meter and switch box automatically read many combinations of current and potential electrodes from short offsets to long offsets starting at one side of the electrode spread and moving toward the opposite end. The short offsets look at the shallow earth, and the longer offsets look more deeply.
At the Minnesota DNR, we typically use either the dipole-dipole or the Wenner-Schlumberger array. The dipole-dipole array gives good horizontal resolution, but may have a poor signal to noise ratio (S/N) because the potential electrodes are outside of the current electrodes. The Wenner-Schlumberger array is more directed for vertical resolution, but it also gives reasonable horizontal resolution. This method has greater S/N than the dipole-dipole method because the potential electrodes are placed between the two current electrodes.
The field data contain apparent resistivity values and geometry information. These data are then inverted to produce a two dimensional (X - Z) plot of resistivity values. This resistivity inversion section is then used to interpret subsurface lithology.
The magnetic method measures the earth's ambient magnetic field. The field strength becomes stronger near large magnetic objects. Therefore, buried magnetic objects can be located by mapping magnetic field values. One example is mapping the location of buried drums on a hazardous waste site.
Because some earth materials are more magnetic than others, this method can also map geologic changes.