Seismic methods measure velocity and density variations in the subsurface by utilizing an active source of mechanical energy that imparts a wave on the surface, which can be thought of as a point-source-earthquake, and records the history of the wave after it has traveled in the subsurface and measured at detector locations on the surface. Seismic methods have been successfully used to locate (and image) favorable geologic conditions for the accumulation of oil and gas in sedimentary rocks.

Seismology is the study of earthquakes and the Earth using seismic waves. Waves generated by earthquakes travel through the Earth and are recorded around the world.  The information from the waves are analyzed to discover the earthquake source and how the waves moved through the earth.  Seismology can then tell us about the entire Earth's structure from the core to the crust.

DC Resistivity is a method that measures variations in the electrical conductivity of the ground. Highly conductive objects in the subsurface (e.g., an ore body), or highly resistive objects (e.g., a void) can cause an anomaly.   It is operationally equivalent to the traditional seismic technique (requires ground contact for deep penetration), and utilizes the same partial differential equation for modeling as the gravity method (LaPlace’s equation, which can be derived from the Wave Equation by setting the wave frequency to zero). In fact, theoretical models developed for gravity can be adapted for use with DC Resistivity, and vice versa. The major shortcoming of this method is the requirement for ground coupling with electrodes.

Low Frequency Electromagnetic (EM), or electromagnetic induction (EMI), methods have been successfully used on the surface and in the air by the mining industry for detecting conductive ore bodies. In addition, surface and airborne EM methods are being developed by the SERDP as one of the primary means for detecting UXO’s. Theoretically, the method is modeled by using the Diffusion Equation, which is derived from the Wave Equation by setting the electrical permittivity to zero (which is a valid approximation at very low frequencies). Like gravity, magnetic, and DC resistivity methods, EMI is a detection (non-imaging) method.  In some circumstances, EMI methods have been successful at detecting voids in the subsurface.

High frequency EM (GPR) methods are undisputedly an excellent subsurface imaging method when it is used in ground-based mode. However, other than ice-sounding radar, GPR has not been successfully adapted to an airborne platform. And, under all but the most ideal conditions (e.g., homogeneous crystalline rock), GPR is a very shallow- penetrating device (a few meters, at best), with extremely low penetration where surface soils contain a high clay content.

Heat Flow is a method that studies the movement and placement of heat within the earth.  Temperature gradients and surface temperature maps are used to study the subsurface.

Borehole Geophysics is the science of taking measurements in wells or test holes.  Probes lowered into a borehole collect data which is then displayed as a log.  Borehole geophysics are used for many things including rock lithology, fractures, permeability, porosity, and water quality.  Common logs are caliper, gamma, single-point resistance, spontaneous potential, normal resistivity, electromagnetic induction, fluid resistivity, temperature, flowmeter, television, and acoustic televiewer.

Geophysical Societies
SEG: http://seg.org/  AGU: http://www.agu.org/  GSA: http://www.geosociety.org/
SPWLA: http://www.spwla.org/