Geophysicists process the survey data to form an image and to estimate the physical characteristics of the subsurface. This requires two steps: develop a 3D velocity dataset, or volume, to produce a smooth estimate of the spatially varying velocity with which the seismic waves propagate in the Earth—a process called tomography—then, with the help of this velocity dataset, locate the subsurface layers from which the seismic waves were reflected, a process called migration.
The resulting 3D representation of the Earth is called a structural image, or volume. The reflecting surfaces are interpreted as the interfaces between rock layers, some of which may have been folded, cracked, faulted or eroded over geologic time. It can be sliced vertically to obtain a cross section or horizontally to map the depths of the rock layers beneath the survey area. The operator can use these interpretations to help determine suitable drilling targets. Modern seismic surveys routinely produce detailed 3D images of these reflecting surfaces to depths of 10 km [6 mi].
Additional information about the characteristics of the rocks can be extracted from seismic data. For example, by studying the size, or amplitude, of the reflections and how the amplitude changes with the angle at which the seismic waves hit the reflectors, geophysicists may be able to determine whether the pores within the rocks contain gas, oil or water. This step, known as amplitude versus offset (AVO), often has a higher level of uncertainty than does structural imaging.
Although most seismic work uses active sources designed to create seismic waves, the detection of weak seismic waves generated during hydraulic fracturing is of increasing interest. These faint signals are used to determine the locations of microseismic events, which can indicate the position and extent of the hydraulic fractures.
To reduce the interpretational uncertainty remaining after seismic surveying, geophysicists can choose from several techniques. The most common are electromagnetic (EM) methods, which leverage the fact that some important subterranean formations have strong EM signatures. For example, rocks saturated with hydrocarbons often have much higher electrical resistivity than those containing water and is the basis of wireline resistivity logging. Salt deposits have both a high seismic velocity and a high electrical resistivity. Their high velocity makes seismic imaging beneath them problematic, but their high electrical resistivity makes them easy to detect using EM surveys.
Geophysicists have two distinct methods for acquiring information about the electrical characteristics of rocks at depth. They can use either a high-powered EM source or fluctuations in the Earth's magnetic field induced by the solar wind as a natural EM source. In both cases, the response of the Earth is detected via an array of receivers deployed on, or near, the surface. The first technique is called controlled-source EM (CSEM) and was developed in the 1980s. It is most commonly used in marine settings, where anthropogenic noise, for example, radio signals or power line noise, is less problematic than on land. The second EM technique, magnetotellurics (MT), was introduced in the 1950s. Some modern systems can acquire CSEM as well as MT data when the controlled source is not active (Figure 2).