Geophysics is the science of applying the principles of physics to investigations related to the structure and properties of the earth. It has been said that geophysics essentially is the measurement of contrasts in the physical properties of materials beneath the surface of the earth and an attempt to deduce the nature and distribution of the materials responsible for these observations. Geophysical exploration has been utilized in numerous fields to study a wide range of targets within the earth from discovering the deep structure of the earth at thousands of meters to near surface structures and properties at depths of a few tens of meters. Some of the latest geophysical technologies are targeted at studying engineered material, such as pavements and bridge decks at only a few centimeters depth. Geophysical surveys are conducted on the ground surface, within drill holes, and from the water and air. A partial list of applications includes: mineral and petroleum exploration and production, environmental contaminant mapping, mapping subsurface ground conditions for civil engineering projects, solution cavity detection, hydrological mapping, mapping near surface utilities, detection and mapping of unexploded ordnance, and archeological and forensic investigations.

The application of geophysical imaging and NDT methods has been increasing in site Characterization and geotechnical investigations throughout state and federal highway departments. Transportation personnel use geophysical methods to assist in highway design, construction, repair, and maintenance phases. Detailed knowledge of unforeseen, highly variable subsurface ground conditions reduces project risk and costs associated with "change of conditions" claims, and improve construction and safety.

In this web manual, the term NDT is used to refer to condition evaluation of engineered structures, including concrete, asphalt, masonry, timber, and steel structure. Condition evaluation includes integrity assessment for defects and corrosions, and the determination of unknown geometry, such as foundation depths or extent of foundations.

A broad range of practical methods, including almost all traditional geophysical methods, have been adapted to provide NDT solutions more specific to a variety of engineering problems. For example, conventional geophysical methods have been used to evaluate the physical properties of soil and rock, such as seismic methods, which are used to calculate the depth to bedrock or locate underground voids. In NDT applications, the same seismic methods have been adapted for condition evaluation of concrete structure with great success, producing a technique known as the Ultraseismic test method. In this method, the concrete structure itself is used instead of the earth for the transmission of the acoustic energy with reflection events coming from either the bottom of a structure, as in a bridge foundation, or a defect zone. Similarly, the GPR geophysical method, originally developed for high resolution imaging of the subsurface, is now used routinely in condition evaluation of pavements, concrete slabs, and walls.

In general, the subsurface Characterization provided by geophysical exploration methods is valuable for highway evaluation for the following reasons:

1. They allow nondestructive investigation below the surface of the ground, pavement, bridge deck, or other structure.
2. They provide information between and below standard geotechnical borings most common to transportation projects.
3. They allow collection of data over large areas in very much shorter times than most destructive methods.
4. They cost less per data point than most invasive methods.
5. They can offer accurate and timely information for design quality and performance.

Although geophysical methods provide the above advantages, it is important to remember that the information obtained in geophysical surveys is often subject to more than one reasonable interpretation. Also, depending on specific site-conditions such as geology, target dimensions, cultural interface, and the engineering problem to be investigated, a combination of methods or techniques may be utilized in a given investigation. In other words there is no one, unique interpretation to a set of geophysical data. Also more than one method may be used to solve a particular engineering problem. Therefore, it is recommended that before conducting any geophysical investigation, as much knowledge as possible be obtained about the target and site. For subsurface Characterization for example, this can be accomplished by obtaining geotechnical, geological, hydrological, or other investigative reports. Boring logs, which are normally included in these reports, can be extremely valuable if the borings were performed in the vicinity of the site. U.S. Government literature, including Federal Highway Administration (FHWA) and U.S. Geological Survey (USGS) publications, and U.S. Department of Agriculture (USDA) soil surveys, can also provide useful information. In essence, a priori knowledge of geological conditions and all previous surveys completed in the vicinity of the site should greatly assist engineers in designing a geophysical survey and selecting a suitable method(s). Additionally, it will help assure the most reasonable interpretation of the results once these data are collected.