Use of geophysical methods to get information for civil engineering. The aim is usually to describe not only the geometry of the subsurface but also its nature (for example, its elastic characteristics as determined by measurements of seismic velocities and densities). Shallow seismic reflection and refraction, gravity, magnetic, electric, and sampling methods are commonly employed to find the depth of bedrock and the strength of sediments for foundation purposes, to determine the rippability (q.v.) of rock, to measure the degree of fracturing, to detect underground cavities, to detect near-surface gas pockets, to locate buried-pipeline hazards below the sea floor, buried barrels of polutants, the integrity of landfills. In water-covered areas high-powered fathometers, sparkers, gas guns, and other seismic reflection methods employing high frequencies (up to 5 kHz) are used to obtain reflections from shallow interfaces so that bedrock and the nature of fill material can be diagnosed. Such methods are also used to locate large pipelines on, or buried in, the sea bottom by the prominent diffractions that they generate. Usually restricted to shallower than 1000 ft penetration.