A geological survey is to investigate the interested land and its subsurface soils and rocks for environmental concerns, hazard reductions and mining prospecting, etc. The goal of geological surveys usually is to build some base maps or models, including topography, stratigraphy, rock distribution and geological structure delineation, etc.
Depending on the purpose of the geological surveys, the methods involved include visual inspection, land surveying, outcrop studies, soil and rock sampling through borehole drilling, geochemical analysis, remote sensing, and geophysical surveys, etc. For example, in order to address the landslide problem in a historically problematic area, a base geological map showing the topography and subsurface stratigraphy need to be built through the geological survey. The geological map thus can be the base for further studies such as the landslide mechanism and prediction.
The geophysical survey methods involved in geological surveys include gravity, magnetic, direct current resistivity and induced polarization, seismic refraction and reflection, borehole logging, crosshole tomography, airborne geophysics and remote sensing, etc. Sub bottom profilers or sonars are also used in marine environments. Combined with traditional geological surveys such as outcrop studies and sample analysis which provide accurate information but in limited scales, geophysical surveys can delineate subsurface materials in large areas more efficiently. For example, for groundwater exploration, geophysical surveys can delineate underground water reservoirs in a large area more efficiently. Based on the map provided through the geophysical surveys, more informative decisions can be made on choosing water well locations.