Near-surface geophysics
[3][4] In studies of the solid Earth, the main feature that distinguishes geophysics from geology is that it involves remote sensing.
The signal-to-noise ratio may be improved by repeated measurements of the same quantity followed by some sort of averaging such as stacking or signal processing.
P-waves travel faster than S-waves, and both have trajectories that bend as the wave speeds change with depth.
On a regional scale, profiles can be combined to get sequence stratigraphy, making it possible to date sedimentary layers and identify eustatic sea level rise.
Magnetometers are used to search for anomalies produced by targets with a lot of magnetically hard material such as ferrites.
[9] High precision gravity measurements can be used to detect near surface density anomalies, such as those associated with sinkholes and old mine workings,[10] with repeat monitoring allowing near-surface changes over these to be quantified.
[11] Ground-penetrating radar is one of the most popularly used near-surface geophysics in forensic archaeology, forensic geophysics, geotechnical investigation, treasure hunting, and hydrogeology, with typical penetration depths down to 10 m (33 ft) below ground level, depending upon local soil and rock conditions, although this depends upon the central frequency transmitter/receiver antennae utilised.
[1] Bulk ground conductivity typically uses transmitter/receiver pairs to obtain primary/secondary EM signals from the surrounding environment (note potential difficulty in urban areas with above-ground EM sources of interference), with collection areas depending upon the antennae spacing and equipment used.
They do experience significant changes with soil moisture content, a difficulty in most site investigations with heterogeneous ground and differing vegetation distributions.
Geophysical methods can be used to find or map an archaeological site remotely, avoiding unnecessary digging.
In surveys of a potential archaeological site, features cut into the ground (such as ditches, pits and postholes) may be detected, even after filled in, by electrical resistivity and magnetic methods.
[14] Methods that included side-scan sonar, magnetic surveys and seismic profiles uncovered a story of bad site location and a failure to protect buildings against geohazards.
[15] In addition, they helped to locate structures that may be the lost Great Lighthouse and palace of Cleopatra, although these claims are contested.
Civil investigations are more often trying to determine the location, amount and (more tricky) the timing of illegally dumped waste, which include physical (e.g. fly-tipping) and liquid contaminants (e.g. hydrocarbons).
[1] In rural areas conventional SI methods may be employed but in urban areas or in difficult sites, targeted geophysical techniques can rapidly characterise a site for follow-up, intensive surface or near-surface investigative methods.
