Georadar
Georadar measurements along a river-dam © Klaus Löcker

One of the most recent methods in geophysical near-surface prospecting is known as “georadar”. By using an antenna emitting electromagnetic signals in the range of 1 – 2400 MHz, reflections can be recorded from nearby buried objects or inhomogeneities. The method mimics the principle of reflection seismics, except for the studied signal, which is not a compressional wave but an electromagnetic wave.

The main advantages of the method are

  1. it's small scale nature, which permits measurements in confined spaces, like underground mines or tunnels,
  2. and the detail of recordings (“radargrams”) at which structures can be resolved (down to cm’s).

The method is employed in Archaelogy, detection of cavities, bomb and mine-detection, waste deposits and polluted areas, cables and pipes detection and in Engineering Geology. The penetration depth depends on the chosen radar frequency (e.g. for objects buried at very shallow depth, GHz-antennas are used). Usually, penetration depths of 2 – 6 m can be resolved. In very dry rock penetration depths of more than 20 m were be accomplished.

See https://en.wikipedia.org/wiki/Ground-penetrating_radar for more information.