A group of profiles for which the measuring system dimensions are successively increased, designed to obtain information on the layering pattern in an area. Also called sounding, expander, and depth profile. 1. A refraction depth probe determines approximate depths and velocities of refraction markers; also called refraction test. 2. An electrical-surveying technique of exploring vertically down into the earth by employing an orderly horizontal expansion of the interelectrode interval or by changing the frequencies. Data from a depth probe are interpreted to give the depth to a resistivity contrast or anomalous IP material if horizontal layering exists. Also called vertical electrical sounding (VES). See two-dimensional plot.