Measuring a property as a function of depth; a depth probe or expander. Especially a series of electrical resistivity readings made with successively greater electrode spacing while maintaining one point in the array fixed, thus giving resistivity-versus-depth information (assuming horizontal layering); electric drilling, probing, VES (vertical electric sounding). Also used with electromagnetic, magnetotelluric, and other types of geophysical surveying. See also geometric sounding and parametric sounding. As opposed to profiling, where the objective is to ascertain lateral rather than vertical variations.