Sounding (also known as Vertical Electrical Sounding or VES) is a survey type which has as its objective to obtain a distribution of resistivity with depth. Sounding is often contrasted with profiling, which has as its objective to obtain a distribution of the lateral distribution of resistivities. Of course, modern hardware typically can collects data which has both sounding and profiling information, but there is often still interest in VES surveys, specifically for water investigation studies.
In a VES study a symmetric array configuration is used - either a wenner or a schlumberger array. The midpoint of the array remains constant, but the size of the array increases. As the size increases, the depth to which the measurement is sensitive increases. The exact sensitivity depends on the distribution of electrical conductivities. Each measurement results in a value which can be plotted on a graph. Historically this graph was interpreted in terms of distributions of electrical properties by manual curve fitting, but nowadays numerous numerical codes exist which can do this.