The term "sonar" is typically used when a sound (compression) wave is propagated in water and the term "seismic" is associated with a family of techniques that infer information on buried objects or geological strata changes by propating compression or shear waves into the ground. When sound waves are emitted and encounter an object, a fluid boundary or a significant change in geologic strata, some of the wave energy will be reflected back in the direction of the emitter. As the transmitter/receiver is moved over the ground or water surface, reflections will first appear from a longer, diagonal path from an object. When the transmitter/receiver is directly over the object, the reflected signal will appear at the shortest time delay. This minimum time delay (coupled with the typically strongest reflected signal) allows the horizontal position of the object to be inferred. The time delay of the reflected signal arrival, together with a knowledge of the approximate wave transmission speed through the medium, allows an estimate of the depth of the object or boundary to be made. Other seismic methods are not described here and further information can be found in the technical literature under geophysical exploration methods.