A acoustic-penetrometer probe and method for determining the presence of materials in a region proximate the penetrometer probe through measurement of reflected acoustic waves, with the probe having a driving head on one end to permit forcing the driving head into the soil, with the probe including an acoustic generator mounted in the probe for generating a below-ground acoustic wave that normally travels away from the acoustic generator, and an acoustic detector located in the probe to measure acoustic waves reflected from objects or materials located proximate the probe to thereby provide a user with information on the presence of materials proximate a radial region located around the probe.