This invention relates to a non-invasive method for easily locating an underground septic tank, distribution box, field line, conduit, or the like. In the preferred embodiment, prescribed acoustic signals are injected into an underground cavity via an accessable port and remotely detected by a time synchronized detector in contact with the ground which cross correlates the detected signal with a duplicate of the prescribed acoustic signal in order to reject background noise, and thus locate the underground cavity. A frequency sweep is conducted to exploit cavity resonance modes and thus further enhance the signal to noise ratio.