Abstract: A method for detecting a subsurface anomaly at a near-surface depth, comprises positioning an electromagnetic sensor configured to measure a component of a planetary electromagnetic field such that the electromagnetic sensor is suspended just above a ground-air barrier and does not contact a ground surface; selecting an electromagnetic frequency by calculating a function of properties of the ground that include relative permittivity, relative permeability, and resistivity; moving the electromagnetic sensor over the surface of the ground; repeatedly measuring intensity of the component of the planetary electromagnetic field at the frequency to obtain a set of measurements; and comparing at least a first measurement in the set of measurements to at least a second measurement in the set of measurements to identify a change in the intensity of the component of the planetary electromagnetic field that is indicative of a presence of a subsurface anomaly.

Abstract: Methods and apparatus are provided for passively detecting the presence of near-surface human-scale underground anomalies using earth field measurements. A sensor is used to measure at least one electric or magnetic component of the Earth's electromagnetic field at a frequency of 5 kHz or greater in proximity to the Earth's surface for a given area. The measured intensities are used to identify variations indicative of the presence of a near-surface human-scale underground anomaly. Measuring the intensity of at least one component of the electromagnetic field at a plurality of frequencies of 5 kHz or greater can be used to determine the depth and characteristic of a near-surface human-scale underground anomaly.