DE2240041

A space monitoring gradient system including at least a plurality of transducers arranged to produce at least two pairs of transducers with the spacing of the transducer of each pair being less than one wavelength of the received signals produced by a noise source at the highest receivable frequency. The axes of the two pairs of transducers are perpendicular to one another with the axis of one pair being parallel to the ground and the axis of the other pair being perpendicular to the ground. A separate automatic gain control (AGC) amplifier is connected in series with the output of each of the transducers, and first and second difference circuits are provided for forming respective gradient voltage signals from the outputs from the respective AGC amplifiers of each pair of transducers. A first comparator compares the gradient voltage signal at the output of the first difference circuit, which is associated with a pair of transducers parallel to the ground, with a first settable comparison voltage and produces an "L" signal at its output whenever the output of the first difference circuit is equal to or less than the first comparison voltage. A second comparator compares the gradient voltage signal at the output of the second difference circuit with a second settable difference voltage and produces an "L" signal output whenever the output of the second difference circuit is greater than the second comparison voltage. The outputs of the first and second comparators are fed to an AND gate which produces an "L" signal at its output when an "L" signal is present at each of its inputs.