A surveying instrument that is a type of proton gradiometer, working on the same principle as the magnetometer. When two detector bottles are used, one near ground level and one about 2 m above, small magnetic anomalies underground affect the lower, nearer, bottle more strongly than the upper. The signals from the two get out of step, and their sound signal is broken into a series of 'bleeps'. It is unaffected by large scale disturbance, an advantage over the magnetometer, and is much simpler, cheaper, and more portable than either magnetometer or gradiometer.