Detecting Linear Features Using the Directionality of the HLEM Response

The horizontal-loop electromagnetic (HLEM) response is sensitive to the orientation of the instrument relative to buried objects. Linear features have maximum responses when they are are oriented perpendicular to the long axis of the HLEM instrument, especially for features with relatively sharp boundaries, such as pipes and archæological targets. Thus, HLEM responses are often acquired using two orientations, parallel and perpendicular to the survey line direction, and the responses are then combined, usually by summing or averaging the bi-directional responses. If the ratio or the difference between the two responses are used instead, the contrast between the directionality of any linear anomalies is emphasised. Two examples are presented to illustrate the utility of this approach: (1) an early to mid-19th century archæological site; and (2) a soccer pitch where shallow drainage pipes and an old stream channel are located. In both examples, the quadrature response for the two orientations, separately and averaged, shows indications of linear features, but the response from other features, such as buried stream channels, obscures the linear anomalies. When the ratio or the difference are used, the object locations become clear and correlate well with results from other methods.