How Important Is The Orientation Of A Horizontal Loop Em System? Examples From A Leachate Plume And A Fault Zone

The coupling between a horizontal loop EM (HLEM) system such as the Geonics EM31@,<br>and a buried object, e.g., a buried pipe, is well-established. The response from a buried pipe or<br>from a vertical lamp-post will depend on the orientation of the transmitter-receiver loop system<br>relative to the object in question. However, diffuse boundaries do not respond like discrete buried<br>objects. In particular, evidence from a survey of a landfill leachate plume and from two lines<br>across the Alpine Fault of the Southern Alps of New Zealand illustrates that the difference is<br>generally negligible. The leachate survey was carried out in an area of heavily-vegetated coastal<br>dunes adjacent to a landfill site. Survey lines criss-crossed the area, using cut trails for access. The<br>line crossings were used for analysis, and the net difference was 0.30 + 0.55 mS/m (less than<br>2.5% on average). The Alpine Fault EM surveys were carried out in advance of and in support of<br>a ground penetrating radar survey. The survey line followed a 6 m wide farm access road that was<br>bordered on both sides by wire fences. The EM survey was completed twice, using two<br>orientations - one parallel to the road (and fences) and the other perpendicular to the road. The<br>results indicate that the response differed by only 0.14 f 0.03 mS/rn, except for the gates and<br>fence corners where different responses would be expected. Therefore, the HLEM system<br>orientation is not significant when surveying features that are not sharp, discrete objects.