Exploration Depth of Multifrequency Helicopter EM Systems
Abstract
Due to the high resolution of hilicopter electromagnetic (HEM) systems, they are being widely used for shallow earth resistivity mapping problems. The traditional investigation of the exploration depth of a HEM system is based on the model of a singlefrequency coil array over a layered earth. In this paper we extend the study to the multifrequency HEM systems. We first determine for each frequency channel of a HEM system the maximal depth of a target, beyond which it cannot be identified from the EM signal. This is mathemically realized by assuming that the abnormal signal from the target is three times larger than the noise level of the HEM channel. Since each frequency channel of an HEM system has a different noise level and for different frequency channel the EM field has different penetration depth, we choose the biggest value of these depths as the depth of exploration. Different models are implemented in the study of this paper, including a layered earth model, a dipping plate or a dyke, a 3D ore body, etc. We use as example the Fugro DIGHEM system with three horizontal coplanar (HCP) coils (380 or 900, 7200, 56kHz) and two vertical coaxial (VCX) coils (900, 5500Hz). The following conclusions are obtained: 1. Except for a steeply dipping sheet, the HCP coil array has a larger depth of exploration than the VCX coil array; 2. The depth of exploration may be obtained from different frequency channels for different target geometries and different conductivity contrasts between the target and host rocks. This means that for a specific target geometry and conductivity contrast, we need to search such a frequency channel that offers the maximal value for depth of exploration; 3. Among the factors that influence the depth of exploration, the noise level of the HEM system is the key. The other factors include the geometry of the target and the conductivity contrast between the target and the host rocks, and the relative location between the HEM system and the target; 4. We have assumed in this paper three times the noise level as the threshold in defining the depth of exploration of a HEM system. If the HEM data have a high quality, e.g. with good system calibration and data processing, this factor might be reduced, so that the depth of exploration can be greatly improved.
 Publication:

AGU Spring Meeting Abstracts
 Pub Date:
 May 2004
 Bibcode:
 2004AGUSMNS33A..01Y
 Keywords:

 0910 Data processing