Effect of differential transmitter/receiver motion on airborne transient EM interpretation

Abstract

NO NO 400 300 600 600 The potential bias or error associated with changes in system geometry are never quantified in such interpretation procedures. The questions to be answered are (1) how much does the change in geometry change the amplitude of the ground response, and (2) how much does the change in geometry affect the ground step function response or transfer function, and (3) how is the spatial shape of the response from discrete conductors altered. The first component of the problem is addressed by analyzing the basic aerodynamics of a towed-bird system. The natural modes of motion of a towed-bird with respect to the aircraft must be determined. This analysis consists of a theoretical and numerical analysis of the aerodynamics of the bird combined with visual observation information. To date, excellent correlation has been found between visual observations and the numerical analysis. The result of this investigation indicates the natural frequencies and magnitudes of motion of the towed-bird system are very system construction dependent. This information then provides the basis by which the ground response changes can be estimated. The Night Hawk geophysical test range is located in Thomas township near Timmins, Ontario. The cover is composed of glacio-fluvial outwash sands, and to a lesser extent, gravel and boulders. The depth of overburden is known to be on the order of 90 m. Detail geology is not available but it is known from limited drilling that EM anomalies in the area are associated with a short. wide graphite zone. Attempts at obtaining a consistent simple interpretation for EM data over this zone have failed. This is probably because the conductive zone is composed of a number of lenticular conductors. Helicopter Input data obtained in this area confirm the difficulty in interpretation using simple plate or sphere models. A preliminary inspection of available fixed wing Input data, however. shows that it may possibly be interpreted using scale model results obtained over thick finite blocks.