Technical Papers - Airborne Electromagnetics

Robust estimation of the band-limited inductive-limit response from impulse-response TEM measurements taken during the transmitter switch off and the transmitter off-time: Theory and an example from Voisey's Bay, Labrador, Canada.

Richard S. Smith, Fugro Airborne Surveys
2060 Walkley Road, Ottawa, Ontario, Canada, K1G 3P5 

Stephen J. Balch, Inco Technical Services Limited
Highway 17 West, Copper Cliff, Ontario, Canada. P0M 1N0

Abstract

Modern transient electromagnetic systems are able to take measurements in the transmitter on-time. Integrating measurements taken during the transmitter switch-off and those collected in the transmitter off-time yield an estimate of the primary field plus the secondary inductive-limit response. If the transmitter loop position is known and the position and orientation of the receiver dipoles are known, it is possible to calculate the primary field. When the theoretical primary field is subtracted from the measured inductive-limit-plus-primary response, the inductive-limit response can be isolated. An anomalous inductive-limit response is a diagnostic feature of highly conductive ore bodies.

On- and off-time PROTEM data collected in a drill hole proximal to the Reid Brook Zone (one of the Voisey's Bay deposits in Labrador, Canada) shows a strong inductive-limit anomaly corresponding to an off-hole conductor. A drill hole targeted to test this conductor intersected 20.4 m of mineralisation, including 8.25 m of massive sulfide.

GEOPHYSICS, VOL. 65, NO. 2 (MARCH-APRIL 2000); P. 476-481, 8 FIGS., 1 TABLE.  Manuscript received by the Editor October 9, 1998; revised manuscript received August 2, 1999. ©2000 Society of Exploration Geophysicists. All rights reserved. 

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