Pulse EM tests at the Elura massive sulphide body using several methods with the crone PEM equipment

Abstract

The first series of tests with the Crone PEM equipment used the horizontal moving coils method. The coil spacings were varied from 50 meters to 300 meters and the transmit loop size from 6 meters in diameter to 100 meter square. The flat upper surface of the Elura body is detected as a marginal negative anomaly with the 50 meter coil separation survey. With the 200, 250 and 300 meter coil separations, the vertical surfaces of the body dominate the response producing definite positive anomalies. With the 100 and 150 meter coil separations, the opposing negative and positive responses from the flat and vertical surfaces tend to cancel each other, severely reducing the effective response. From the above data the ring current psoitions in the weathered layer are determined. From this the apparent resistivity (assuming a half-space) is calculated to be 43 ohm meters. Since the 100 meter thick weathered layer is known to have a resistivity of approximately 10 ohm meters it is obvious the PEM survey is penetrating well below the weathered surface. This penetration is achieved in sample times as early as 300 msecs. The borehole survey, down the shaft, 45 meters west of the deposit, detects the body from all 4 transmit loops laid out to the north, south, east and west of the shaft. The strongest response is obtained with the loop directly over the body illustrating the directional capabilities of this borehole method. In-Loop Pulse EM tests (receiver position at the centre of transmit loop) were run on four lines. The survey on the line 100 meters south of the deposit does not detect the body. The sulphide body is however detected from the line 50 meters south of the deposit and is strongly detected from two lines cutting across the body. The method has three advantages over the moving horziontal coils method; (1 (improved coupling, (2) additive response from vertical and horizontal surface, (3) the receiver is remote from the ring currents. This makes the In-Loop system a superior method for exploration under Elura type conditions for wide conductive targets. The responses obtained by the various PEM surveys over the Elura orebody are exactly what would be expected and no unusual effects were observed. Variation of the responses with different coil configurations can be explained by the changes in coupling between transmitted field to the conductive body and coupling of the resultant secondary fields to the receive coil.