The operation of an airborne time-domain electromagnetic system (TDEM) involves the measurement of decaying secondary electromagnetic fields induced in the ground by a series of short current pulses generated from an airborne transmitter. Variations in the decay characteristics of the secondary field are analyzed and interpreted to provide information about the subsurface geology. The response of such a system utilizing a vertical-axis transmitter dipole and a horizontal-axis receiver coil has been documented by various authors including Palacky and West (1973, Geophysics, v.38, p.1145- 1158).
The principle of sampling the induced secondary field in the absence of the primary field (during the "off-time") gives rise to an excellent signal-to-noise ratio and an increased depth of penetration compared to conventional continuous wave (frequency domain) electromagnetic systems. Such a system is also relatively free of noise due to air turbulence.
Free-air model studies using Plate and Layered Earth programs have shown that the "depth of investigation" depends upon the geometry of the target (McPHAR’s P-THEM Time Domain Helicopter-borne EM System routinely detects anomalies at 300 to 400 metres depths). In addition to substantial penetration, time-domain systems respond to a wide range of conductors. With measurements taken during off-time, significant effects are seen, thus responding to the majority of geological features deemed relevant in most exploration projects and many mapping applications. The method also offers very good discrimination of conductor geometry. This ability to distinguish between flat-lying and vertical conductors combined with excellent depth penetration results in good differentiation of bedrock conductors from surficial conductors.
P-THEM Time Domain Helicopter-borne EM System
P-THEM is a versatile state-of-the-art time-domain helicopter-borne EM button-on system. It features a powerful transmitter powered by a motor generator and an upgraded state-of-the-art PC-based receiver. P-THEM is adaptable, well suited for shallow soil mapping, ground water and/or ore-body exploration. The system features a loop-transmitter assembly, a digital receiver and a 3-axis receiver suspended on the tow-cable midway between the transmitter assembly and the helicopter.
The standard base frequency for the P-THEM system is 25 Hz, for use in areas of the world where 60Hz power is common the base frequency is 30 Hz.
P-THEM is a versatile state-of-the-art time-domain helicopter-borne “button-on” electromagnetic system. It features a powerful transmitter, a state-of-the-art receiver and an advanced digital signal processing software package. The P-THEM system comprises an 8.5 metres diameter (57 m2) 4-turns transmitter loop assembly and a 3-axis receiver suspended on the tow-cable midway between the transmitter assembly and the helicopter. Power for the P-THEM system is provided by a motor generator mounted on the transmitter assembly.
P-THEM systems transmit a half sine wave 4ms pulse with 400,000 NIA peak moment. It is easily transportable and rapidly deployed; typically in half-a-day.
|P-THEM 57 m2 Transmitter Assembly with a motor generator mounted on the transmitter structure.||Looking down from the helicopter in-flight - the 3-axis (X, Y, Z) receiver (yellow pod) is suspended on the tow-cable midway between the transmitter assembly and the helicopter.|