GPX Geophysical Exploration Services
Instrument : Electromagnetic Imaging Technology (EMIT) SMARTEM Receiver
 
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SMARTEM DATA EXAMPLE - COURTESY OF EMIT

SMARTem Receiver System
Example - Borehole TEM
Nickel Exploration in Western Australia
Why is ‘Signal Processing’ So Important?

In 1995, as part of the commissioning of the SMARTem I receiver system , a series of borehole TEM surveys was carried out at Leinster, Western Australia.  The surveys were in close proximity to two operating nickel mines with a good deal of associated infrastructure.  Significant interference was observed from the electrical power grid and from an in-mine low-frequency communications system (PED).  The PED (Personal Emergency Device) signal is a system for one-way communication to underground miners.  It has a central frequency of 374 hz and a series of tones between 350 and 400 hz are transmitted.

The survey used as an example here utilised a 2 turn 200m x 200m transmitter loop with a transmitter current of 29 amp.  A base frequency of 2.083 hz was used.  An MCI axial component borehole probe was used as the receiver antenna in this survey.  The raw waveform recorded on the SMARTem receiver at a depth of 95 m in the hole is shown here in a screen taken directly from the SMARTem operating software.  The waveform is digitised continuously through the on and off time for the duration of the reading.  The primary field spikes from the transmitter switch-on and switch-off are several hundred millivolts in amplitude at this depth and a background level of interference can be observed.
 

 
At an expanded scale, the nature of the interference can be observed.  Interference at 50 hz (20 millisecond period) and around 370 hz (PED) can be seen at amplitudes of about 50 millivolts.  The magnitude of these noise waveforms is significant in terms of the expected amplitudes of secondary TEM responses.  We expect to resolve TEM signals several orders of magnitude smaller in size.
 
 

Software was written for the SMARTem receiver that allows an ‘intelligent’ filtering of the spectrum of stacked waveforms during the survey.  Data from contaminated frequencies around 370 hz is removed and replaced by an estimate based on interpolation of ‘clean’ data outside the affected frequencies.  The result has been used  successfully for several years in surveys of this type.
 

This graph (above right) shows a spectrum of the signal on the receiver antenna in the range 0 to 1000 hz.  This spectrum was calculated over several full cycles of the transmitter and thus shows the strength of the primary field.  The dominant features are the 50 hz and PED interference and the primary field.  The aim of the survey is to measure the secondary field (typically several orders of magnitude smaller than the primary field) during the off-time, but the presence of the PED and power line interference during the off-time will clearly present a problem.

The two plots below show borehole TEM profiles from this survey.  With only standard stacking and windowing the data is badly affected by the PED and powerline interference.  With specialized processing, the data is of good quality.  Both profiles are made from the same raw data, the only difference is in the processing applied.  The new processing can be used in real-time or after the survey.
 

 
E-mail gpx@gpx.com.au