Tianyou Chen

Geobandwidth: comparing time domain electromagnetic waveforms with a wire loop model

We compare time domain systems of different waveform shape, power and receiver sampling times using a wire loop conductor model to define a comprehensive ‘geobandwidth’ that shows the strength of the response over a range of time constants, analogous to a range of conductance. Frequency domain EM responses can also be calculated as a function of time constant for a wire loop model, giving a consistent comparison method for all time domain waveforms and frequency domain. Arbitrary waveforms can be modelled as a sum of simple short ramps, and the geobandwidth determined numerically. Peak time constant (time constant of peak response) or equivalent frequency can be determined analytically or numerically. The frequency content of a time-domain EM system can be characterised by the peak time constant or the equivalent frequency. The results of these calculations are used to compare response amplitude across a wide range of geological target conductance. Systems can be compared on the basis of signal or signal/noise ratio. The effect of time domain EM waveform, power and receiver sampling times are effectively compared for a wide range of time constants using a wire loop conductor model. Peak time constant and equivalent frequency can be determined analytically or numerically. Arbitrary waveforms can be modelled as a sum of simple short ramps.

MULTIPULSE – high resolution and high power in one TDEM system

An airborne time domain electromagnetic (TEM) system with high resolution and great depth of exploration is desired for geological mapping as well as for mineral exploration. The MULTIPULSE technology enables an airborne TEM system to transmit a high power pulse (a half-sine, for instance) and one or multiple low power pulse(s) (trapezoid or square) within a half-cycle. The high power pulse ensures good depth of exploration and the low power pulse allows a fast transmitter current turn off and earlier off-time measurement thus providing higher frequency signals, which allows higher near-surface resolution and better sensitivity to weak conductors. The power spectrum of the MULTIPULSE waveform comprising a half-sine and a trapezoid pulse clearly shows increased power in the higher frequency range (> ~2.3 kHz) compared to that of a single half-sine waveform. The addition of the low power trapezoid pulse extends the range of the sensitivity 10-fold towards the weak conductors, expanding the geological conductivity range of a system and increasing the scope of its applications. The MULTIPULSE technology can be applied to standard single-pulse airborne TEM systems on both helicopter and fixed-wing. We field tested the HELITEM MULTIPULSE system over a wire-loop in Iroquois Falls, demonstrating the different sensitivity of the high and low power pulses to the overburden and the wire-loop. We also tested both HELITEM and GEOTEM MULTIPULSE systems over a layered oil sand geologic setting in Fort McMurray, Alberta, Canada. The results show comparable shallow geologic resolution of the MULTIPULSE to that of the RESOLVE system while maintaining superior depth of exploration, confirming the increased geological conductivity range of a system employing MULTIPULSE compared to the standard single-pulse systems. The MULTIPULSE technology airborne TEM system transmits a high power pulse and low power pulse(s) (trapezoid or square) within a half-cycle. The high power pulse ensures good depth of exploration and the low power pulse allows higher near-surface resolution and better sensitivity to weak conductors as confirmed by field results.

HELITEM detects the Lalor VMS deposit

CGG deployed HELITEM, a helicopter-borne time domain electromagnetic (TDEM) system over the Lalor Deposit and other proximal deep conductors of Hudbay Minerals in northern Canada. The Lalor Deposit is ~570 m deep and is a difficult target for airborne EM systems. The system was configured with a dipole moment of up to 1.9 MAm2 at base frequencies of 15 Hz and 30 Hz. The results of this survey have been used to characterise the TDEM response from deep volcanogenic massive sulphide (VMS) deposits within the region. HELITEM is the only airborne controlled source TDEM system to have detected the Lalor Deposit. CGG deployed HELITEM, a helicopter-borne time domain electromagnetic (TDEM) system over the ~570 m deep Lalor Deposit in Canada. The results have been used to characterise the TDEM response from deep volcanogenic massive sulphide deposits within the region. HELITEM is the only airborne controlled source TDEM system to have detected the Lalor Deposit.

Airborne Geophysics, Remote Sensing, UAV (Drone)-based Surveys and Mining Geophysics

Knowledge of occurrence and extent of quick clay is vital regional for hazard zonation and in detail for infrastructure projects. Quick clay poses a serious geohazard in Scandinavia and Canada (amongst others) as it practically liquefies at failure and thus leads to serious, retrogressive slides. To this end, geotechnical drillings and samples are analyzed, to indicate sensitive clay in an area. As quick clay has a higher resistivity than marine clays, geophysical methods looking at resistivity distribution provide a valuable tool in addition to geotechnical assessment. So far, this has mostly focused on electrical resistivity tomography (ERT) for detecting these subtle changes. Supporting a recent road development project close to Oslo AEM was suggested in order to link drill sites and fill the data gaps between them. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings. Especially where the sediment layer is thick, variations in electrical resistivity within this layer can be resolved. These subtle changes can be linked to quick-clay extend by comparison with borehole data and ground-based geophysical methods (ERT and IP). We discuss results from the combination of these methods and outline the possibilities and limitations of quick-clay mapping using AEM.