b'Minerals geophysics Minerals geophysicsthe electrodes and the ground itself.using stacked repeat readings, data Resistance in the wires can be reducedassessment to remove unwanted outliers, by using thicker wire, but there may beetc., will come into play. IP measurement handling costs in lay out and retrieval.parameters may also need to be tweaked In practice, most attention is devoted toto minimise unwanted EM coupling reducing the contact resistance of theeffects.electrodesusing larger and/or multiple plates, pre-soaking the electrode pits,Finally, there is the interaction between using saline water and detergent, orthe transmitter and receiver circuits (the even drilling to position the electrodesarray) to consider. This interaction is beneath blanketing cover. In the past,quantified by the mathematical formula Ive used electrodes placed in augeredfor resistivity which is the intrinsic rock holes to bypass badly cracked black soilproperty that we are actually trying to Terry Harveylayers, and gelignite to fracture silcreterecover. In general terms this formula Associate Editor caps to increase permeability and wateris Apparent Resistivity = Receiver for Minerals geophysicspenetration. Ground resistance wouldVoltage divided by Transmitter Current terry.v.harvey@glencore.com.au appear to be out of our control, butand multiplied by a Geometric Factor. sometimes strategic re-positioning ofThe expression makes the necessary the electrodes such as interchanging theassumption of a uniform half-space, transmitter and receiver circuit locationshence the term Apparent Resistivity, Force and finesse can help. and the Geometric Factor takes into account the spatial disposition of all four The maximum available voltage outputelectrodes.Galvanic electrical geophysicalwill be dictated by generator and exploration methods typically employtransmitter hardware, and by safetySo, as well as optimising the electrical two completely separate circuits: one forconsiderations. Equipment limitationsparameters of the transmitter and injecting electric current into the earthmay take the form of maximum allowablereceiver circuits, we will need to (the transmitter circuit) and a secondvoltage, maximum allowable current,strategically position the transmitter for measuring the voltage gradientor maximum allowable power (voltagecircuit electrodes to maximise the electric generated by the passage of this currentmultiplied by current). However, it is nocurrent flowing through the target zone through the earth (the receiver circuit).longer a matter of just building biggerand strategically position the receiver Each circuit consists of appropriatetransmitters - increasing governmentcircuit electrodes to best sample the instrumentation connected by wires toregulation of electrical equipment usedvoltage differences generated by this the ground via specialised electrodes.in mineral exploration is restricting thecurrent flow. There is a range of standard The electrical properties of both circuitsuse of higher power units. electrode arrays to choose from, or can be described by Ohms Law (Voltageyou could design your own, perhaps = Current multiplied by Resistance), butThe application of Ohms Law to theinformed by forward modelling of your the magnitudes of voltages and currentsreceiver circuit is a bit more problematic.environment and target.differ markedly between the two circuits. We dont want to affect what were measuring by the measurement processHaving sorted out the measurement With the transmitter circuit, the aim isitself, so current flow through theparameters, we can then systematically to maximise the electric current passingreceiver must actually be minimised, notmove the array to undertake a series through the target zone, therebymaximised. To achieve this, instrumentof contiguous readings to cover the generating the maximum possibleresistance is made as high as practicable. area of interest. Such a survey would voltage difference for the receiveraim to map the spatial distribution circuit. With the receiver circuit, the aimMeasurement of the resulting reducedof resistivities (and IP effects) within is to optimally measure this voltagesignal strength requires accuracy andthe ground and hence throw light on difference to most effectively sample theefficacy, particularly when the secondthe sub-surface geology. In modern target zone. Simply put, the transmitterorder IP effect, where signal strengthsgeophysical practice, the results from circuit utilises force, the receiver circuitmay be orders of magnitude smaller, issuch a systematic survey are typically demands finesse. also being measured. The receiver circuitprocessed with an inversion routine to needs to be as stable as possible, sofacilitate this process.Ohms Law applied to the transmitterattention will be directed to optimising circuit tells us that to maximise thethe electrodes with pre-watering toClearly there are both theoretical and electric current injected into the ground,minimise SP drift and using porouspractical factors to consider when we will need to minimise the circuitpots to eliminate unwanted electrode- designing and executing a resistivity (or resistance and maximise the appliedrelated IP effects. We also need to beIP-resistivity) survey.voltage. particularly clever in how we go about the measurements themselves. SignalForce or finesse - which factor do I think The transmitter circuit resistance is theprocessing techniques such as coherentis more important? No fence sitting here, sum of the resistances of the wires,signal enhancement and noise reductionId want both!FEBRUARY 2020 PREVIEW 30'