Subsurface Imaging

Induced polarization (IP) surveys are used to detect disseminated mineralization and to simultaneously measure electrical resistivity. Originally developed for porphyry exploration, gold deposits also respond well to this technique since many are hosted within bodies of disseminated sulphides. Dipole-dipole surveys provide the best signal to noise ratio for detecting discrete targets but in cases where the primary voltage at the receiving electrodes is too low, pole-dipole surveys are often employed. Gradient surveys provide much more rapid coverage but this is at the expense of target resolution. The IP decay curve can be analysed to derive spectral IP parameters tau and C which are influenced by chargeable grain size and the type of chargeable source.

Electromagnetic (EM) Surveys

Electromagnetic induction (EM) measures theelectrical conductivity of the subsurface. A primaryalternating electric current of known frequency and magnitude ispassed through a sending coil creating a primary magnetic fieldin the space surrounding the coil, including underground. The eddy currents generated in the ground masker as signalin the receiver coils The secondaryfield is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondarycurrents is proportional to the terrain conductivity. Thedepth of penetration is governed by the coil separation andorientation.

Unlikeconventional resistivity techniques, no ground contact isrequired. This eliminates direct electrical couplingproblems and allows much more rapid data acquisition. Forshallow profiling (up to 6m), a Geonics, Inc. EM-31 TerrainConductivity meter is used. One person can collect as manyas 10,000 data points per day with this instrument. AnEM-34 is used for depths of investigation between 10 and 60m. This instrument requires two people to operate, and up to 500data points per day can be collected under good conditions. These tools are extremely sensitive and accurate, capable ofdetecting variations in conductivity of as little as 3%. Data are automatically stored in an electronic data logger forlater transfer to a computer.

TheEM-61 instrument is a high resolution, time-domain device fordetecting buried conductive objects. It consists of apowerful transmitter that generates a pulsed primary magneticfield when its coils are energized, which induces eddy currentsin nearby conductive objects. The decay of the eddycurrents, following the input pulse, is measured by the coils,which in turn serve as receiver coils. The decay rate ismeasured for two coils, mounted concentrically, one above theother. By making the measurements at a relatively longtime interval (measured in milliseconds) after termination ofthe primary pulse, the response is nearly independent of theelectrical conductivity of the ground. Thus, theinstrument is a super-sensitive metal detector. Due to itsunique coil arrangement, the response curve is a single welldefined positive peak directly over a buried conductive object. This facilitates quick and accurate location of targets. Conductive objects, to a depth of approximately 3m can bedetected.

TheEM 31 uses an alternating electromagnetic field, which fills thespace, below and above ground, surrounding the transmittingcoil. When the electromagnetic field coupleswith a conductor,for example a steel pipe under the ground, AC eddy currents areinduced to flow in the pipe. This generates a secondary magneticfield, which is sensed by the co–planar (12’ offset) receivercoil. Due to phase lag the computer on board can discriminatebetween the primary and secondary fields and outputs themeasurements of the secondary field (thus, a conductive zone is
sensed by the induced secondary magnetic field).