In a ground gravity survey, the Earth's gravity field and the location and elevation of each station are precisely measured. These measurements at different locations are used to map the variations in rock density over the selected survey area and, after appropriate processing and interpretation, the subsurface geological structures and probable locations of mineral bodies of economic interest may be determined.

Gravity is widely used with magnetics in geological mapping and exploration, and has prime application in base metal, gold and diamond exploration. From a geotechnical viewpoint, underground tunnels and cavities can be detected because of the associated mass deficiency.

Fugro Ground Geophysics uses both Scintrex (CG-3 and CG-3M) and LaCoste & Romberg (Model G) gravity meters for surveys, depending on the specific requirements.

Positioning for gravity surveys is exclusively done with geodetic grade dual frequency GPS receivers, with the operational mode changing to suit the parameters of the survey. Surveys at close station spacings are generally performed using Real Time Kinematic (RTK) systems, with accurate results available instantly, while surveys over larger spacings i.e. from 250m up to 4km are post-processed due to the longer baselines and operational delays involved with maintaining the telemetry link over longer distances.

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Time-domain electromagnetic (TDEM) methods are based on the principle of using electromagnetic induction to generate measurable responses from sub-surface features. When a steady current in a cable loop is terminated a time varying magnetic field is generated. As a result of this magnetic field, eddy currents are induced in underground conductive materials. The decay of the eddy currents in these materials is directly related to their conductive properties, and may be measured by a suitable receiver coil on the surface.

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Induced Polarization (IP) is an electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical chargeability (dielectric constant) in the Earth at low frequencies. Induced Polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks.

This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages.

It is most often used in exploration for disseminated sulphides, and may also be used in groundwater exploration. Chargeability can also be estimated by recording the phase difference between transmitted current and measured voltages, in the method known as Complex Resistivity, which is often used in areas where coupling effects distort the results from a conventional IP survey.

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In a Magnetics survey, the Earth's magnetic field and the magnetic responses due to magnetic minerals are measured. Naturally magnetic minerals such as magnetite occur in rocks and in varying percentages. Other minerals have a high magnetic susceptibility resulting in induced fields. It is both the remnant and induced magnetic responses that are used to map an exploration area and calculate the susceptibility of rock types.

Because of its speed, the ease of the physical measurement and its economy, magnetics is the most widely used and popular geophysical exploration method. From a detailed study of an anomaly, it is possible to calculate magnetic susceptibility, length, width, depth, dip, and the remnant magnetism of the causative body.

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MagnetoMetric Resistivity (MMR) and Magnetic Induced Polarisation (MIP) are advances on normal Induced Polarization methods. MMR is typically used for borehole mapping to locate bodies offset from the hole, with a TDEM probe used to measure the signal decay. MIP is suitable for use in areas where highly conductive layers inhibit the collection of normal dipole-dipole or gradient array IP data.

Seismic

FGG carries out small scale seismic surveys for engineering applications i.e. downhole and crosshole, plus refraction and reflection surveys as required for various targets.

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The CSAMT electromagnetic sounding technique uses an artificial signal source (typically in the range 0.1Hz to 10 kHz) in addition to the natural MT fields. This provides a stronger and more reliable signal and enables imaging of shallower targets than would otherwise be possible with low frequency natural signals alone. By measuring electric (E) and magnetic (H) fields at the earth's surface, geophysicists can generate electrical resistivity models of the earth.

Natural electromagnetic waves are generated in the earths atmosphere by a range of physical mechanisms. As these travel into the Earth's interior they decay at a rate dependent upon their wavelengths.

• High frequency signals originate in lightning activity
• Intermediate frequency signals come from ionospheric resonances
• Low frequency signals are generated by sun-spots

Considering the above sources, the use of a controlled source allows much more efficient surveying, particularly for the relatively shallow targets of most interest in mineral exploration (~1000m). The low frequency signals travel further into the earth than high frequencies, and natural MT surveys are capable of penetrating more than 100 kilometres.

Conductivity Mapping

Conductivity Mapping maps geological variations, groundwater contaminants or any subsurface feature associated with changes in the ground conductivity using electromagnetic inductive techniques that make the measurements without electrodes or ground contact. With this inductive method, surveys can be carried out under most geological conditions including those of high surface resistivity such as sand, gravel and asphalt.

Typical conductivity mapping equipment such as the Geonics EM31 have an effective depth of exploration of about six meters, making them ideal for many geotechnical and groundwater contaminant surveys. Important advantages of such instruments over conventional resistivity methods are the speed with which surveys can be conducted, the precision with which small changes in conductivity can be measured and the continuous readout and data collection while traversing the survey area. The in-phase component is especially useful for detecting shallow ore bodies and buried metal hazardous waste.

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