Discriminating desired resources from their host ground can be done by measuring various geophysical phenomena (see the table at the bottom of the page for a summary).
Different rock types have different iron levels (magnetite), which will vary the local magnetic field. Magnetic anomalies are often associated with desired mineralisation (see case studies).
Gap Geo offers the capability to create high resolution maps of the local magnetic field strength, either via airborne or surface techniques, at potentially sub-metre intervals (see High Resolution Mapping for examples).
If an electrical current is introduced into the ground, it will channel through areas of higher electrical conductivity. By using a sensitive magnetometer, and removing from the measured signal the magnetic fields that would come from the wire and homogeneous ground, we are left with the total field magnetometric resistivity value - with features indicative of potential mineralisation targets.
To perform a TFMMR survey, a high power transmitter outputs current through a wire that ends at electrodes set into the ground - positioned to best energise a target, when possible.
By setting up a current carrying wire on the surface, and then suddenly cutting off the power, a brief eddy current will be induced in the sub-surface terrain, and create a transient magnetic field - the nature of which will depend on the conductivity distribution. This electromagnetic effect can be used to detect conductors in resistive terrain, or resistors in conductive territory, and is typically used when seeking features associated with massive sulphides.
The magnetic field (or sometimes the change in the magnetic field) is typically measured via stationary surface measurements for deep ground penetration, or can be via a borehole probe to get the sensor closer to the mineralisation.
Soil chargeability & Apparent resistivity
Soil chargeability and apparent resistivity are particularly useful to determine the location of mineralisation associated with disseminated sulphides - such as gold or copper, but can also be used to determine boundaries between fluids and surrounding soils, and so map leakages, for example.
These values can be calculated through a survey process called Induced Polarisation, in which transmitter electrodes deliver current to the ground, and then receiver electrodes measure the potential difference. Following some calculations, 'pseudo-sections' of the ground are developed - mapping the survey values to cross sectional planes.
Our proprietary SAM technique collects both electrical and magnetic information simultaneously, thereby providing a significant data advantage over traditional surveys. By using a highly sensitive total field sensor in a moving survey format, we achieve high spatial resolution with an emphasis on efficiency. Where deeper penetration is important, a stationary survey mode (SAMSON) can be used. For larger survey areas, airborne deployment becomes more suitable.
||Resistivity • density
||Resistivity, Seismic, Gravity
||Resistivity, Seismic, CSAMT, IP
|Faults and fractures
||Abrupt changes in resistivity and density
||Resistivity, Seismic, Gravity, SAM, SAMSON, Magnetics, CSAMT, TEM, FEM
||0.2 to 5% minerals • weak conductor • low resistivity • moderate to strong IP
||Resistivity, IP, SAM, Magnetics, Seismic
||Greater than 10% mineralization • strong conductor • very low resistivity • moderate to weak IP
||Resistivity, CSAMT, SAM, SAMSON, TEM, FEM, Magnetics, Seismic, IP
||High or low resistivity depending upon alteration type • weak magnetics • moderate to weak IP
||Resistivity, SAM, SAMSON, IP, Magnetics, CSAMT, TEM, FEM, Radiometric
|Silicification (dike-like) features
||Large resistivity contrasts • potassium
||Resistivity, CSAMT, Seismic, Radiometric
||Low resistivity • weak magnetics • potassium
||Resistivity, SAM, Magnetics, Radiometrics, Seismics, IP
||Low over high resistivity • weak magnetics
||Resistivity, SAM, SAMSON, CSAMT, TEM, FEM, Seismics, Magnetics, IP, Radiometrics
|Buried stream channels
||Subtle resistivity changes • magnetic sands • density changes
||Resistivity, SAM. SAMSON, Magnetics, Seismics, TEM, CSAMT
|Leaking dams and ponds
||Resistivity changes • streaming potentials
||Resistivity, SAM. SAMSON, CSAMT, SP, TEM, FEM