What is borehole geophysics?
Using borehole geophysical methods to explore at depth
In mineral exploration, ground geophysics can start to hit its limits, especially when targets are deep or hidden beneath conductive overburden like clays. Borehole geophysics offers a way to get closer to the source, increasing your chances of detecting mineralized zones that would be invisible from surface.
What types of borehole surveys are available
There are several borehole geophysical methods used in mineral exploration for which each has its strengths and ideal use cases. At Abitibi Geophysics, we commonly use Electromagnetics (EM) and Induced Polarization with Resistivity (IP/Res) methods. Less common, but quite effective, is Gravity and Cross-Hole Radio-Tomography.
Borehole Electromagnetic (BHEM) surveys are good for detecting deposits such as VMS, uranium, or magmatic Ni-Cu-PGE. Using a surface loop with a transmitter and a downhole sensing probe, the depth of investigation will depend on the borehole’s length and loop size, whereas the off-hole distances can reach up to ~800 m.
With BHEM, a current is sent through the surface loop, generating a magnetic field that penetrates the ground and ‘couples’ (that is, it triggers a response) with metallic conductors like massive sulfides or graphite. These conductors, in turn, induce electric currents and generate a secondary magnetic field. The probe in the borehole senses this secondary field. The larger the loop and the higher is the current the greater is the investigation depth.
Borehole Induced polarization and resistivity (BHIP) is useful for detecting deposits such as orogenic gold, VMS, MVT Pb-Zn, porphyry Cu, uranium, and magmatic Ni-Cu-PGE. The typical investigation radius around a BH is up to ~200 m and up to 2000 m along the BH. The setup and configuration vary according to the goals and available BH’s.
The standard BHIP setup involves lowering a train of receivers and a transmitter down the borehole, injecting current, and measuring responses along the way. This produces a 2D “slice” or pseudosection of how the rock’s electrical properties change around the hole. This configuration is more sensitive to sources nearer to the BH.
Abitibi Geophysics developed another method where the current is applied at the surface (3D hole-to-hole IP survey H2H-3D-IP). When two semi-parallel boreholes are present, electrodes are lowered into both holes to measure the electrical response between them. The geometry of the survey means we can tell whether the source of an anomaly is located between the boreholes, outside of them, below them, or very close to them. This allows for better positioning of the targets and is more exploratory than the prior mentioned configuration.
Our DasVision method places continuously recording receivers at the surface while current is injected at surface and also into the boreholes. It creates a network of active recordings along the entire borehole length.
Borehole gravity surveys are well-suited to map density contrasts, aiding geological interpretation and direct targeting of massive sulfides, iron formations, and voids. With borehole gravity, you’re not injecting anything, you’re simply measuring how much the rock “pulls” on the GraviLOG Slim-Hole sensor. The depth of investigation can reach up to ~2300 m, beyond which high pressure and heat makes it difficult for the sensor to operate. The denser the rock, the higher the measured gravity. Even small, dense lenses that might be too subtle for surface gravity surveys can be clearly detected.
Cross-hole radio-tomography maps contrast in electrical permittivity, helping identify conductive bodies, and their continuity and shape between boreholes. It is useful in exploration contexts such as those targeting Ni-Cu or VMS deposits. It works by sending radio waves from one borehole to another. The travel time and signal strength reveal subtle changes in rock properties between holes, at depths of up to 3,000?m. The system fits standard drillholes and can image features that surface methods might miss.
Choosing the right borehole geophysics method
Each borehole method has trade-offs. EM is often the first choice, especially for sulfide-rich systems, and is attractive because of its excellent utility in only a single BH, while IP/Res is excellent for more subtle or disseminated targets and often requires many BH’s or pairs of BH’s to be surveyed to produce a very spatially precise model. Although gravity is less commonly used in boreholes, it’s highly effective when density contrasts are expected. Cross-hole radio-tomography, on the other hand, excels at imaging conductive zones and their continuity between drillholes, making it useful in exploration.
The best results come from understanding what you're looking for, the physical property contrast it might generate, and what boreholes or surface access are available. Used wisely, borehole geophysics is a powerful way to see what surface methods might miss.
