Geophysical Methods
IP (Induced Polarization)/Resistivity:
IP/Resistivity Survey methods can be carried out on SURFACE or though BOREHOLES.
Surface
- We use Spectral parameters MIP, Tau and ‘c’ to obtain information about quantity of sulphides, grain size, and uniformity of grain size. This helps to characterize and prioritize otherwise similar chargeability and resistivity anomalies
- 2D in-line modes or in 3D with remote receivers are options to consider.
- 3D surveys detect relatively small (e.g., 20m) lenses.
- See also:
Borehole
- Cross-hole Surveys use transmitter electrodes located at infinity across strike and potential electrode pairs located across two boreholes.
- The cross-hole pairs can range from a couple meters apart for geotechnical investigations to hundreds of metres apart for mineral exploratoin
- We inversion model and review the results at the end of each field day so that we can plan the following day’s cross-hole pairs.
- UBC GIF 2D and 3D inversion model software inversion models the IP and Resistivity data.
- 2D results present as stacked pseudosections of chargeability, apparent resistivity, Spectral and 2D inversion model depth sections. Magnetic profiles from available airborne or ground surveys plot as profiles on these presentations.
- 2D results collected on a regular grid also inversion model in 3D.
- 3D inversion model results present as plan maps for various model depths and depth sections for any pertinent orientation
- See also:
CSAMT (Controlled Source Audio-frequency Magneto-Tellurics)
- The CSAMT method builds on the AMT and MT methods. Their main applications are for mineral/oil/geothermal exploration, geologic mapping and groundwater investigations. These methods scan a range of frequencies. We can then produce a depth section resistivity model of the ground. The main benefit of the magnetotelluric methods is that depth penetration can be quite significant.
- Phoenix receivers and a TXU-30 transmitter collects the data. Software picks the optimum transmitter location but typically they are over 20 km away from the receiver lines. The transmitter line is at least 2 km between electrodes which consist of stainless steel rods and aluminun foil pits in swampy or wet locations.
- Zond software inversion models the results.
- See also:
TDEM (Time Domain Electro-Magnetics)
- Large Loop and Moving Loop TDEM surveys locate conductive targets such as those associated with base metals, graphite and uranium.
- A PROTEM receiver with 3D coil receiving from a TEM57 transmitter boosted by two TEM67 power modules can power large 10-gauge copper wire loops for high currents and relatively short turn-off times.
- Borehole BH43-3D and MAG43-3D EM and Fluxgate probes can detect weak and ‘super conductor’ sources.
- Lamontagne MultiLoopIII and Maxwell software model the 3-component data.
- See also:
IMAGEM operates in walking-mode or snowmobile-mode. It records 200 channels of on- and off-time EM measurements.
The transmitter applies approximately 700 amps through a single-turn and ground-portable platform. This system, which is based on airborne survey designs, replaces 50-m and less inter-coil MaxMin readings. The fixed receiver and transmitter configuration provides data that are easier to interpet particularly in variable/rough topography. We acquire readings at 1-second intervals in walking-mode and 10x per second in snowmobile-mode for much higher resolution than MaxMin or Promis surveys.
We calculate time constants and import to modeling software such as Maxwell.
Gravity
- Gravity Surveys help explore for for gold, base metals and diamonds, for example. For geotechnical applications gravity surveys detect buried bedrock escarpments, for example.
- Scintrex CG-6 Gravimeter with Trimble R12I rover and Trimble R12 base with 35W repeater for acquiring high precision data through uncut survey lines and rough bush. A Trimble TSC7 datalogger helps navigate and record the GPS data.
- We calculate Bouguer Gravity with Earth Tide corrections, Free Air Corrections, Bouguer corrections and Latitude corrections.
- The corrected data inversion model using UBC-GIF v6 software.
- See also:
Magnetics
- We carry out many cesium magnetometer surveys without cut lines. The onboard GPS navigates and positions the readings. Cesium Magnetics Data record at 10x per second. Therefore, lines space 50-metres apart in areas where drills are standing by. This allows for the interpretation of ‘breaks’ in the mag which can act as conduits for gold and other economic mineralization. We position IP/resistivity survey lines to ‘target’ specific areas (e.g., breaks in the magnetics contours) to reduce costs by exploring more efficiently.
- Scintrex EnviC Cesium Magnetometers and GEM System Overhauser Magnetometers apply to the rover and base station units respectively.
- Snowmobile-mode magnetometer surveys use Scintrex NavMag Cesium Magnetometer sensor and Trimble AgGPS132 sensor mounted on a custom built aluminum sleigh. They separate by 1.7 metres. Post processing accounts for this layback and a 0.3 second latency for highly precise and lower-than-walking-mode heading errors. Coverage averages 60 km per day in typical arctic terrain. Joe Mihelcic carries out these surveys and has been doing so since 1998 with well over 10,000 km of coverage completed.
- We inversion model airborne and ground magnetics data with UBC-GIF Mag3D v6 and MVI v3 software.
GPR (Ground Penetrating Radar)
some GPR applications include:
- diamond exploration
- UST (underground storage tanks) locates
- depth to water table
- depth to bedrock
- Private Locates
- stratigraphy mapping
- contaminant mapping (in special cases)
- road-bed studies
- grave site mapping
- See also:
GPR works best in low conductivity (high resistivity) areas. Conductive materials (e.g., clay) attenuate the GPR signal to the point that very little depth penetration is achieved. Penetration is greatest in unsaturated sands and fine gravels. When possible, additional geophysical methods, such as the Geonics EM61 metal detector for UST locates, or the Geonics EM31 for depth to bedrock mapping, are used. Physical evidence from boreholes and test pits are commonly used to complement the geophysical data to provide a more robust quantitative interpretation of the broad coverage geophysical results.
GPR can also be used to detect contaminants and water main leaks, if the conditions are right. The reflection delay time is controlled by the dielectric properties of the material. The signal velocity (m/ns) is approximately equal to one third the value of the dielectric constant. Some contaminants have different dielectric properties compared to the host geology. For example, GPR signals travel at 0.01 m/ns through sea water compared to 0.06 m/ns through clean, saturated sand.
The penetration depth of the instrument is also dependent on the chosen base frequency. The higher the frequency, the less the penetration. The trade-off is that the lower frequencies lose target resolution. Typically, 100 MHz and 50 MHz are used for bedrock detection, 500 MHz and 250 MHz antennae are used for tank locates, and 1000 MHz is used for rebar checking.
- We use Sensors & Software Noggin 100 MHz, 250 MHz, 500 MHz and Conquest 100 instruments.
- PulseEKKO gear use lower frequencies (e.g., 50 MHz and 25 MHz) required for diamond exploration, overburden studies, etc.
- often run in ATV- and Snowmobile-mode to better cover large areas of ground.
- production rates of a few km per day are typical.
EM31, EM34, EM39, EM61
- The Geonics EM31 is industry standard and most commonly used in the ‘vertical dipoles mode’ for penetration up to approximately 6 metres deep.
- The apparent conductivity and inphase results present as colour-contour plan maps. The inphase, when interpreted with the corresponding apparent conductivity response, are useful for detecting buried metal, fill, former structures, pipes/utilities, slag, landfill extents, etc.
- EM31 results combine with EM34 results for deeper penetration and pseudo-depth sections,
EM31 Horizontal dipoles | Penetration Depth 3m | Plot-point Depth 2m
EM31 Vertical dipoles | Penetration Depth 6m | Plot-point Depth 4.5m
EM34 Horizontal dipoles, 10m coil separation | Penetration Depth 7.5m | Plot-point Depth 5m
EM34 Horizontal dipoles, 20m coil separation | Penetration Depth 15m | Plot-point Depth 10m
EM34 Vertical dipoles, 10m coil separation | Penetration Depth 15m | Plot-point Depth 11.25m
EM34 Horizontal dipoles, 40m coil separation | Penetration Depth 30m | Plot-point Depth 20m
EM34 Vertical dipoles, 20m coil separation | Penetration Depth 30m | Plot-point Depth 22.5m
EM34 Vertical dipoles, 40m coil separation | Penetration Depth 60m | Plot-point Depth 45m
- EM39 borehole surveys typically consist of a conductivity probe and a gamma probe
- This combination is useful for environmental investigations
- Clean sandy soils typically have low gamma counts and low conductivity values.
- Contaminated soils typically have high conductivity while the gamma counts remain unchanged.
- Clayey soils typically have high gamma counts and high conductivity.
- See also:
The Geonics EM61 detects large or small metallic objects. In the hand-held mode, the smaller transmiiter and receiver coils can penetrate ~1m deep. It detects small objects the size of a pop can. For larger objects and deeper penetration, the EM61 with standard 1 m x 0.5 m coils has a penetration of 2-3 metres deep. The recorded results for the top and bottom coil and the difference between these two responses plot as colour contoured plan maps.
Seismic
- We complete seismic Refraction surveys to determine depth to bedrock and other features. ClearView Geophysics uses hammer sources or its specially designed Seis-Gun.
- MASW (Multichannel Analysis of Surface Waves) surveys determine shear wave velocity and ‘stiffness’ results.
- We use the Geometrics Geode with 4.5 Hz or 14 Hz geophones for land surveys, and DHA-7 hydrophones for borehole and water-borne surveys.
- Seismic refraction analyses of the same MASW seismic survey data determines compression wave velocities (Vp). The travel-time/distance from the first-arrival shock source graph has different slopes depending upon the velocity that the shock wave travels through the ground. Note that refraction will not work if a “slow” layer (e.g., sand) is located below a “fast” layer (e.g., clay). We calculate the Shear Modulus using the MASW deduced shear wave velocity (Vs) and corresponding mass density. Mass density approximates from calculations using the compression wave velocity (Vp). Vs and Vp values are factors to calculate Poisson’s Ratio.
- IXRefraX and ParkSEIS software edit, analyze and present the Refraction and MASW results respectively.
- See also:
MaxMin & VLF-EM
- MaxMin reads frequencies as low as 110 Hz to 56k Hz. Intercoil separations can range from 10m/12.5m to 250m and larger
- Trimble R12i rover / R12 base with 35W receiver record the transmitter and receiver locations so that post processing can correct for spacing errors, especially through bush and with uncut survey lines.
- VLF-EM surveys use low-frequency communication transmitters located around the world.
- Geonics EM16 receivers measure responses for the chosen transmitter.
- The VLF method is a relatively inexpensive way to obtain electromagnetics data that can be post-processed to obtain resistivity information.
4-Pin Wenner Array and Resistivity Imaging
4-Pin Wenner Array surveys are carried out as per ANSI/IEEE Std. 81-1983 and others as a guide (i.e., IEEE 80-2000 and ASTM G57 for soil resistivity test). The work can be carried out for many applications such as to aid with appropriate electrical grounding design for planned facilities within highly congested industrial areas.
A Phoenix 3 kW transmitter with Scintrex IPR12 is used as the transmitter and receiver respectively. The transmitter, which is powered by a Honda motor-generator, provides switch-selectable transmitter voltages that provide sufficiently high Vp’s at the receiver where required (e.g., for the larger “a”-spacings and more conductive settings). New custom designed and manufactured stainless steel grade 316 electrodes measuring 12 inches long are used to keep burial depths consistent at all accurately measured and positioned injection and receiver points.
Typical instruments used for 4-Pin Wenner Array surveys for grounding design are battery operated. The 3 kW MG operated transmitter used by ClearView is able to produce higher sustainable transmitter currents and higher Vp’s where required for the larger “a”-spacings and in more conductive areas. Therefore, results are ‘cleaner’ compared to battery-operated systems because the measured amplitudes are larger for the MG operated Tx. That is, minor fluctuations in measured transmitter currents and receiver voltages caused by factors such as electrode contact, near surface moisture, etc. are less significant for large values possible with the MG powered transmitter compared to smaller amplitude measurements that would be expected from a battery operated system.
Several additional geophysical surveys can be carried out to characterize the sub-surface in the area: GPR at 250MHz and 100MHz, Geonics EM31 Ground Conductivity (vertical dipoles mode), and Geonics EM61 metal detection (standard coils). Private locates with the Radiodetection gear is also completed. The results from these additional surveys are important because buried facilities could interfere with and skew the 4-Pin Wenner Array results if located along or relatively near the spread.
Iris Syscal equipment acquires resistivity imaging data. This battery operated combined transmitter/receiver device can collect data for 96 electrodes spaced up to 10 metres apart. Pre-set Schlumberger and Wenner arrays cycle through for the desired exploration depth. The results model and present with both Loke and UBC 2D inversion modeling software.
Radiodetection
The Radiodetection PTLG+ instrument consists of a 10-watt transmitter with selectable frequencies. The transmitter energizes pipes and cables so that the receiver operator can trace them. It can also be operate in the passive mode to detect live hydro lines and certain secondary radio signals emitted by various lines (e.g., telephone, electrical). Results can be digitally recorded with the receiver. We trace detected lines with spray paint on the ground surface and photograph them for further reference.
Radiodetection is just one of several instruments used for SUE (Subsurface Utility Engineering). Others include GPR, EM31 and EM61
Borehole Logging
Boreholes can be surveyed with many types of probes:
- Caliper for mapping fractures and borehole diameter changes
- Induction Conductivity used with gamma to map formation conductivity and potential contamination
- Gamma for mapping geologic changes
- Single Point Resistivity for mapping geologic changes
- Spontaneous Potential for mapping geologic changes
- Fluid Conductivity useful for locating potential flow zones
- Fluid Temperature, which is useful for locating fractures that might flow sources
Borehole Time-Domain EM Surveys are also carried using a 3D EM probe.
IP/Resistivity Surveys use stainless steel and copper electrodes to acquire cross-hole survey data: Cross-hole IP/Resistivity Surveys for Gold & Voids
Video logs are also useful for further characterizing features detected within the borehole.
