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Clearview Geophysics

Geophysical Methods

IP (Induced Polarization)/Resistivity:

IP/Resistivity Surveys can be carried out on SURFACE or though BOREHOLES.

Surface

  • Spectral parameters MIP, Tau and ‘c’ are used 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
  • Surveys can be carried out in 2D in-line modes or in 3D with remote receivers.
  • 3D surveys are preferred where the target consists of relatively small (e.g., 20m) lenses that could be missed by traditional 2D surveys.

Borehole

  • Cross-hole Surveys are completed with 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
  • At the conclusion of each survey day, the results are inversion modeled and reviewed so that the following day’s cross-hole pairs can be planned and completed.
  • UBC GIF 2D and 3D inversion model sofware is used to inversion model the IP and Resistivity data.
  • 2D results are presented as stacked pseudosections of chargeability, apparent resistivity, Spectral and 2D inversion model depth sections. Magnetic profiles from available airborne or ground surveys are included as profiles on these presentations. Note that 2D results collected on a regular grid can also be inversion modeled in 3D.
  • 3D inversion model results are presented as plan maps for various model depths and depth sections for any pertinent orientation

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 so that, with post processing, a depth section resistivity model of the ground can be produced.  The main benefit of the magnetotelluric methods is that depth penetration can be quite significant. 
  • Phoenix receivers and TXU-30 transmitter are used to collect the data. Software is used to pick 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 are used to inversion model the results.

TDEM (Time Domain Electro-Magnetics)

  • Large Loop and Moving Loop TDEM surveys are completed to 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.
  • A BH43-3 EM probe is used to log boreholes.
  • Lamontagne MultiLoopIII software is used to model the 3-component data.

Gravity

  • Gravity Surveys are used for many applications, including gold, base metal and diamond exploration.  For geotechnical applications it can be used to 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 is used to navigate and record the GPS data.
  • Bouguer Gravity is calculated using Earth Tide corrections, Free Air Corrections, Bouguer corrections and Latitude corrections. 
  • These corrected data can then be inversion modeled using UBC-GIF v6 software.

Magnetics

  • Many cesium magnetometer surveys are carried out without cut lines.  The onboard GPS is used for navigation and positioning.  Cesium Magnetics Data are recorded at 10x per second.  Therefore, lines are spaced 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. IP/resistivity survey lines can be ‘targeted’ to specific areas (e.g., breaks in the magnetics contours) to reduce costs by exploring more efficiently.
  • Scintrex EnviC Cesium Magnetometers and GEM System Overshauser Magnetometers are used for the rover and base station units respectively.
  • Snowmobile-mode magnetometer surveys are carried out with a Scintrex NavMag Cesium Magnetometer sensor and Trimble AgGPS132 sensor mounted on a custom built aluminum sleigh. They are separated 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.
  • UBC-GIF Mag3D v6 software is used to inversion model airborne and magnetics data.

GPR (Ground Penetrating Radar)

some GPR applications include:

  • diamond exploration
  • UST (underground storage tanks) locates
  • depth to water table
  • depth to bedrock
  • Private Locates
  • stratigraphic mapping
  • contaminant mapping (in special cases)
  • reinforced concrete testing
  • road-bed studies
  • grave site mapping

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.

  • Sensors & Software Noggin 100/250/500 and Conquest 100 are used. PulseEKKO gear are used where lower frequencies (e.g., 50 MHz and 25 MHz) are required (e.g., diamond exploration, overburden studies, etc.)
  • often run in ATV- and Snowmobile-mode to cover large areas of ground economically.
  • production rates of a few km per day are typical.

EM31, EM34, EM39

  • The Geonics EM31 is an industry standard device most commonly used in the ‘vertical dipoles mode’ for penetration up to approximately 6 metres deep.
  • Results are presented as colour-contour plan maps for the apparent conductivity and inphase. 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.
  • for deeper penetration and pseudo-depth sections, results from an EM31 survey can be combined with results from an EM34 survey.

EM31 Horizontal dipoles | Penetration Depth (m) – 3 | Plot-point Depth (m) – 2
EM31 Vertical dipoles | Penetration Depth (m) – 6 | Plot-point Depth (m) – 4.5
EM34 Horizontal dipoles, 10m coil separation | Penetration Depth (m) – 7.5 | Plot-point Depth (m) – 5
EM34 Horizontal dipoles, 20m coil separation | Penetration Depth (m) – 15 | Plot-point Depth (m) – 10
EM34 Vertical dipoles, 10m coil separation | Penetration Depth (m) – 15 | Plot-point Depth (m) – 11.25
EM34 Horizontal dipoles, 40m coil separation | Penetration Depth (m) – 30 | Plot-point Depth (m) – 20
EM34 Vertical dipoles, 20m coil separation | Penetration Depth (m) – 30 | Plot-point Depth (m) – 22.5
EM34 Vertical dipoles, 40m coil separation | Penetration Depth (m) – 60 | Plot-point Depth (m) – 45

  • EM39 borehole surveys typically consist of a conductivity probe and a gamma probe. This combination is useful for environmental investigations because clean sandy soils typically have low gamma counts and low conductivity whereas contaminated soils can have high conductivity while the gamma counts are unchanged. Clayey soils typicaly have high gamma counts and high conductivity.

EM61

  • The Geonics EM61 is used for metal detection of large or small objects. In the hand-held mode, the smaller transmiiter and receiver coils can penetrate ~1m deep. It can be used to detect 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. Results are recorded for the top and bottom coil and the difference between these two responses are plotted as colour contoured plan maps.

Seismic

  • Seismic Refraction surveys are commonly carried out 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 are carried out to determin shear wave velocity and ‘stiffness’ results.
  • Geometrics Geode with 4.5 Hz or 14 Hz geophones are used for land surveys, and DHA-7 hydrophones are used for borehole and water-borne surveys.
  • Seismic refraction analyses of the same MASW seismic survey data can be used to determine 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).  The Shear Modulus can be calculated using the MASW deduced shear wave velocity (Vs) and corresponding mass density, which in turn can be approximated from the compression wave velocity (Vp).  The Vs and Vp values can then be used to calculate Poisson’s Ratio.
  • IXRefraX and ParkSEIS are used to edit, analyze and present the Refraction and MASW results respectively.

‘IMAGEM’

  • IMAGEM was designed for walking-mode or snowmobile-mode surveys. It records 200 channels of on- and off-time EM measurements.
  • Approximately 700 amps are transmitted 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.  Readings are collected at 1-second intervals in walking-mode and 10x per second in snowmobile-mode for much higher resolution than MaxMin or Promis surveys.
  • Time constants can be calculated and the data can be imported to modeling software such as Maxwell.

VLF-EM

  • VLF-EM surveys use low-frequency communication transmitters located around the world.
  • Geonics EM16 receivers are used to measure responses for the chosen transmitter.

MaxMin

  • MaxMin can be carried out with frequencies as low as 110 Hz to 56k Hz. Intercoil separations can range from 10m/12.5m to 250m and larger
  • When used on uncut survey lines, Trimble R12i rover / R12 base with 35W receiver are used to record the transmitter and receiver locations so that post processing can correct for spacing errors, especially through bush.

4-Pin Wenner Array

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.