Resistivity & Induced Polarization
Resistivity (DC) and Induced Polarization (IP) is an electrical survey method used for measuring the electrical properties of the subsurface. In a DCIP survey, a time-varying current is injected into the ground using two current electrodes and the resulting potential field voltage distribution is measured. The measured response is then used to calculate the apparent resistivity of the ground. When the current is removed, materials capable of holding a charge (i.e. polarizable), will have residual currents that decay over time. The resulting change in voltage over time represents the chargeability of a material.
Key features and benefits of IP
Used to identify the subsurface distribution of electrical properties: resistivity and chargeability
- Wide variety of survey arrays and configurations that are selected based on the survey objectives: pole-dipole, dipole-dipole, and 2D, 3D
- 2D and 3D inversion algorithms are used to generate subsurface models,
- Commonly used for mapping stratigraphy, structure, and alteration of rocks and soils,
- Direct detection of disseminated sulphides using chargeability (i.e. porphyry copper deposits).
Services provided by SJ Geophysics
- Ground acquisition of IP data:
- Volterra-IP
- 2D & 3D data acquisition / data inversion
Borehole
Borehole surveys involve the acquisition of geophysical data from within a borehole. They can be separated into two types: those measuring the physical properties of rocks near the borehole and those measuring the properties of objects located further away from the borehole.
Key features and benefits of borehole
- Suited to measure the response from targets that are too deep, small, or complex to identify with surface surveys,
- Wide variety of borehole survey types possible: electromagnetic, resistivity/induced polarization, magnetic, radiometric, etc.,
- Used to provide data complementary to surface surveys,
- Added value to drilling programs.
Services provided by SJ Geophysics
- Borehole-EM
- Borehole-Mag
- Borehole-IP (cross-hole, surface to borehole, combined borehole and surface 3DIP surveys) Unique to SJ Geophysics We can survey below the drill bit.
Utilizing our unique
Volterra borehole acquisition system, all data is logged within the acquisition unit downhole eliminating the need for expensive cables. The borehole survey occurs as the drill rods are removed from the hole. This means drillholes at high risk of collapse can be surveyed reducing the risk of not collecting data.
Controlled Source Electromagnetic
Controlled source electromagnetic surveys use the principals of electromagnetic induction to measure the electrical properties of the ground. EM data is acquired by transmitting a time-varying signal that induces currents in the ground which are then detected by a receiver unit. The induced currents in the ground are generated by the transmitter using either a surface loop or a grounded wire dipole. The measured response provides information on the subsurface distribution of rocks and minerals whose electrical properties are known to span many orders of magnitude.
Key features and benefits of EM
- Low impact survey method. Based on the principals of induction, no ground contact is required by the receiver units,
- Wide variety of EM techniques are available including both ground and airborne options,
- Ideally suited to identify conductive bodies within resistive host rocks,
- EM is successfully used to identify conductors such as graphite and metallic bodies, massive sulphide deposits, and buried metallic cultural objects.
Services provided by SJ Geophysics
Ground acquisition of electromagnetic data:
- Volterra-EM (fixed-loop, moving-loop, in-loop soundings etc.)
- Volterra-CSAMT
- Max-Min
- VLF
Magnetic
Magnetic surveys are a passive potential field method that measures variations in the Earth’s magnetic field (due to induced secondary fields caused by magnetic/ferrous materials). These variations can be used for the direct detection of metallic ore bodies or the identification of structural trends and geological framework.
Key features and benefits of magnetic surveys
- Low impact survey method. Acquisition is logistically simple and cost effective,
- Availability of both ground or airborne surveys,
- Direct detection of the presence or absence of magnetite (pyrrhotite, hematite),
- Suited to identify different rock units (sedimentary vs volcanics) based on the characteristics of the magnetic response,
- Used to map the distribution of geologic structures and rock units,
- Focused on identifying magnetic bodies such as magnetic ores or buried cultural objects (eg. tanks and drums).
Services provided by SJ Geophysics
Ground acquisition of magnetic data:
- GEM magnetometer
- Volterra BH-magnetic
Gravity
Gravity surveys are a passive potential field method that measures variations in the earth’s gravitational field. These variations are the result of changes in the subsurface density distribution of rocks and minerals. Gravity data requires careful data reduction procedures to remove influencing factors such as the earth’s ellipticity, lunar tidal forces and local geographic effects. After corrections have been made, subtle features can be identified in the data.
Key features and benefits of gravity surveys
- Suited to indicate regions of mass excess or mass deficiency. A region of mass excess could be caused by the presence of base metals (high density) while a region of mass deficiency could be caused by voids or graphite (low density),
- Density distribution gives information on the subsurface geology,
- Used to map dense mineral deposits such as volcanogenic massive sulphides and kimberlites,
- Focused on identifying subsurface caverns.
Services provided by SJ Geophysics
- Ground acquisition of gravity data
Magnetotellurics
Magnetotelluric surveys are a unique electromagnetic technique that utilizes natural energy as its source. Measurement in the variation of these natural magnetic B(t) and electrical E(t) fields can be used to derive the electrical properties of the subsurface material.
Key features and benefits of Magnetotelluric
- Low impact survey method. MT only requires the use of magnetic sensor and a few electrodes,
- Natural source (hence no need for use of transmitter and generator),
- Expands investigation depth greater than galvanic electrical methods,
- Integrated MT and DC Resistivity/IP data acquisition.
Services provided by SJ Geophysics
- Tensor
- Scalar
- CSAMT