Techniques

The use of Geophysics relies upon exploiting the physical properties of the subsurface to solving a variety of problems. The choice of geophysical techniques is varied as methods which work well in one environment will fail in another e.g. GPR maps well in resistive ground but is attenuated in conductive etc.

Seismic refraction

Measured parameters: Traveltimes of refracted seismic energy (P - or S - Waves)
Physical property: Acoustic velocity (function of elastic moduli and density)

Typically, acoustic pulses are generated at predetermined source locations along the length of the refraction seismic profile. The travel times of acoustic energy that have been critically refracted at horizons of interest is recorded at predetermined receiver locations. The recorded travel time information is used to generate a velocity–structure profile of the shallow subsurface along the length of the refraction profile.

Seismic Reflection

Measured parameters: Traveltimes and amplitudes of reflected seismic energy (P - or S - waves)
Physical property: Density and Acoustic Velocity

Typically, acoustic pulses are generated at predetermined source locations along the length of the reflection seismic profile. The travel times and amplitudes of reflected acoustic energy is recorded at predetermined receiver locations. The recorded travel time–amplitude information is used to generate a reflection seismic profile. These data can be transformed into a velocity–structure profile.

Crosshole Seismic Tomography

Measured parameters: Traveltimes of seismic energy (P – or S - waves)
Physical property: Acoustic velocity (function of elastic moduli and density)

Typically, high-frequency acoustic pulses are generated at predetermined source locations in the source borehole. The amplitude and arrival time of direct arrivals (and others) is recorded at predetermined receiver locations in the receiver borehole. The recorded travel time–amplitude data are statistically analysed and used to generate a velocity–attenuation cross-sectional model of the area between the source and receiver boreholes.

Multichannel Analysis of Surface Waves (MASW)

Measured parameters: Traveltimes of surface waves generated using an active source
Physical property: Acoustic velocity

Surface wave ( generally Rayleigh wave) energy, generated using a nearby acoustic source, is recorded at predetermined receiver locations. A dispersion curve (phase velocity versus frequency), generated from the acquired field data, is inverted and used to generate a 1-D shear wave velocity profile (generally tied to the physical center of the receiver array). If additional MASW data sets are acquired at adjacent locations, 2-D or 3-D shear-wave velocity models can be created.

Refraction Microtremmor (ReMi)

Measured parameters: Traveltimes of passive surface wave energy
Physical property: Acoustic velocity

Surface wave (generally Rayleigh wave) energy, generated using a passive (background) acoustic source, is recorded at predetermined receiver locations. A dispersion curve (phase velocity vs. frequency), generated from the acquired field data, is inverted and used to generate a 1-D shear wave velocity profile (generally “tied” to the physical center of the receiver array). If additional ReMi data sets are acquired at adjacent locations, 2-D or 3-D shear-wave velocity models can be created. ReMi data can be used in conjunction with MASW to generate a shearwave model to greater depth and with higher resolution than with one technique alone.

Ground Penetration Radar (GPR)

Measured parameters: Traveltimes and amplitudes of reflected EM energy
Physical property: Dielectric constant, magnetic permeability, conductivity and EM velocity

Typically, pulsed EM energy is generated at predetermined station locations along the length of the GPR profile. The travel times and amplitudes of reflected EM energy are usually recorded by a monostatic transmitter–receiver. The recorded travel time–amplitude information is normally used to generate a GPR profile (2-D time–amplitude image). These data can be transformed into a 2-D velocity–depth model.

Borehole Radar

Measured parameters: Traveltimes and amplitudes of reflected EM energy
Physical property: Dielectric constant, magnetic permeability, conductivity and EM velocity

Using the Geomole Borehole Radar system pulsed EM energy is generated along the length of a borehole. The travel times and amplitudes of reflected EM energy are recorded and the travel time–amplitude information is used in conjunction with additional geological and structural information to generate a 3D model from this non – directional antenna. The Geomole BHR antenna uses a frequency range of 10 – 110 Hz and has a penetration depth typically less than 50 m.

Electromagnetics

Measured parameters: Response to EM energy
Physical property: Electrical conductivity

EM tools are used to measure the Earth’s response to natural or anthropogenic EM energy. Measurements can be made in either the time or frequency domain. Some tools are used to locate metals or utilities; others are used to create conductivity–depth models of the subsurface.

Electrical Resistive Imaging (ERI)

Measured parameters: Potential differences in response to induced current
Physical property: Electrical resistivity

Typically, current is induced between paired electrodes. The potential difference between paired voltmeter electrodes is measured. Apparent resistivity is then calculated. If the current electrode spacing is expanded about a central location, a resistivity–depth sounding can be generated. If the array is expanded and moved along the surface, 2-D or 3-D resistivity–depth models can be created. With modern equipment this has been automated by the use of multichannel systems capable of recording up to 12 readings at once from 64 electrodes placed in the ground, iterating through a sequence of readings before moving the system along.

Induced Polarisation (IP)

Measured parameters: Polarisation voltages or frequency dependant ground resistance
Physical property: Electrical Capacitivity

Two types of IP data are acquired: frequency domain and time domain. Frequency domain IP data are generated by comparing the apparent resistivities determined for two variable frequency input currents. Time domain data are generated by measuring rate of decay of the measured potential difference after current flow is terminated. IP measures the capacitive properties of the ground, and is used to qualitatively–quantitatively estimate the concentration– distribution of clay or metallic mineralisation.

Self Potential (SP)

Measured parameters: Natural electrical potential differences
Physical property: Natural electrical potentials

SP tools are used (mostly) to measure:
(a) natural potential differences arising from oxidisation–reduction of metallic bodies straddling the water table and (b) streaming potential associated with flowing groundwater. SP data are usually interpreted in a qualitative manner, and are routinely used to locate zones of seepage in earth fill dams and levees.

Magnetics

Measured parameters: Spatial variations in the strength of the geomagnetic field
Physical property: Magnetic Susceptibility and remanent magnetisation

Magnetometers are designed to measure variations in the magnetic field of the Earth. These are usually caused by the presence of magnetically susceptible material of natural or human origin (typically magnetite or iron, respectively). In certain instances, magnetic data can be interpreted quantitatively, and transformed into constrained geologic models.

Gravity

Measured parameters: Spatial variation in the strength of the gravitational field of the earth
Physical property: Bulk density

Gravimeters are designed to measure variations in the gravitational field of the Earth, and are typically used to generate 2-D or 3-D density–depth models of the subsurface.