OVERVIEW OF OUR GEOPHYSICAL TECHNIQUES

SEISMIC REFRACTION

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source (S) located on the surface. Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (L1) as refracted arrivals (R1, R2, etc.) before returning to the surface. This energy is detected on the surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.

If external constraints are available, the velocity–depth profile can be transformed into a geological model. The conversion of observed travel times can be carried out using several techniques. For less complex geological scenarios where fast turn-around schedules are essential, a time-intercept approach can be used as opposed to areas which may present significant lateral heterogeneity, in which case the tomographic inversion approach is recommended.

DOWNHOLE SEISMIC

Downhole seismic surveys are a simple and cost-effective method to obtain high-resolution data on elastic and dynamic properties of the earth. Seismic energy is generated on the surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves are measured at regular intervals down the hole using a string of hydrophones or, in the case of S-wave surveys, a single clamped triaxial geophone that is gradually moved down the hole. The P- and S-wave arrival times for each receiver location are combined to produce travel-time versus depth curves for the complete hole.These are then used to produce total velocity profiles from which interval velocities and the various elastic moduli can be calculated subject to availability of density data from geophysical logging of the borehole or laboratory tests.

SEISMIC REFLECTION

CROSS-HOLE SEISMIC TOMOGRAPHY

Cross-hole seismic surveys involve measurement of the travel time of seismic energy transmitted between two or more boreholes to derive information on the elastic properties of the intervening materials. One hole is used to deploy the source (S) whilst the placement of receivers in other borehole(s) is used to detect the arrival of the seismic energy. The travel times of the seismic waves are derived from the first-arrivals identified on the seismic trace for each shot-receiver position and are used with the known distance(s) between the shot/receiver boreholes to calculate the apparent velocities (P and S) for each depth interval. This data is then used to derive a vertical profile of the various elastic moduli.

MULTI-CHANNEL ANALYSIS OF SURFACE WAVES

The Multi-channel Analysis of Surface Waves (MASW) technique makes use of surface wave (Rayleigh wave) energy, generated using a nearby acoustic source, which is recorded at predetermined receiver locations (R1, R2, etc.). 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 centre 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. If external constraints are available, the shear wave velocity models can be transformed into geological models.

REFRACTION MICROTREMOR

In Refraction Microtremor (ReMi), surface wave (Rayleigh wave) energy, generated using a passive (background) acoustic source, is recorded at predetermined receiver locations (R1, R2, etc.). 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 centre 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. If external constraints are available, these shear wave velocity models can be transformed into geological models.

The ReMi technique is often used in tandem with the more traditional MASW technique as it enables the ability to gain information on subsurface dynamic moduli and shear wave velocity to greater depths than are achievable using MASW. However, as the depth of investigation increases, vertical resolution decreases.

GROUND PENETRATING RADAR

MICRO GRAVITY

Microgravity profiling is a passive technique that involves the highly accurate measurement of relative changes in the earth’s gravitational field. Measurements are made using a gravity meter, which comprises a very sensitive temperature stabilized spring balance. Subtle changes in gravity result from variations in the density of materials within the subsurface and the method can, therefore, be used to successfully locate voids or buried features such as underground storage tanks. The effects of tidal and instrument drift that would otherwise mask any subtle anomalies are overcome by repeat readings at a fixed base station throughout the survey. Accurate topographic leveling is carried out at each station in order to correct for the effects of terrain.

MAGNETICS

Magnetic profiling is a passive method that involves measurement of localized variations in the amplitude of the geomagnetic field resulting from buried ferrous targets, such as underground storage tanks and pipes and variations in the magnetic susceptibility of near-surface materials. The amplitude and shape of the anomaly caused by buried ferrous objects will depend on the target’s shape, orientation, and susceptibility. In certain instances, magnetic data can be interpreted quantitatively, and transformed into constrained geological models. More typically, however, magnetic data are interpreted qualitatively and simply used to verify the presence or absence of magnetically susceptible materials or features.

ELECTRICAL RESISTIVITY

Measurement of ground resistivity involves passing electrical current into the ground using a pair of steel or copper electrodes and measuring the resulting potential difference within the subsurface using a second pair of electrodes. These are normally placed between the current electrodes. Typically, current (I) is induced between paired electrodes (C1, C2). The potential difference (ΔV) between paired voltmeter electrodes P1 and P2 is measured. Apparent resistivity (Δa) is then calculated (based on I, ΔV, electrode spacing’s).

Resistivity soundings involve gradually increasing the spacing between the current/potential electrodes (or both)
to increase the depth of investigation. The resistance data collected in this way are converted to apparent resistivity readings that can then be modelled to provide information on the thickness of individual resistivity layers within the subsurface.

Another form of electrical resistivity technique is,2D resistivity imaging. This is a fully automated technique that uses a linear array of up to or beyond 72 electrodes connected by a multicore cable. The current and potential electrode pairs are switched automatically using a laptop computer and control module connected to a ground resistivity meter (that provides the output current). In this way, a profile of resistivity against depth (‘pseudo-section’) is built up along the survey line. Data is collected by automatically profiling along the line at different electrode separations.

The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased by the minimum electrode separation (the physical distance between electrodes which remains fixed throughout the survey) and the process repeated to provide an increased depth of investigation.

The maximum depth of investigation is determined by the spacing between the electrodes and the number of electrodes in the array. However, as the spacing between the active electrodes is increased, fewer and fewer points are collected at each ‘depth level’, until on the final level only a single reading is acquired. In order to overcome this, the array is ‘rolled-along’ the line of investigation to build up a longer pseudo-section.

The raw data is initially converted to apparent resistivity values using a geometric factor that is determined by the type of electrode configuration used. Many 2D resistivity imaging surveys are carried out using the Wenner Array. In this configuration the spacing between each electrode is identical. Once converted, the data is modelled using finite element and least-squares inversion methods to calculate a true resistivity versus depth pseudo-section.

INDUCED POLARISATION

Two types of IP data are acquired: frequency-domain and time-domain. Time-domain IP surveys involve measurement of the magnitude of the polarisation voltage (Vp) that results from the injection of pulsed current into the ground. Polarisation voltages primarily result from electrochemical action (ionic exchange) within the pores and pore fluids of the material being energized. The current is applied in the form of a square waveform, with the polarization voltage being measured over a series of time intervals after each current cut-off using non-polarizing electrodes. The measured value of Vp is divided by the steady voltage (observed whilst the current is on) to give the apparent chargeability of the ground. This provides qualitative information on the subsurface geology. TD-IP is primarily used in mineral exploration surveys.

Spectral IP surveys involve measurement of the magnitude and relative phase of the polarisation voltage that results from the injection of alternating current into the ground. Polarisation voltages primarily result from electrochemical action (ionic exchange) within the pores and pore fluids of the material being energised. Measurements of the relative phase shift between the transmitted current and the measured signal and the magnitude of the polarisation voltage are taken over a range of different frequencies. This results in a distinct IP response spectrum or ‘dispersion’ at each measurement position that can be used to determine capacitive parameters such as relaxation time and chargeability, and can be interpreted qualitatively or modelled quantitatively in 2D and 3D to estimate the distribution of clay or metallic mineralisation.

SPONTANEOUS POTENTIAL

The spontaneous potential (SP) method is a passive electrical technique that involves measurement of naturally occurring ground potentials. These can be generated from several different sources although all require the presence of groundwater to some degree. The two main sources of interest in environmental and engineering studies are streaming potentials, due to movement of water through porous subsurface materials, and diffusion potentials resulting from differing concentrations of electrolytes within the groundwater.

SP measurements are made using a pair of non-polarising electrodes. These normally comprise a pot containing a copper electrode immersed in a saturated copper sulphate solution. A porous base to the pot enables the electrolyte to percolate out and make contact with the ground. The potential difference between the two pots is measured using a high impedance voltmeter.

SP is acquired with a stationary reference electrode (SE) and moving the measuring electrode (ME) along lines or grids to measure variation in potential. Data i usually interpreted qualitatively and is routinely used to locate zones of seepage in earth-fill dams and levees, assessing seepage from dams and embankments, fluid migration pathways in landfills, mapping coal mine fires and for the study of drainage structures, shafts, tunnels, and sinkholes. SP measurements can also be quantitatively inverted in a stochastic sense to generate models of charge distribution potential which are highly effective at identifying vertical features such as mine shafts and sinkholes.

FREQUENCY DOMAIN ELECTROMAGNETICS

Frequency domain electromagnetic profiling utilises a time-varying electromagnetic field (the primary field) to induce eddy currents within subsurface conductors. These currents result in a secondary magnetic field that is measured together with the originally transmitted signal, using a receiver coil on the EM instrument. The secondary field is then separated into two orthogonal components, the real and imaginary (quadrature) components, representing respectively the vector components of the field in-phase and 90 degrees out of phase with the primary. The quadrature component provides a measure of the apparent ground conductivity whilst the real (in-phase) component is responsive to buried metallic objects.

The depth of penetration attained is dependent on several factors including the ground conductivity, the loop spacing and the orientation of the primary field (dipole orientation). A number of EM instruments are available which together provide a depth of investigation range of between 0.5m and 30m. The use of three or more loop spacings in both dipole orientations enables quantitative modelling of the depth to individual conductive layers. This is commonly known as EM depth sounding.

TIME DOMAIN ELECTROMAGNETICS

This active method measures the bulk electrical resistivity of the ground by inducing eddy currents in subsurface conductors using pulsed electromagnetic energy transmitted from a square loop of wire laid on the ground. The decay of these induced currents results in a decaying secondary magnetic field which is measured over a series of time gates immediately after termination of the transmitter pulse. Measurement of the secondary field can be made using either the transmitter loop or more commonly with a separate receiver coil located at the centre or to the side of the transmitter loop.

In the case of horizontally layered materials, the induced current loop will diffuse outwards and downwards with time whilst gradually decaying in amplitude. The speed of this diffusion and the amplitude of the secondary magnetic fields are related to the conductivity of individual subsurface layers. As a consequence thin resistive layers are generally invisible to TDEM soundings.

The depth of investigation of a TDEM survey is dependent on the moment of the transmitted signal together with the conductivity of the subsurface layers. A larger moment (achieved through an increase in the loop size and/or transmitter current) and an increase in ground resistivity will result in increased signal penetration.

Time-domain electromagnetic data can be acquired using airborne, surface and borehole receiver geometries. Modelling of the subsequent TDEM data can be carried out to produce time slice images of conductivity or to directly model decays of sub-surface conductors as metallic mineralisation or clay pods.

RADIOMETRIC SURVEYS

Radiometric surveys involve the measurement of gamma radiation resulting from natural radioactive sources. Instruments are available to measure either total count or provide spectral information on individual elements such as uranium, thorium and potassium to identify specific sources of radiation associated with geological units such as granites. Modern multispectral meters capable of measuring up to 256 channels are being increasingly used in the environmental mapping. Radiometric measurements are primarily used in mineral exploration but can also be applied to the detection of faults and mapping contamination.

BOREHOLE LOGGING

In the same way that surface geophysics makes it possible to “see” beneath the ground surface, borehole geophysical logging makes it possible to see beyond the walls of a boring or well. To perform borehole logging, sensors (or probes) that measure most of the different physical properties described above of the formation around the boring are lowered down the hole to record continuous data (or logs). A multi-conductor cable on a motorised winch controls the sonde, and transmits data back up the hole to a computer and graphic display. Often, multiple logs (called a suite) are recorded for a single boring – each measuring a different property – to allow more complete knowledge of subsurface conditions.

Typical borehole logs include: