Near surface geophysics

Electrical Techniques

  • Overview

    • 1D Resistivity Sounding
    • Resistivity measurements are made by passing an electrical current into the ground using a pair of electrodes and measuring the resulting potential gradient within the subsurface using a second electrode pair (normally located between the current electrodes). Resistivity sounding involves gradually increasing the spacing between the current/potential electrodes (or both) in order to increase the depth of investigation. The data collected in this way are converted to apparent resistivity readings that can then be modelled in order to provide information on the thickness of individual resistivity units within the subsurface.
    • Electrical Resistivity Imaging (ERI)
    • Electrical Resistivity Imaging (ERI) uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a laptop computer and relay box. 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 and the process repeated in order to provide an increased depth of investigation.
  • DC Resistivity applications
  • Resistivity methods may be used to map lateral changes and near-vertical features (e.g., fracture zones) and to determine depths to geoelectrical horizons (e.g. depth to saline water).
    DC Resistivity is commonly used to…
    • delineate aggregate deposits for quarries
    • measure earth impedance for electrical grounding circuits
    • estimate depth to bedrock
    • estimate depth to water table
    • detect and map geologic features
    • define mining targets as part of an induced polarization survey

Transient Electromagnetic or Time-Domain EM

  • Overview
  • The transient electromagnetic (TEM) method, alternately called time-domain EM (TDEM) or pulse EM (PEM), is a commonly-used, non-intrusive, geophysical method for obtaining subsurface resistivity-conductivity data.
    Because rock conductivity strongly correlates with rock properties, TEM is a powerful tool for mapping changes within rock or soil: clayey layers restricting groundwater flow, conductive leachate in groundwater, and seepage in earthen embankments for example.
    This active method involves inducing eddy currents within subsurface conductors using pulsed electromagnetic (EM) energy transmitted from a square loop of wire located on the ground. The decaying secondary EM signal induced by these eddy currents is measured over a series of time windows immediately after the transmitted signal is shut-off using the transmitter loop, or more commonly, a smaller second receiver coil located at the centre or to the side of the transmitter loop. TDEM soundings are capable of providing information on the conductivity of different layers within the subsurface to depths of between 3-1000m.
  • Applications
  • TEM techniques are used to map geologic structure in search of geothermal sources, groundwater, and aggregate deposits. Environmental and engineering uses range from delineating salt-water intrusion and contaminant migration to determining permafrost and depth to bedrock
    The depth of investigation can vary from 10s of meters to over 1000 meters (30 to 3,000 feet), depending upon the size of the transmitter loop used, available power from the transmitter, and ambient electromagnetic noise.
  • TEM advantages over DC resistivity
  • The TEM/TDEM method has several advantages over the DC resistivity technique. TEM does not require long electrode arrays and so is less sensitive to lateral changes in soils. DC resistivity requires long electrode spreads with lengths that are typically three to five times the depth of exploration.
    Thus, investigating to depths of 200 feet with DC resistivity requires an area of uniform horizontally-stratified soils with a lateral extent in excess of 600 feet. In contrast, the TEM method can obtain depths of exploration of several 100 feet with a 50-foot transmitter loop.
    TEM has better depth resolution than DC resistivity, particularly for mapping conductive aquitards (confining layers) in resistive sections. Whereas the DC technique has difficulty mapping strata below a resistive layer, TEM can easily map conductive strata beneath a thick resistive section.

Seismic Refraction

  • Overview
  • 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 ('shot') 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 (refracted arrivals) before returning to the surface. This energy is detected on 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.
    The time at which the energy is received at the surface is analyzed for structure and velocity. Seismic data may be modeled using either layer-based or tomographic techniques. Each method has its own strengths and weaknesses, and data collection parameters, thus should be determined prior to data collection to meet the objectives of the project.
    Data is typically presented in simple cross section highlighting the interface between soil and hard rock. In complex geologic environments, 3D refraction data (3D tomography) reduces ambiguity and allows integrated models to be created combining the seismic data with borehole and other ground-truth data.
  • Applications
  • Common applications include:
    • Mapping depth to bedrock and bedrock topography
    • Providing elastic properties of the subsurface for engineering design
    • Calculating the subsurface velocity profile
    • Mapping subsurface water table in sediments
    • Identifying fault locations and weak rock zones
    • Determine rippability of hard rock prior to construction
    • Mapping slide planes of active landslide

Magnetotelluric Method

  • Overview
  • Magnetotellurics (MT) and Audio-frequency MT (AMT) are electro-magnetic survey and imaging techniques that use naturally-occurring ionospheric current sheets and lightning storms — passive energy sources — to map geologic structures to depths of 500 meters or more.
    The MT geophysical survey method combines measurements of the earth’s electric field and magnetic field over a wide band of frequencies. Low frequencies sample deep into the earth and high frequencies correspond to shallow samples.