Selecting the right geophysical survey technology, or combination of technologies is critical to successfully imaging and/or mapping key targeted subsurface features.
Electrical Resistivity Tomography (ERT) is an advanced geophysics method used to determine the subsurface’s resistivity distribution by making measurements on the ground surface. ERT data are rapidly collected with an automated multi-electrode resistivity meter. ERT profiles consist of a modeled cross-sectional (2-D) plot of resistivity (Ω·m) versus depth. ERT interpretations, supported by borehole data or alternate geophysical data, accurately represent the geometry and lithology and/or hydrology and/or petrology of subsurface geologic formations.
ERT measures resistivity. Resistivity, measured in Ω·m, is the mathematical inverse of conductivity. It is a bulk physical property of materials that describes how difficult it is to pass an electrical current through the material. Resistivity measurements can be made with either an alternating current (AC) or a direct current (DC). As resistivity measurements are frequency dependent, care must be taken when comparing resistivity values collected using different techniques.
Clay materials, metallic oxides, and sulfide minerals are the only common sedimentary materials that can carry significant electrical current through the material itself. As such, the resistivity of most near surface sedimentary materials is primarily controlled by the quantity and chemistry of the pore fluids within the material. Any particular material can have a broad range of resistivity responses that is dependent on the level of saturation, the concentration of ions, the presence of organic fluids (such as non-aqueous phase liquids, NAPLs), faulting, jointing, weathering, etc.
The general principals that ERT is based on have been in use by geophysicists for almost a century. Recent advances to field equipment and data processing procedures have made rapid 2D surveys routine and 3D surveys possible. Old-style 1D resistivity surveys are still common and are useful on many occasions, but encounter interpretation problems in areas of complex 2D or 3D geology.
Ground Penetrating Radar (GPR) is a general term to describe methods that use radio waves to probe subsurface objects or geologic features. GPR is a non-invasive electromagnetic (EM) geophysical technique for subsurface exploration and characterization. Using radar principals, GPR systems transmit impulse electromagnetic energy (i.e. radio waves) into the ground and detect echoes, or reflected wave front energy at surface. This process is somewhat similar to p-wave seismic reflection methods and theoretical similarities exist between the kinematic properties of elastic and electromagnetic wave propagation.
The concept of using radio waves to probe the subsurface is not new and the GPR method evolved from research projects conducted in the Arctic and Antarctic during the late 1950’s where Radio Echo Sounding (RES) techniques were used to determine the depth of ice sheets and glaciers (Bogordsky et al. 1969). The use of radar to provide subsurface ground information began in earnest in the 1970’s as research activity associated with the Apollo 17 Lunar sounder experiments led to many articles being written on theory and feasibility of the GPR technique. The first practical applications of GPR focused on permafrost soil applications (Annan and Davis 1976) and, as the knowledge base for the strengths and weakness’ of GPR became better understood, so did the diversity of the ground conditions and applications for where it was used (Ulriksen 1982).
Today, ground penetrating radar is one of the more commonly used geophysical devices for obtaining shallow ground information. GPR data is relatively quick and inexpensive compared with other methods. Obtained results provide location and depth information for detected/interpreted subsurface features. GPR has the highest resolution of any geophysical method for imaging the subsurface, with centimeter scale resolution sometimes possible.
Induced Polarization (IP) is a geophysical method used extensively in mineral exploration and mine operations. The IP survey is very similar to electrical resistivity tomography (ERT). Resistivity and IP methods are often applied on the ground surface using multiple four-electrode sites. In an IP survey, in addition to resistivity measurement, capacitive properties of the subsurface materials are determined as well. As a result, IP surveys provide additional information about the spatial variation in lithology and grain-surface chemistry.
IP survey can be made in time-domain and frequency-domain mode. In time domain Induced polarization method, voltage decay is observed as a function of time after the injected current is switched off. In frequency-domain Induced polarization mode, an alternating current is injected into the ground with variable frequencies. Voltage phase-shifts are measured to evaluate impedance spectrum at different injection frequencies, which is commonly referred to as spectral IP.
IP method is one of the most widely used techniques in mineral exploration and mining industry and it has other applications in hydrogeophysical surveys, environmental investigations and geotechnical engineering projects.
Seismic Reflection (both on-shore and marine) is the most common geophysical methodology used for oil and gas exploration and exhibits the highest degree of technical sophistication in terms of both data acquisition and signal processing capabilities. Seismic reflection surveys provide either 2 or 3 dimensional imagery of stratigraphic boundaries and geologic structure (e.g. faults, stratigraphic layering, folding) at depths ranging from 100’s of metres to several kilometres in depth.
Seismic reflections occur at boundaries in rock densities which give rise to “reflected” acoustic energy. For shallow ground exploration, seismic reflection can be an effective method in profiling upper bedrock surface depths as well as deeper litho-stratigraphic features. Compared with other shallow geophysical methods however (e.g. GPR, ERT, SR), Seismic Reflection surveys tend to be more expensive because of comparatively higher equipment and labor requirements.
When required, Surface Search Inc. subcontracts conventional seismic data acquisition and signal processing companies to collect Seismic Reflection data on behalf of our clients. We offer full signal analysis and interpretation services for seismic reflection surveys.
Sub-bottom Marine Profiler (SBP) is a geophysical method which is specifically designed to detect and characterize layers of sediment or rock beneath a body of water. SBP provides continuous seismic reflection profiles in real time by the use of an acoustic wave generating source and an array of hydrophones to receive the energy reflected by the various interfaces.
Sub-bottom profilers are typically used in geological and geophysical exploration surveys, marine construction projects, and route surveys for pipeline laying projects.
Electromagnetic (EM) surveys consist of using either time-domain or frequency-domain electromagnetic field generating systems to measure and map changes in the electrical/magnetic properties of the shallow subsurface. For shallow site investigations EM methods are commonly used to map variations in either soil properties or to detect and map the location of buried man-made objects or altered ground. Applications range from mapping the lateral extent of contaminated soils to locating underground features such as storage tanks, drums, pipelines and utilities.