A Brief Description of Common Surface Geophysical Methods
GTS uses an 8-channel Advanced Geosciences SuperSting to inject automatically an electrical current into the ground between all various pairs of electrodes and then measures the loss in potential (i.e. the resistivity) between various other electrodes. The number of electrodes, the spacing of the electrodes, and the total line length determine the depth of penetration and the minimum size of detectable features. The electrode array is typically laid out as a single long profile, but can be laid out as a two-dimensional grid of electrodes, which can then be used to create a three-dimensional subsurface profile.
After all the measurements are acquired automatically by the SuperSting, a geo-electric resistivity profile is constructed with advanced computer software, and interpreted to reveal subsurface features of interest.
Please click on the links below to see an example of electrical resistivity profiling.
Electrical resistivity is commonly used for:
Locating faults, fractures, fissures
Monitoring groundwater and soil pollution
Mapping building extent for archaeologists
Locating groundwater and mapping the water table
Locating subsurface cavities and abandoned mine shafts
Mapping clay layers, gravel deposits, sand deposits, and lithology
Terrain conductivity is the inverse of electrical resistivity. While electrical resistivity provides a more detailed view of the subsurface, terrain conductivity is a faster method to gather bulk areal data. In many cases, both methods are used to provide complementary data.
Terrain conductivity is determined by inducing an electromagnetic current with a transmitter coil on one end of the instrument and then measuring the secondary electromagnetic field, created by the first transmission, at a receiver coil on the other end of the instrument. The strength of the secondary field signifies the conductivity of the subsurface. At GTS, an EM-31 meter is often used in evenly spaced, parallel lines to provide a detailed "picture" of the subsurface.
Please click on the link below to see an example of how terrain conductivity can be utilized.
Terrain Conductivity: General Concepts
Terrain Conductivity is commonly used for:
Locating buried trenches
Locating shallow mineral deposits
Conducting saline intrusion surveys
Locating and monitoring shallow contaminant plumes
Locating underground storage tanks (USTs), pipes, and other buried metal objects
Delineating karst features or approximate bedrock depth for land development
Very Low Frequency
Very low frequency (VLF) waves are transmitted by high-powered military radio transmitters around the world and can be used by geophysicists quite far from the transmitters themselves. Very low frequency surveys are generally used for spatial mapping of subsurface anomalies rather than for depth determination as depth penetration is limited to a maximum depth of approximately 100 feet.
Please click on the link below to see an example of a VLF survey.
Very low frequency is commonly used for:
Mapping conductive minerals
Locating water-bearing fractures
The spontaneous potential (SP) method has been used for many years and is one of the simplest and cheapest geophysical techniques. Unlike resistivity, which actively injects a current into the ground, SP is a passive method, which measures the differences in ground potential between two points on the surface where porous-tipped electrodes have been placed. Potentials may range anywhere from one millivolt to over one volt and can be either positive or negative values.
Please click on the link below to see a case study where the spontaneous potential method was implemented.
Spontaneous potential is commonly used for:
Locating metal utilities Geothermal investigations
Delimiting spring catchment areas
Detecting and delineating sinkholes
Mapping zones vulnerable to pollution
Locating groundwater, aquifers, and aquitards
Delineating shear zones and near surface faults
Locating leaks in dams and leachate leaks around landfills
To use seismic refraction, energy must be expended at the surface by dropping a weight, hammering a metal plate, or exploding a small charge. Then, a linear array of evenly-spaced geophones record the travel time for the seismic waves to penetrate the subsurface and to return to the surface after encountering a layer with a different velocity. This timing indicates the velocity and the depth to the refracting layer.
Seismic refraction is commonly used for:
Determining depth of landfills
Mapping top of rock and amount of overburden, bedrock structure, and bedrock quality
Determining bedrock rippability by heavy excavation equipment
Ground Penetrating Radar (GPR)
A ground penetrating radar (GPR) system consists of a transmitter, a transmitting and receiving antennae, a receiver, and an onboard computer. The antennae transmits a series of radio waves into the subsurface in a broad beam and then receives some of these waves after they are reflected back by a subsurface feature. The resultant radargram can reveal subsurface anomalies plotted with distance and depth along the transect.
Please click on the links below to see case studies where GPR was implemented.
Ground penetrating radar is commonly used for:
Inspecting architectural facades
Delineating landfills and burial trenches
Locating existing and potential sinkholes
Locating pipes, utilities, underground storage tanks (USTs) and drums
Inspecting concrete and rebar structures (highways, bridges, sewers, railbeds, airport runways)
Locating subsurface features for archaeological and forensics investigations (graves, hearths, soil disturbances)
Detecting unexploded ordinance (UXO)
Locating microphones in concrete walls
Locating tunnels, arms caches, and graves
Underground Storage Tanks
Magnetometers can be used to locate buried ferromagnetic objects because these objects produce localized anomalies in the earth's magnetic field. Although it is fairly simple to conduct a magnetic survey, ferromagnetic objects at the surface such as fences, cars, and buildings produce their own magnetic anomalies which can mask anomalies coming from subsurface objects, so careful interpretation is a necessity.
Please click on the link below to see a case study utilizing magnetometry along with ground penetrating radar.
Magnetometry is commonly used for:
Underground Storage Tanks
Vibrations can be monitored using one or more accelerometers which are designed with a small mass and a stiff spring to record waveforms in terms of velocity, acceleration, and amplitude. Vibration studies can monitor the current effects and predict the future effects of a vibratory source on surrounding structures.
Vibration monitoring is commonly used for:
Determining effects of traffic-induced vibrations on historic structures
Soil or ground resistance testing requires a linear array of four equally-spaced electrodes along with a ground resistance tester and wire. The ground resistance tester emits a current between the two outer electrodes, and the drop in potential (i.e. the resistance) is measured between the two inner electrodes. The distance between pins determines the depth at which the soil resistance is tested. Soil or ground resistance testing is commonly used to design the size, shape and depth of grounding rods or grids for buildings or utilities, or for determining soil corrosivity.
Soil resistivity is commonly used for:
Determining the degree of corrosion potential in a soil
Soil Resistance Testing: Basic Concepts
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