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GPR Geophysical Services
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The following page gives a brief description of these geophysical methods:

 

G.P.R. (Ground Penetrating Radar)

 

Ground Penetrating Radar (GPR) employs low energy radio waves with typical frequencies from 40Mhz to 1000MHz. Depth of penetration is controlled during each survey and can be set from 200mm to 20m depending on site conditions and the particular information required.

The equipment is light-weight, portable, fully digital and controlled by a small field computer using its own 12V power source. It is normally deployed from the back of a 4wd vehicle but can equally easily be carried onto remote or difficult access sites.

GPR is capable of identifying a wide range of subsurface conditions and/or targets. Detectable objects can range in size from around 10mm up to underground cavern proportions. All types of subsurface materials, ferrous and non-ferrous, can be detected and accurately plotted.

The results are available in real time and thus usually able to be reviewed on site at the time of investigation. Collected data can also be stored on the equipment hard drive for later analysis and report preparation.

 

Benefits:

• Cost effective, fast efficient and accurate
• Non-destructive and non-invasive
• Extremely portable and environmentally neutral.
• On-site real time data colour display

 

 

 

Capabilities: GPR is capable of mapping all types of underground services, buried objects, voids, soil stratigraphy, subsidence, leaking pipelines, gravesites etc. It can also be used to detect reinforcing steel in concrete and/or voids under concrete slabs.

GPR can be used for environmental investigations - leachate and contaminant plumes, leaking tanks, sinkholes, forensic and archaeological sites, pavement construction, and many others.

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E.M (Electromagnetic)

 

There are two types of Electromagnetic Induction (EMI) techniques commonly used. These are frequency-domain (FEM) and time-domain (TEM).

1. EMI - Frequency Domain

The EMI (FEM) instrument normally operates at a fixed frequency. The generated signal creates small subsurface eddy currents beneath the instrument. Automatic accurate measurements of the components of the generated subsurface eddy currents are then used to record changes in the subsurface soil conditions.

An example of EMI (FEM) operating at 9,8kHz is the Geonics EM31 (left).

Very small - milli Siemens /metre (mS/m) changes in soil electrical conductivity/resistivity can be detected and recorded. The data is gathered by simply moving the equipment over the site under investigation on a pre-determined grid and no direct contact with the ground is necessary.

Benefits:

• Cost effective - large areas covered quickly and easily.
• Non-destructive and non-invasive
• Extremely portable and environmentally neutral.
• On-site real time data allows some pre information.

Capabilities: EMI (FEM) is capable of detecting and mapping any contaminant plume that is causing even a small change in soil conductivity/resistivity. It can be used to find buried dumpsites, leachate plumes, voids or tomos, underground streams and aquifers and buried metallic/magnetic objects.

2. EMI - Time Domain

The other type of EMI instrument in common use is time-domain EMI (TEM) or “transient EM”. A short low energy electromagnetic pulse from the transmitter coil couples with the ground by and a receiver coil measures the decaying signal induced into the ground with respect to time.

This technique allows very sensitive detection of shallow and deep buried metal objects.

A typical example of EMI (TEM) is the Geonics EM61 (left). In addition to recording the on-site data, the EM61 produces an audible alarm when it crosses a potential target. On completion of each survey the dataset is downloaded onto a PC and a map prepared showing location and approximate size of each detected target.

Benefits:

• Cost effective - large areas covered quickly and easily.
• Non-destructive and non-invasive
• Extremely portable and environmentally neutral.
• On-site real time data allows some pre information.

Capabilities: EMI (TEM) is capable of detecting and mapping the location of both ferrous and non-ferrous buried metallic objects, large or small. It is capable, for instance, of detecting a buried 44gallon drum at a depth of 3m.

The EM61 is currently used by the armed forces and geophysical consultants throughout the world for UXO (Unexploded Ordnance) detection.

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Resistivity

 

Soil Resistivity measurements can be taken using contact electrodes which are in direct contact with the ground.

One of the simplest configurations is the four-electrode Wenner array. Here two electrodes are used to pass an electric current through the ground, and the other two measure the created voltage or potential difference. By varying the electrode separation and location information can be obtained on subsurface changes in soil resistivity with position and depth.

Multi-electrode resistivity surveys using modern equipment allow this basic principle to be automatically and rapidly applied over a large area investigating both shallow and deep soil conditions. As an example soil resistivity information collected in this manner can be used to enhance the results of a previous electromagnetic induction (EMI) survey resulting in a more complete picture of subsurface conditions at the site under investigation.

Benefits:

• Large sites can be investigated.
• Deep soil conditions determined.
• Non-destructive and non-invasive
• Portable and environmentally neutral.

Capabilities: Multi-electrode resistivity surveys allow depth and volume information of closed landfill or similar dumpsites to be determined. Another example would be to determine the extent of saline intrusion at coastal sites.

 

Seismic

 

Seismic surveys, in a similar manner to ground penetrating radar, are based on the principle of energy wave propagation with consequent reflection and refraction through subsurface soil strata.

A low frequency seismic energy source is generated – for shallow surveys,( less than 50m) striking a metal plate placed on the ground with a sledgehammer is commonly used. More complex explosive energy sources are normally needed for deeper surveys.

The energy wave propagating through the subsurface materials, changes velocity at material density boundaries. Energy is reflected or refracted at these boundaries due to the soil/rock density changes. Geophones placed on the surface detect the returning energy waves and the information used to determine subsurface soil strata and rock formation.

While ground penetrating radar and seismic methods are both based on wave propagation principles it is worth noting that GPR is sensitive to changes in dielectric permittivity while seismic detects changes in subsurface material density.

Seismic refraction surveys are commonly used for shallow investigations such as engineering, bedrock contour delineation and environmental studies.

Benefits:

• Simple to set up and carry out.
• Provides information on subsurface material structure.

Capabilities:
Seismic Refraction can be used to plot the depth and contour of underlying bedrock , detect underground caverns, delineation of sand and gravel layers.

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Magnetic Surveys

 

Magnetic surveys are based on measuring and plotting changes in the strength of Earths natural magnetic field. It is the oldest branch of geophysics and has been studied since ancient times.

The most commonly used instruments are proton magnetometers. They operate on a relatively simple principle using a sensor filled with a hydrogen rich liquid like water surrounded by a polarizing coil. The polarizing coil is used to displace the protons out of earth’s magnetic alignment. When the coil is switched off the precession frequency of the protons is recorded and is directly proportional to the strength of earth’s magnetic field at that point.

Gradiometer magnetometers employ two sensors, one above the other, to suppress unwanted global noise effects and to enhance shallow anomalies.

Benefits:

• Large sites can be investigated.
• Simple to set up and carry out.

Capabilities: Archaeological investigations such as location of buried ruins, landfill investigations, detection of buried drums and casings.

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Gravity

 

Variations in gravity field resulting from differences in rock densities may be measured using gravity metres, or gravimeters. Most gravimeters utilise an astatic spring system. These systems generally consist of a zero length spring, in which the tension is proportional to the actual length, and a measuring and/or an auxiliary spring. This diagram illustrates the basic general design of gravimeters.

Data collected by gravimeters are corrected for various effects such as latitude, topographic, etc. Used in conjunction with complementary datasets, such as resistivity and/or EM, gravity data can reveal the structure and properties of the subsurface strata.

The interpretation of gravity data may be used to map sedimentary basins, aquifers, ore-bodies, and various geological structures.

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