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Ground Penetrating Radar (GPR)

Ground Penetrating Radar (GPR) is a non-destructive technology that works by transmitting high frequency radio waves into the ground or structure and analyzing the reflected energy to create a profile of the subsurface features. The reflections are caused by a contrast in the electrical properties of subsurface materials which can be indicative of changes in lithology, water content, void spaces in the ground, rebar or post tension cable corrosion, asphalt deterioration or manmade features such as foundations, pipes and cables, storage tanks, graves, etc.
 
The radio wave is fired from the transmitter at regular intervals as the GPR unit is moved along a survey line.  For each transmit pulse the resulting reflected energy from the subsurface is manifested as a single radar trace. These individual traces are stacked side by side as the data is collected to build up a cross-sectional image or profile of the subsurface.  In the typical graphical presentation, strong, high amplitude reflections are given darker shading; these reflections are indicative of interfaces exhibiting strong contrasts in electrical properties. Weaker reflections (lower reflection contrasts) are manifested on the profile by lighter, grey-scale tones.

High frequency GPR

High frequency GPR antennas (1000 MHz or greater) are most commonly used for scanning concrete structures to detect the location of embedded structural elements (rebar, post-tensioned cables, beams) and embedded utilities (primarily electrical lines, telecom/security cabling).
 
Maximum penetration depth in concrete is generally in the 12-16” range (30-40 cm) for the 1000 MHz system. The low penetration depth is the main limitation of this frequency – if slabs are greater than 16”, scanning from both sides may be necessary.  High-frequency GPR is not effective in wet or uncured concrete, or in concrete containing metal shavings.
 
Applications for High Frequency GPR

  • Cutting and coring applications (safety, object avoidance)
  • Detecting defects in concrete structures (delamination, cracks, voids, spalling)
  • Indoor utility detection and mapping
  • Detecting voids below concrete floors

 
Low frequency GPR 

Low frequency GPR is generally used for geologic investigations and for finding large structures buried at depths greater than 3m.  The targets are either laterally extensive, flat-lying to sub-horizontal reflectors such as bedrock interfaces or stratigraphic horizons, or large structures such as industrial process pipes and sewers.  The equipment for these types of surveys is modular in nature and can be moved manually in rough terrain or mounted on a cart in smoother terrain.  
 
The antennas in these systems are not shielded. Thus, the radar wave can also radiate through the air and reflect off poles, buildings, trees etc.  These airborne reflections can introduce a lot of interference into the radar profile; fortunately, these reflections have a distinctive slope and can be minimized through special processing techniques.

For more on Ground Penetrating Radar please view the following video from our valued equipment supplier, Sensors & Software Inc.



Electromagnetic Induction (EM)

multiVIEW uses electromagnetic (EM) induction tools to locate EM fields that are induced onto an underground utility for utility locating. Types of signal application techniques include:

  • Straight Induction: No connection to utility hardware, the transmitter is placed over the position of the suspected utility and the field is induced onto the target line by “spilling” the electromagnetic field through the earth and onto the utility
  • Direct Connect: A signal lead cable from the transmitter is attached to a piece of utility hardware that is connected to the target line.  A ground lead attached to a grounding stake allows the signal to return to the transmitter, thus setting up an electric circuit (this is required since without a complete circuit, no field is generated around the pipe or cable).
  • Induction Clamp: The clamp is placed around conduits or exposed cables and is generally used for tracing electrical or telecom cabling. Coils of wire in the clamp are energized by the transmitter and produce a magnetic field that is transferred onto the cable. The cable being traced must be fully grounded at both ends of the line in order for this method to be effective.

 
Borehole Geophysics

Borehole geophysics is the science of recording and analyzing measurements of physical properties made in wells or test holes. Probes that measure different properties are lowered into the borehole to collect continuous or point data that is graphically displayed as a geophysical log. Borehole geophysics is used in ground-water and environmental investigations to obtain information on well construction, rock lithology and fractures, permeability and porosity, and water quality. The geophysical logging system consists of probes, cable and drawworks, power and processing modules, and data recording units. State-of-the-art logging systems are controlled by a computer and can collect multiple logs with one pass of the probe.

Borehole-geophysical logging can provide a wealth of information that is critical in gaining a better understanding of subsurface conditions needed for ground-water and environmental studies. Geophysical logs provide unbiased continuous and in-situ data and generally sample a larger volume than drilling samples. It is an ideal testing method for delineating hydrogeologic units, defining ground water quality and determining well construction and conditions.


Time & Frequency Domain Electromagnetics

Time domain electromagnetics is a geophysical technique that is used to detect geological conductors, airborne anomalies, examine Brownfields, map intrusions and uncover a wide range of minerals, conductivities and environments.

The method works by employing a transmitter that drives an alternating current through a square loop of insulated electrical cable laid on the ground. The current consists of equal periods of time-on and time-off, with base frequencies that range from 3 to 75 Hz, producing an electromagnetic field. Termination of the current flow occurs over a very brief period of time (a few microseconds) known as the ramp time, during which the magnetic field is time-variant.

The time-variant nature of the primary electromagnetic field creates a secondary electromagnetic field in the ground beneath the loop that is a precise image of the transmitter loop itself. This secondary field immediately begins to decay, in the process generating additional currents that propagate downward and outward into the subsurface like a series of smoke rings. Measurements of the secondary currents are made only during the time-off period by a receiver located in the center of the transmitter loop.

The depth of the investigation depends on the time interval after shutoff of the current, since at later times the receiver is sensing currents at progressively greater depths. The intensity of the currents at specific times and depths is determined by the bulk conductivity of subsurface rock units and their contained fluids. Depending on subsurface resistivity, current induced, receiver sensitivity and transmitter-receiver geometry, TEM/TDEM measurements allow geophysical exploration from a few metres below the surface to several hundred metres of depth.

Seismic Reflection/Refraction

Seismic refraction involves measuring the travel time of the component of seismic energy which travels down to the top of a rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface as a head wave along a wave front. The shock waves which return from the top of rock are refracted waves, and for geophones at a distance from the shot point, always represent the first arrival of seismic energy.

Seismic refraction is generally applicable only where the seismic velocities of layers increase with depth. Therefore, where higher velocity (e.g. clay) layers may overlie lower velocity (e.g. sand or gravel) layers, seismic refraction may yield incorrect results. Seismic refraction is commonly limited mapping layers only where they occur at depths less than 100 feet.

Greater depths are possible, but the required array lengths may exceed site dimensions, and the shot energy required to transmit seismic arrivals for the required distances may necessitate the use of very large explosive charges. In addition, the lateral resolution of seismic refraction data degrades with increasing array length since the path that a seismic first arrival travels may migrate laterally off of the trace of the desired seismic profile. Recent advances in inversion of seismic refraction data have made it possible to image relatively small, non-stratigraphic targets such as foundation elements, and to perform refraction profiling in the presence of localized low velocity zones such as incipient sinkholes.

Seismic reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts. Reflected seismic energy is never a first arrival, and therefore must be identified in a generally complex set of overlapping seismic arrivals.  Therefore, the field and processing time for a given lineal footage of seismic reflection survey are much greater than for seismic refraction.  However, seismic reflection can be performed in the presence of low velocity zones or velocity inversions, generally has lateral resolution vastly superior to seismic refraction, and can delineate very deep density contrasts with much less shot energy and shorter line lengths than would be required for a comparable refraction survey depth.

Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) where the inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land.