Ground-Penetrating Radar VS. Electromagnetic Locating

Ground-Penetrating Radar (GPR) and Electromagnetic (EM) Locating:

The two procedures of underground locating both operate on the electromagnetic spectrum. However, their behavior can be very different.  Some locating companies will tell you that one method is superior to another.  This is not the case.  We here at Trinity Subsurface use a variety of tools to perform a utility locate.  If used properly, these tools complement one another, helping us to perform a thorough comprehensive locate.

Ground-Penetrating Radar (GPR

This Geophysical method that uses Radar pulses to produce an image of the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, asphalt, metals, pipes, cables, or masonry. This Method uses a form of electromagnetic radiation in the Microwave band of the radio spectrum.  A GPR device emits and detects reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, pavements, and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids.

GPR uses high-frequency (usually polarized) radio waves, usually in the range 10 MHz to 2.6 GHz. A GPR transmitter and antenna emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different dielectric properties, it may be reflected or refracted or scattered back to the surface. A receiving antenna can then record the variations in the return signal. 

The electrical conductivity of the ground, the transmitted frequency, and the radiated power all may limit the effective depth range of GPR investigation. Increases in electrical conductivity attenuate the introduced electromagnetic wave, and thus the penetration depth decreases. Because of frequency-dependent attenuation mechanisms, higher frequencies do not penetrate as far as lower frequencies. However, higher frequencies may provide improved Detail and resolution. Thus, operating frequency is always a trade-off between resolution and penetration. Dry sandy soils or massive dry materials such bedrock, and concrete tend to be resistive rather than conductive, and the depth of penetration could be up to 49 ft. However, in moist or clay-laden soils and materials with high electrical conductivity, penetration may be as little as a few inches.

Ground-penetrating radar Antennas are generally in contact with the ground for the strongest signal strength; however, GPR air-launched antennas can be used above the ground.

Electromagnetic (EM) Instruments.

EM instruments work on the principle, discovered by Michael Faraday in 1831, that an electric current can be converted to a magnetic field and the same field can be converted into an electric current.  EM instruments can be either digital or analog.  They work by inducing a small electric current along a conductive pipe or conduit in the ground.  This current creates a magnetic field that, in turn, can be detected on the surface with a specially designed receiver. This is similar to the way a radio picks up a transmission from an antenna miles away. Electro-Magnetic locators can only find Metallic objects in the ground.  

Utilities such as PVC pipe, some types of Fiber-optic cable, Clay pipe, and HDPE pipe are not locatable using EM methods as they are not conductive.  There are ways to locate nonmetallic lines however.  This usually involves inserting a locatable rod or fish tape into the conduit and locating that instead.  Locating Technicians at Trinity subsurface Engineering carry a full suite of instruments to every job.  This enables them to overcome most, if not all, limitations commonly found in the field.  

 

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