Utility Locating and Concrete Inspection Services
Electromagnetic Induction (EMI) is the most widely used technology for locating buried utility services, and is very effective in most soil types. With EMI, you can locate and trace a facility, as well as estimate its depth. Electromagnetic Induction consists of two steps. First, a transmitter is used to transfer an alternating electrical current to the pipe or wire to be located. Next, a receiver is used to analyze the transmitted signal, and localize the position and depth of the facility. The transmitter can transfer the signal to the facility either by a direct connection, or by inducing a signal.
The direct connect method introduces a signal into pipes or cables (or the fluids within pipes) that is radiated from the facility to aid its detection and location. For example, a metal pipe may be used to complete the AC circuit and the resulting electromagnetic field generated is used to locate the pipe. This method requires the utility to be known in advance, and the utility to be accessible at various locations so that the signal can be introduced to the line. The surface-induced method generates a signal at the ground surface that will induce a response in the cable, pipe, or tracer wire underground. For example, the creation of a fluctuating electromagnetic field into the ground will induce a current in a metal pipe. The field due to the induced current can be used to localize the pipe. Unknown pipes can be located using this technique. Also, no direct connection to the pipe is required. Electromagnetic Induction is very complementary to Ground Penetrating Radar.
Limitations of EMI Technology
There are certain limitations involved with EMI technology. First, EMI cannot induce a signal in a non-metallic pipe. Second, EMI signals generally cannot travel down a broken tracer wire or if the metal pipe does not have good metal-to-metal contact. Inducing a higher frequency has the potential to jump over a broken tracer wire, but higher frequencies also bring the risk that the signal may jump (or bleed) over to another nearby underground facility.
Ground Penetrating Radar (GPR) is a geophysical method that uses radar pulses to image below the surface. This non-destructive technique makes use of electromagnetic energy in the UHF and VHF bands of the radio spectrum by detecting reflected signals from subsurface structures. The transmitting antenna of a GPR system radiates short pulses of high-frequency radio waves into the ground. When the wave hits a buried object or a boundary with different dielectric constants, the receiving antenna records a variation in velocity and amplitude within the reflected return signal. The principles involved are similar to reflection seismology.
The depth range of GPR is limited by the electrical conductivity of the medium, and the transmitting frequency. High frequencies do not penetrate as far as lower frequencies, but higher frequencies give better resolution. Optimal depth penetration is achieved in dry sandy soils or massive dry materials such as granite, where the depth of penetration is up to 50 feet. Radar wave travel time is the only direct measurement obtained using GPR equipment in the field. Using an estimate of wave propagation velocity through the ground, the depth (or distance) to buried interfaces or features can be reliably calculated. Often, direct measurements to buried objects are used to calibrate the depth estimates.
Limitations of GPR Technology
There are certain limitations involved with any technology. Radar waves cannot penetrate through metal. Thus, if steel storage tanks are present on the site, GPR will not be able to see inside them, nor assess their condition. Further, objects behind steel may be obscured and thus not observable. Radar performance is driven by the characteristics of the sensor, the targets, and the competing background clutter. An object may be observable at a shallow depth, but the same object may be invisible at a greater depth.
High Resolution Concrete Radar scanning and imaging services is now offered at Consumer Markouts, to quickly locate and document what is beneath concrete to help contractors avoid costly mistakes when cutting concrete. Often contractors refer to concrete scanning as concrete x-ray but this is not the correct term. We use Ground Penetration Radar (GPR) equipment which uses radio waves that are safe and not gamma rays used in concrete x-ray equipment. GPR equipment is used on one side of a floor, wall or ceiling, where x-ray equipment requires access on both sides.
Drilling or cutting through concrete floors, walls or ceilings can be dangerous and very costly if you don’t know what is under the surface. Our GPR technicians can accurately and quickly locate rebar, find conduits in concrete structures and identify post-tension cables before it’s too late. It is essential to research what is embedded in concrete before cutting or coring concrete slabs, walls, ceiling, roofs and other structures. Every year contractors spend many thousands of dollars unexpectedly repairing damaged conduit or cables, replacing expensive drill bits and replacing concrete that they have damaged.
Consumer Markouts use the latest technology in High Resolution Ground Penetrating Radar (GPR) Systems. The portability of modern Radar Imaging systems allows us to move easily about the job site and record a large amount of data in a short period of time.