Ground penetrating radar (GPR) is a non-destructive geophysical technique used to determine the cross-sectional profiles of subsurface geological features and objects without digging, drilling or probing. Ground penetrating radar equipment can detect several parameters including depth, size, location, and other characteristics of underground objects.
GPR is used in many industries and is especially valuable in engineering, mining, archaeological, road maintenance, and environmental applications to name a few. (Read The View Underground: Ground Penetrating Radar.)
In trenchless construction, ground penetrating radar equipment is used to determine the characteristics of underground utilities, such as gas lines, water lines, sewers, culverts, etc. The data obtained from GPR methods allow contractors to plan trenchless construction activities with minimal damage to existing underground utilities.
GPR operates by transmitting pulses of radio waves (electromagnetic energy) through the ground surface to create a picture of subsurface features. Ground penetrating radar equipment typically consists of three main elements: a control unit, an antenna, and a survey encoder. The most critical component, the antenna, comprises of a transmitter and a receiver.
The transmitter emits the electromagnetic signal which bounces off of buried objects. The reflected signal is then collected by the receiver which measures and records several parameters including the two-way travel time and the amplitude (strength) of the returning signal.
The data obtained from the receiver is sent to the control unit which registers the reflected information and displays the relevant profile data on the screen in real-time. GPR equipment used in trenchless construction is usually mounted on a trolley, where it can be pulled along the ground by hand or behind a vehicle.
Underground features are identified by the presence of hyperbolic readings on the screen of the control unit. Objects are located at the top of the hyperbola; therefore, the highest band of the GPR data can be used to determine the depth of the object. GPR operators track hyperbolic readings that they suspect belong to a single utility by running the equipment in a series of parallel lines.
If the same hyperbola is observed on the other lines, operators can be confident that an underground utility line exists at the given location.
The depth of penetration of GPR equipment depends on several factors, including the frequency of the antenna used and the type of soil or rock in the survey area. Low-frequency GPR antennas, with frequencies between 100 MHz and 200MHz, can detect subsurface features at depths of up to 100ft or more in low conductivity soil or rock.
Low-frequency antennas, however, produce low-resolution output. High-frequency antennas (300Mhz to 1500MHz), on the other hand, can obtain reflections at shallow depths (up to 14 ft in low conductivity soil or rock) with relatively higher resolutions.
The penetration depth and output resolution of both low- and high-frequency antennas decrease as the conductivity of the soil increases.