Attractive lawns and healthy green spaces are important to homeowners and municipalities alike but accurate imaging and mapping of utilities is essential. Nearly 30 million acres of lawn blankets the United States. This grass not only provides a pleasant view and place for kids to play barefoot, it also enhances the environment by providing oxygen, reducing noise and air pollution, control soil erosion, protects groundwater and balances synthetic and organic compounds.
Most importantly, though, an attractive lawn brings aesthetic value to a home and neighborhood. Homeowners, apartment managers and groundskeepers all cherish a perfect lawn, free from holes and piles of dirt. In fact, two-thirds of homeowners support the idea of lawn and tree care. The mere thought of digging up the lawn to locate utilities is enough to panic these lawn enthusiasts.
SoftDig® Uses GPR for a Small Imaging Footprint
At SoftDig®, we leave the ground just as we found it – no destruction, no despair. SoftDig® uses ground-penetrating radar (GPR), a geophysical locating method that uses pulses of energy to capture images below the surface of the ground in a nondestructive way. We use GPR because it allows us to pinpoint the location of underground utilities without disturbing the ground.
GPR uses electromagnetic radiation in the microwave band. GPR has two main pieces of equipment – a transmitter and a receiving antenna. The transmitter sends electromagnetic energy into the soil or other substrate. If the electromagnetic impulse hits an object or boundary, the density of the object reflects, refracts and scatters the signal. The receiver detects the returning signals and records variations within them. Software within the GPR then assembles these signals into images of objects lying in the subsurface.
Imaging devices look for changes in composition. The major differences between various imaging devices are the changes the devices detect and the way the devices detect the changes. These differences allow the devices to make images in a variety of substrates.
The principles of GPR are similar to those of seismology, except the methods of ground-penetrating radar uses electromagnetic energy rather than acoustic energy of seismic waves. Seismology refraction surveys record signals that bend within the ground and arrive back at the surface. Increasing seismic velocity in the ground, related to the ground’s elastic properties and density, bends these acoustic signals back towards the surface. Seismology refraction is popular for mapping horizontal structures beneath the ground but not very effective for characterizing vertical features.
GPR uses electromagnetic energy in the form of high-frequency radio waves, usually in the range 10 MHz to 1 GHz, which effectively detect changes in electrical properties below the surface. Seismic energy, on the other hand, detects changes in subsurface mechanical properties.
An In-Depth Look at How GPR Works
As with all types of radar imaging, GPR does not work in all conditions. The technology may provide false readings, particularly in coastal areas featuring high salt content in the soil, and in tightly packed clay soils. Rocks can sometimes interfere with GPR location of utilities.
The ground itself can limit how deep GPR signals penetrate, which is usually less than 10 meters. The ground has electrical resistivity, which means it opposes the flow of electric current to some degree.
Dielectric permittivity of the substrate is also a factor. Certain materials can become polarized in the presence of an electric field. Dielectric permittivity is the ease with which materials become polarized. The quantity of water present in the material greatly affects dielectric permittivity.
GPR works by sending a tiny pulse of energy into the ground then recording the time it takes for reflected signals to return to the receiver, and recording the strength of those signals. A scan consists of a series of pulses over a single area. As the energy pulse enters a material with dielectric permittivity or other electrical conduction properties different from the one it left, it produces a reflection. The strength, or amplitude, of the signal is the result of the contrast in the dielectric constants and conductivities between the two materials. A pulse moving from wet sand to dry sand will produce a very strong reflection, for example, in comparison to the relatively weak reflection produced by moving from dry sand to limestone.
While some of the GPR energy pulse reflects back to the receiving antenna, some energy continues to travel through the material until it dissipates or the scanning session simply ends. The rate of signal dissipation varies widely, depending on the properties of the materials.
GPR can have applications in a variety of media, including all types of soil, rock, ice, fresh water, pavements and even structures. Concrete scanning GPR can detect embedded utility conduits and structural elements in concrete floor and wall slabs up to 20 inches thick. SoftDig® employs both standard GPR units to detect utility lines up to depths of about 10 ft., and a concrete scanning GPR.
SoftDig® provides exceptional imaging results for the location of utilities across a variety of media – and without ever disturbing the ground. If you have a job that could benefit from GPR, contact us today at (800) 545-1531 or use our online estimate request form.