Geophysical surveys can be the bedrock (pardon the pun) of today’s environmental projects, from locating abandoned underground storage tanks (USTs) and utilities, to complex mapping of geology in remedial investigations, to finding landfill boundaries and other buried unknown problems. In the past few decades, a variety of non-destructive testing methods have gained in popularity over expensive and time-consuming drilling and digging for environmental projects. Among these, the method of pairing ground penetrating radar (GPR) with electromagnetic (EM) induction instruments is one that shows great promise in significantly reducing survey time and costs.
In the past, most environmental scientists and geologists relied on destructive technologies, including drilling and excavating test pits. Depending on the site (and project budget), a survey may require drilling or digging one or two holes for a small site, or as many as 20 to 30+ holes for large sites. On average, each borehole into the ground on an environmental site costs about $5,000 to $10,000, so costs for drilling or soil sampling can be very high. And not only are these methods slow and costly, they merely produce point measurements, rather than a continuous profile.
In response to the expense and safety of destructive surveys, and concern about the accuracy of relying on point measurements, companies have more recently come to rely on a variety of other nondestructive survey methods.
Chief among these is ground penetrating radar (GPR), which works by sending a tiny pulse of energy into a material via an antenna. A computer records the strength and time required for the return of any reflected signals. Subsurface variations will create reflections that are picked up by the system and stored on digital media. These reflections are produced by a variety of materials, including geological structure differences and man-made objects like pipes and foundations. GPR is considered the most accurate, highest resolution geophysical technology.
In general, GPR works best in dry sandy soils with little salt content, but dense clay-based soils are difficult to penetrate with GPR. In some situations, penetration depth may be limited to a few feet or less within clays, whereas targets residing in sandy soils could be detected at depths of 30 feet or more. The photo on page 19 shows a GPR unit surveying the location of buried tanks at a gas station and looking for other underground obstructions.
A GPR survey can be done at a cost of $1,000 to $2,000 per day, which means one can cover an entire site with GPR for less than the cost of a single borehole. In light of these clear cost advantages, GPR is now often the preferred method on environmental and construction sites. Instead of boring three to four holes, companies can bore one hole, then use GPR to match the results and correlate data across the remainder of the site.
Another tool for the measurement of subsurface conditions is use of the seismic refraction method, which requires a seismic energy source, trigger cable (or radio link), geophones, geophone cable, and a seismograph. Seismic equipment is useful for finding larger features such as soil layers and bedrock depths, especially when deeper information is required. It works well in clay soils, where GPR is not effective, but it is quite time consuming. To set up and collect the data and then analyze it, you may only collect two to four single transects per day, which gives you vertical cross sections into the ground at those locations. By comparison, with GPR one could collect data using 1.5-metre spacing in two directions and cover two hectares per day in the same amount of time.
Along with seismic refraction, a different tool widely used for mapping the depth of soils and rock is electrical resistivity imaging (ERI), which involves placing stakes in the ground and measuring electrical resistance. This tool also works well in clay soil. However, similar to seismic equipment, it takes longer and costs more to get the required data coverage. Technicians must set up a row of about 24 to 48 sensors (metal stakes) along the ground typically in a straight line. The line can be as long as required, but you are only getting the information along that one line. One can collect 80 or more profiles of similar length with GPR in the same time it takes to collect three to four profiles using this technique.
Magnetometers measure the strength and sometimes the direction of a magnetic field. By detecting irregularities in the earth’s magnetic field, a magnetometer can indicate the location of old tanks and drums, but only those that are made of ferrous material; they won’t locate plastic or concrete utility pipes or fibreglass tanks. Some types of magnetometers, also known as pipe and cable locators, feature a transmitting wand that is waved back and forth over the ground’s surface looking for a signal. It does a good job of finding ferrous objects but does not provided accurate depth information like GPR.
Also useful as a reconnaissance technique is the use of electromagnetic induction (EM or EMI) devices, which are based on the measurement of the change in mutual impedance between a pair of coils on or above the earth’s surface. Most EM instruments are comprised of two or more sets of coils. These coils are electrically connected and are separated by a fixed distance. EM devices can simultaneously examine soil conditions and locate objects found beneath the surface of the earth spatially, but do not provide good depth information.
One of EM’s limitations is that it cannot be used in close proximity (1.5 to six metres), depending on manufacturer) to aboveground obstructions like buildings, cars, and fences. This makes it less useful for smaller urban sites like gas stations, where there tend to be numerous aboveground obstructions.
GPR gaining in popularity for geophysical surveys
Among all these options, GPR equipment has become considerably more popular in the last 10 years for environmental projects. It is commonly used for locating old USTs, oil tanks, and gas tanks, as well as 189-litre waste drums filled with chemicals that were routinely dumped on sites in the 1970s and 80s. It is also an important tool for mapping utility and product lines, old landfill boundaries, debris pits, buried environmental targets, or waste. Finally, GPR is used in remediation investigations to map soil layers and depth to top of water table or bedrock. Contaminants mainly pool either on top of the water table or bedrock, so environmental scientists need to map changes in these features to plan their borings.The upsurge in GPR’s popularity is largely driven by cost and safety – it is far cheaper, and much safer, to do a quick geophysical survey than drill numerous holes in the ground at a significantly higher cost. Cost has come down relative to other technologies, and it is easier to use. Older GPR units required a trained geophysicist to operate – with today’s equipment, users can virtually push a button and start scanning.