September 3, 2015
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 are gaining 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-plus 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 non-destructive survey methods.
Chief among these is GPR, which works by sending a tiny pulse of energy into a material via an antenna. An integrated 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 ft or more. Figure 1 shows GPR units looking for underground obstructions while surveying the location of buried tanks at a gas station.
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 and 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 basically gives you vertical cross sections into the ground at those locations. By comparison, with GPR one could collect data using 5-ft spacing in two directions and cover an acre 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 however 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 two 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 fiberglass 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 proximity (5 to 20 ft, 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.
Figure 2 shows technicians using GSSI’s Profiler EMP-400 frequency domain, EM profiling system. The Profiler, which can get within 5 ft of aboveground obstructions, can be used to both locate metallic targets and also measure soil conductivity – even at tighter urban sites.
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 50-gal waste drums filled with chemicals that were routinely dumped on sites in the 1970s and 1980s. 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.
GPR + EM = $avings
On some sites, there is a definite cost advantage to combining use of EM with GPR. On smaller projects, like locating tanks at gas stations, GPR and/or pipe and cable locators can easily get the job done alone. But for larger multi-acre projects, for example, multiple areas at old factory sites or large governmental cleanup sites, there can be huge cost-savings by using EM first and then focusing the GPR only on “hot spots” or anomalous areas located using the EM.
There are three main reasons EM is a good first tool on large sites: you can collect data much faster; the device does not have to be in contact with the ground (or as close to it) as does GPR; and the EM device’s scanning swath is slightly wider.
Because the EM is carried, rather than pushed or dragged like a GPR device, users can walk faster and maneuver around obstacles more quickly. It is far easier to collect data with the carried EM on sites where there may be overgrown grass, tall weeds or rocks. Also, an EM can very easily be adapted to be pulled behind an ATV with a trailer or sled setup — again making data collection faster. Most EM systems also have either built-in or plug-in global positioning systems (GPS), so one does not have to waste time setting up a physical grid (measuring, spray paint, pin flags, and the like).
After using an EM device as the first, fast survey, the user quickly plots that data (this can be done in the field on a laptop in about 10 minutes) and looks for anomalies, targets, and/or potential soil issues. EM cannot provide exact information on the target’s depth, shape, and orientation but the data is easy to view, process, and even immediately overlay on maps such as Google Earth. GPR surveying is narrowed to only those target areas found with the EM, and the GPR is used to provide information on depths, size (dimensions/shapes), and orientation of targets in either 2D or 3D imaging.
This combo EM/GPR surveying method saves a lot of time and money on environmental and construction sites when looking for old USTs, illegally disposed of or buried 50-gal drums, old foundations, possible debris pits or former landfills, other various types of targets, and even some contaminant plumes.
Dealing with constantly varying soil or aboveground conditions from site to site is another reason why it pays to have both types of equipment in the company “tool bag.” For example, EM works much better (images deeper) in clay soils than the GPR, and GPR works much better (images deeper) in sandy soils than the EM. Also, as noted, an EM system cannot be used within about 5-plus ft of cars, buildings, and fences, whereas the GPR has no issue with these features, so having both available would provide more site coverage.
Users often bring both tools to an unknown site; if the soils are overly clayey they can mainly use EM, and if the soils are very sandy they mainly use GPR. If conditions lay somewhere in the middle they may want to use both tools. In this way, they are gathering multiple data sets and optimizing their potential results for their clients, along with reducing their liability and improving their confidence that they are locating everything on site. Finding an anomaly or target in both EM and GPR would be considered the equivalent of a geophysicist’s homerun!
Lastly, each tool may be better at locating a specific target type — GPR can find both metallic and non-metallic objects, whereas EM is mainly good at finding metallic objects. EM is good at mapping soil changes (either geologic or disturbances such as UST graves), mud pits, and potential contaminant plumes, whereas it is tougher to identify those items with GPR. In some instances, GPR can have difficulties with corrugated metal drain pipes because of the design and the way the GPR energy is scattered, whereas EM works well finding such large metal targets. Therefore, using both tools together offers a higher likelihood of success in finding everything the client is looking for (or not looking for).
The cost benefit of pairing the two tools is based on the simple fact that EM is five times faster than GPR on the front end, which lets users focus use of GPR only on identified hot spots. Let’s look at this calculation using a 5-acre site. GPR takes about a day to do one acre well, so it would take five days to complete the entire survey. EM can complete about 5 acres in one day. Starting with EM would therefore allow completion of an initial survey in one day, leaving the second day for use of GPR. This cuts the project from five days to two.
The resulting savings can range from 30 to 60 percent, depending upon the site. Since GPR/EM consulting fees are typically in the $200 to $400 per hour range, the three-day savings could be on the order of $5,000 to $10,000. From the consulting perspective, one might think — “Why would I want to shave all that profit from my jobs?” The answer is that by doing this, one can build better relationships with clients; allow more time for other projects or office work (what small company ever has enough time to get all their office work done?); and get your name and services in front of more clients to grow the business.
In addition, GPR and EM integrate well with GPS systems, which have exploded in popularity in the past decade. The ability to map out and locate targets with GPS coordinates paired with the GPR/EM results and overlying them on Google Earth, geographic information systems (GIS), and AutoCAD is a huge advantage. The ability to streamline reports and simplify the final product for client interpretation is crucial to success in this day and age, especially given ever-increasing client expectations and decreased patience for waiting for reports.
Overall, using these technologies and adding to business capabilities instantly differentiates one from competitors, expands service offerings in a wide range of applications (construction, environmental, geotechnical, engineering, forensics, and more), and offers the potential to significantly increase revenue for only a small investment.
Brian Jones is a geophysical and environmental application specialist with GSSI.