Monitoring, the Non-Destructive Way
In geophysical surveys, ground penetrating radar and electromagnetic induction technologies offer advantages over expensive and time-consuming drilling
Pardon the pun, but geophysical surveys can be the bedrock of today’s environmental projects.
Applications range from locating abandoned underground storage tanks 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 nondestructive testing methods have been gaining in popularity over expensive and timeconsuming 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 30-plus holes for large sites. On average, each borehole into the ground on an environmental site costs $5,000-10,000, so the expense for drilling or soil sampling on a project can quickly rise. 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, 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 manmade 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.
A GPR survey can be done at a cost of $1,000-2,000 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 or 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-foot 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, 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 24-48 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. In the same time it takes to collect two to four profiles using this technique, one can collect 80 or more profiles of similar length with GPR.
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 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 (a.k.a. EMI) 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 (5-20 feet, 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.
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 underground storage tanks, oil tanks and gas tanks, as well as 50-gallon 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 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 the top of the 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.
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, such as 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
- 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 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 and orientation of targets in either 2D or 3D imaging.
This combo EM/GPR surveying method saves time and money on environmental and construction sites when looking for old underground storage tanks, illegally-disposed-of or buried 50-gallon drums, old foundations, possible debris pits or former landfills, 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 images deeper in clay soils than the GPR, whereas GPR works much better in sandy soils. Also, as noted, an EM system cannot be used within close proximity of cars, buildings and fences, while 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; 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 home run.
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 underground storage tank graves), along with 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. By comparison, 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 or not looking for.
The cost benefit of pairing the two tools is based on the simple fact 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 five-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 five 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 range of $200-400 per hour, the three-day savings could be on the order of $5,000-$10,000. From the consulting perspective, one might think, “Why would I want to shave all that profit from my jobs?” Well, you can build better relationships with clients, allow more time for other projects or office work, and get your name and services in front of more clients to grow your 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 and overlying them on Google Earth, AutoCAD and various GIS products 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, and offers the potential to significantly increase revenue for only a small investment.