The Use of Geophysics for Optimizing Environmental Site Characterization and Remediation

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Introduction

Recently, special guest speakers John A. Mundell, P.E., L.P.G.,

P.G. and Ryan Brumbaugh, L.P.G. of Mundell & Associates,

Inc. presented a webinar with REGENESIS in which they

discussed the use of geophysical survey applications to

enhance standard environmental site characterization

practices and improve the design of remedial alternatives

for complex site and geologic conditions.

Their talk provided a summary of the use of various

geophysical methods applied to environmental projects,

focusing on such methods as electromagnetic metal detection, electromagnetic conductivity, two-dimensional

electrical resistivity, ground penetrating radar (GPR), seismic, microgravity and downhole logging.

The following Q&A is a result of the questions posed during the presentation from Mundell & Associates and REGENESIS.

Q: Are there any geophysical methods that are effective for steep slopes and/or wooded areas?

A: Obviously, a lot of these geophysical techniques are used under many different kinds of site-specific

conditions. For example, 2D Resistivity can be used along a steep slope as long as you can walk up the slope

and are able to put the electrodes in the ground and have them seat properly.

Resistivity profile of a slope area.

In non-environmental cases, we’ve used 2D Resistivity to actually map landslides and use those for engineering

projects because those are often caused by excessive water problems and drainage issues.

Ryan BrumbaughJohn A. Mundell

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You can use almost any of these as long as you can carry the equipment. There comes a point where they’re

very dangerous. It comes down to just what you can walk safely with.

Q: In trying to locate areas of historical excavation and fill, what methods are most appli-cable for a site that has multiple utilities, active tanks, and other metal interferences?

A: With the combination of EM techniques and ground-penetrating radar, we’ve had a lot of success because

either heavy metal detection like EM61 or other EM methods really helps locate those metallic utilities. They

help locate tanks.

And if you have clay at a site, you may not penetrate with ground-penetrating radar. But if there are trenches

with utilities filled with gravel and granular materials, those are easily imaged with the ground-penetrating

radar. So, both that and tank pits that are filled with granule material show up very well with good data density

with GPR. So, I would say that combination is probably the best that we could offer.

GPR survey indicating sub-base materials beneath a site.

Q: In the urban example, did you detect the chlorinated solvents or just the permeable zones?

A: Oftentimes people ask this question: Can you find low PPB of chlorinated solvents? And you can’t really.

As far as chlorinated solvents go, sometimes, if they’re so intense, they may affect the conductivity. But in

these cases, we’re really looking for pathways. We’re really trying to understand the geology. And we found

that if you understand the geology, you have a pretty good idea of where the migratory pathways for those

kinds of chemicals go.

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In chlorinated solvents cases, they can sink. They can sit down on top of the low bottom of a low spot in a

bedrock, in permeable bedrock or clay layer. In the case of petroleum, they can still find those low spots and

those higher permeable highly weathered areas.

So, really, the only thing I would say is this: When you’re thinking about petroleum hydrocarbons, occasionally,

you can pick up halos of bioactivity going on that changes the conductivity of the groundwater because you’ve

got a lot more dissolved solids in the water. You might be able to pick up zones of heavily impacted petroleum

and impure product. But as far as tracking chlorinated solvents plumes, you’re looking at the geology mainly.

Q: What would you recommend from munitions sites?

A: A lot of people use flyover kind of surveys with specialized electromagnetic equipment that may be carried

by planes or drones. That’s a whole specialty area of people who deal with those areas. And you have to do

them very carefully. We’ve been in sites where we knew that that was a situation and we had to really steer

clear because you have to do it very safely. Active munition impact sites need to be surveyed on a regular

basis and are kind of done annually at least in our experience.

Q: What are the typical costs associated with these technologies?

A: They range from very low and simple uses cases that take less than one day, for example using GPR to check

and find a trench for under $1,000, to highly complex150-acre sites that span multiple data collection events

over multiple years, amounting to $100,000 costs for geophysics.

But if you want a ballpark really and you’re looking at like a multiple-approach characterization of a site, we’re

looking maybe in a range of $7,000 to $10,000 per day. That includes everything, including the report. For

example, for a site where you take three days, it may be between $20,000 and $30,000.

That sounds like a lot obviously, but when you get into drilling and bedrock and understanding what’s going

on and having failures, it’s actually these kinds of expenditures during site characterization that really have

helped save a lot of money and have helped direct-injection programs like the people at REGENESIS where

you have to know where you want to inject and the depth you want to inject. And in those cases, they’ve saved

money, because the remediation practitioner is able to inject where you should and avoid injecting where you

shouldn’t. I think they pay for themselves when they’re done correctly.

Q: What are the depth limitations of a GEM-2 survey? Does your firm have your own e-logging tools?

A: We’ve used GEM-2 equipment in the past. We don’t currently use it because of the various operational things

you encounter when you use equipment for a long period of time. You’re typically in the maybe 20-foot to

30-foot range maximum with something like that.

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We own a variety of EM instruments and as you could see from the techniques that we showed during

the presentation, each EM instrument configuration has a particular zone where you’re looking: the top

5 feet for some instruments, 10 to 20 feet for other instruments, and with the EM34 which allows you to

actually space out the receiver and the source, greater depths are possible. You make it up to 40 meters with

something like that.

Q: How well does the resistivity testing work in an area with dense utilities?

A: Sometimes, not very well. And that’s a really good question because you may go and mobilize at a site and

have scoped out 2D Resistivity, and you get to the site, and you find along corridors of right-of-ways where

we get access. You may have key utility lines you’re facing. What we normally try to do is incorporate an

alternative technique, which is often seismic to kind of go along. And since we own our own equipment, we’ll

bring that oftentimes along, or we scope it in. And if we find one line has a lot of electrical interference, we’ll

do a seismic line to at least try to get a good detail, the top of bedrock.

Resistivity Figure of a Plume

What we’ve also found sometimes is that you can interpret sometimes electrical interference on the 2D ERI

lines where they’ll give you perhaps geometric shapes, but the absolute value of the resistivities will be off

because of the electrical influence. In most cases, we have seen that with careful data collection techniques,

resistivity data is robust and it is rare that a dataset is completely unusable. So, they’re still useful in some

cases because of the shapes of the geology and other things, but the absolute values might be off. So, you can

really have limited success with those if you don’t watch it.

Q: How would you approach a large site that has little prior information and has a limited assessment budget?

A: We always look at those as doing desktop studies. If someone asked us to take a quick look at something,

we may be able to do a desktop study for a couple thousand dollars and pull up all the geology we know to

understand where we think things could be. We could also look at a number of historical aerial photos that

might give us some credence.

If it’s a large site, we may pick a technique like an EM or GPR where we’ll drag those instruments behind

the gator and really cover acres and acres of land in a few days. That’s one of the most cost-effective ways

of getting something done for a lower price. For some clients, limited budget means under $1,000, and for

others, it means under $50,000.

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Using Global Positioning Systems, towing equipment and in some cases drones can really cut cost. With

geophysics, many decades ago, getting the survey set up and collecting data was a big deal. We couldn’t

collect very intense data at that time. And over the last 10 to 20 years with the Global Positioning Systems

and newer equipment, we can collect a lot of data over acres and acres of land in a few days.

Q: For areas with buried treated wood pilings and/or concrete debris, would ERI or electro-magnetism be better? A shallow groundwater table can interfere with GPR surveys.

A 2-D ERI profile indicating presence of highly weathered bedrock zones (purple in color)

compared to more competent zones (green).

A: In a specific case where we had buried wood pilings and concrete debris, we used EM61 and EM31 or equivalent

EM mapping technique. And those show up. Wood shows up very well oftentimes as higher conductivity

zones. The concrete debris shows up at high resistivity or low conductivity zones, and so using those kinds of

two surface instruments, and then after you do that, doing a line or two of 2D ERI across the anomalies.

Doing the 2D line right over those anomalies can show you the depth of those and also, the character and

maybe estimate of volume. So, we’ve had a lot of success with those kinds of techniques. GPR energy is rapid-

ly dissipated when transmitted into conductive materials. Generally, shallow groundwater will limit the depth

of imagery because of the increased electrical conductivity associated with saturated ground conditions.

Q: Is the current EM, for example, EM31 or EM61, technology a lot less sensitive to cultural interferences?

A: I wouldn’t say they’re less sensitive. I would say there are ways of correcting for them since modern

electronics enable the newer instruments to collect more precise measurements. That precision allows for

more advanced processing to filter out unwanted signals from cultural noise and still retain a weak signal of

interest. For example, with the EM61, we have coil settings where you send a signal to the ground and then

you listen. In that case, you have coils at different depths above the ground, and you can basically filter some

of the cultural noise that you may get with very near surface metallic debris that is not significant. You can

filter out and actually see deeper.

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So yes, the EM techniques are particularly affected by cultural noise and new sensors or electronics will not

change the physics principles at work. For example, it is very difficult to survey parking lots filled with cars or

dumpsters or other metal things. You may need to get things of the way so you can look beneath the ground.

So you always will be faced with those kinds of things when using EM approaches, but some have filtering

techniques to really help you understand, are you seeing near-surface small stuff? Are you seeing buried

materials deeper? The metallic materials are more significant.

Modern instrumentation is a good thing. You may have a better signal-to-noise ratios than what we have in

the past because of the technologies. So, it’s improved.

Oftentimes, if you stay 10 feet away from something, for example, they may not be affected, whereas before

maybe you would have been affected by more noise. That’s the beauty of using multiple techniques because

you’re gonna find that some of the EM techniques, you can’t see anything in a certain area of the site and with

another one, like ground-penetrating radar, you might pick up something.

Q: How effective is the 2D Resistivity and/or seismic refraction in a CA6 and CA1 material overlying clay till, when you are most concerned with tagging the top of a clay and water table, and the deepest excavated area is approximately 30 feet below ground surface?

A: For seismic refraction, you have to have a strong enough contrast in density to kind of pick up on those

contrasts of those materials. In the same way, for resistivity, the same thing happens as far as resistivity goes.

You need to have a strong enough contrast.

Rippability Analysis Using Seismic Refraction and 2-D Resistivity

When you have very closely spaced electrodes in 2D ERI, you can pick up a lot of subtle near-surface things

as long as your data density is great enough. And so we’ve been able to find differences in material that are

very subtle in the top 10, 20 feet with a good spacing of 2D ERI.

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With seismic, when you get that shallow of material changes, you may get into difficulties in interpreting the

data because it is shallow, and if you don’t have a strong enough contrast in material density, for example, you

may not see that. In that case, I would pick 2D ERI, in that case, to try to look at that.

In addition, we use an instrument like a CMD Explorer where it looks at EM at different depths. Sometimes,

looking at different depths below the surface, you can pick up patterns, and you can also see material

contrasts in the soils that could be picked up by something looking at, not just a single depth like an EM31,

but it depends on what kind of orientation you have for your instrument. But looking at multiple depths can

also help you interpret that.

Q: Is it possible to monitor in situ cleanup methods using geophysics, for example, in situ chemical oxidation?

A: We’ve looked at a few opportunities to try to do that. The beauty of that is once you set up a profile line like a

2D ERI profile line, you can just time that as an injection goes on, for example, to just take readings at different

times. So, there’s published literature on a few cases where people have done that. And we’ve gone back to

a couple sites on a different time and redone a line or two at a different time. And sometimes you get maybe

subtle changes in that case.

So, definitely, this is an opportunity for the future. We’d love to support anybody who is thinking about doing

that because we have very clear idea of how we might go about doing that and really getting good data on the

radius of influence of the ejection and overtime where the various injected materials are dispersed.

Q: What geophysical methods would be most effective to identify historic narrow stream channels at various depths below a site?

A: It depends on how big they are and how thick they are. For example, we used an EM31 one time to map

shallow drain tiles in a field that were filled with sand and gravel. There’s certain processing you have to do in

order to accomplish that.

Once we process that data we were able to map those shallow trenches that were only two foot by three foot

deep. But if you have good enough data density, you could actually find shallow filled tiles.

With a shallow stream or a stream that cuts through, I think with the right data density, for example in the

upper 30 feet, you could collect enough EM data to maybe GPR data if there’s not too much clay overlying

the sand and gravel.

But definitely, EM data would pick up on lower conductivity materials. Seismic or resistivity profiling may

also be able to image the stream channel depending on the nature of the surrounding materials. I think it’s

possible. We’d have to get in the details there, but I wouldn’t say no at this point.

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Summary

The questions summarized as part of the REGENESIS “Distinguished Speaker” webinar series were

provided by our guest presenters, John A. Mundell and Ryan Brumbaugh in response to questions

fielded throughout the webinar presentation. REGENESIS is grateful to both Mr. Mundell

and Mr. Brumbaugh for sharing their expertise. REGENESIS is dedicated to providing relevant,

industry-leading content in support of client partners globally. Any use or reproduction of the

contents of this FAQ document must be approved by REGENESIS and/or Mundell & Associates.

COPYRIGHT & TRADEMARK

Content may not be copied, reproduced, transmitted, distributed, downloaded or transferred in any form or

by any means without REGENESIS prior written consent, and with express attribution to REGENESIS and

Mundell & Associates.

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