## ABSTRACT

Oil on the water surface represents just the American Petroleum Institute API > 10 gravity component of any crude oil spill or well blowout, and is identified and tracked by conventional remote sensing means. However, the API ≤ 10 components of the hydrocarbons are not readily accessible by these means. UV sensors on underwater vehicles can sample just a few cubic centimeters at a time and are subject to fouling. Side-scan sonar, under certain conditions, can “see” gas bubbles on the near outer shell of a subsurface plume if they exist early on during a blowout, but cannot assess the entire volume. Oil sequestered in bed sediments in oceans or rivers is not visible to UV sensors, nor is it visible to divers. It is apparent that this sensing gap problem needed to be addressed. A new technology developed by the US Geological Survey, working closely with Williamson & Associates of Seattle, holds promise for rapid mapping and characterization of below-surface hydrocarbons. Crude oil drifting in the deep ocean water column, oil blanketing the seafloor, and oil sequestered in seafloor and river bottom sediments can now be quickly mapped in 3D. If drifting in the seawater column, dispersed oil can be tracked as its distribution evolves over time. This technology also is potentially useful for mapping combined storm-water overflow (CSO, or sewage) deposits, as well as Superfund sites in Puget Sound and other sensitive rivers, bays, and estuaries close to cities that pose serious hazards to both humans and wildlife. The technology is based on a surface-sensitive electro-physical property known as induced polarization (IP). This surface-sensitivity means that highly dispersed IP-reactive materials have more surface area exposed to surrounding water, and are thus more responsive than undispersed materials of the same mass. IP technology has been used successfully for many decades to map disseminated porphyry sulfide deposits on land, but has only been applied commercially at sea since 2007. Recent laboratory and Puget Sound experiments have verified that the IP response of oil dispersed in water and sequestered in sediments is unusually strong: at least 20 times greater than a strong “hit” in an IP survey for sulfide minerals on land. The marine IP system has been towed behind a small boat in as little as 1–2 meters water depth, while one version has been tested (using a towed sled) to 3,500 meters depth. Depending on the cable-streamer design, the depth of detection of chargeable materials in sediments can be greater than 20 meters. It can be used to monitor active drill platforms for leaks. Finally, IP is also strongly reactive to buried pipelines; in the Gulf of Mexico there are over 43,000 miles of poorly-located, often hidden, corroding pipe. We can now map it precisely.

## INTRODUCTION

Induced polarization is most commonly used with at least four electrodes: one pair constitutes a transmitter dipole, and one or more pairs function as non-polarizing receiver dipoles. On land, this requires the tedious emplacement of individual electrodes. In water, because of the natural ionic conductivity, electrodes are not manually emplaced, but instead towed in a single streamer or an array of streamers, behind a ship (figure 1). The transmitter dipole is used to inject galvanic current into the ground or seawater, typically using a square-wave signal. At sea, as little as 14 volts will cause 2 – 6 amps of current to cascade into the water. Increasingly-distant receiver dipoles detect a modified form of this signal. Laboratory testing with a USGS-developed electrical geophysical technology called marine induced polarization (“marine IP”) has shown that it can detect oil and oil-like substances in river and ocean water in concentrations as low as 2 ppm (figs. 1, 2, and 3; Wynn et al., 2016). This is well below the level of oil that might be recoverable. Moreover, marine IP can map these substances in the deep-water column as well as in the sub-sediment (fig. 4) as fast as a ship can tow the geophysical streamer through or over them. The reach and the resolution of this system are controlled by the dipole size and separation (greater depth reach means lower resolution), and somewhat limited by the conductivity of the water (with slightly deeper sediment penetration in rivers, for instance). Modeling for an array used offshore of South Africa has shown that a 15-meter dipole system could detect polarizable material more than 20 meters below the seafloor (Wynn, et al., 2012). Worldwide experience at this point shows that Marine IP can detect sulfides (including very fine pyrite in anoxic sewage-derived sludge), ilmenite (iron-titanium oxide in beach sands), and oil-like materials (including crude oil and derivatives), in order of increasing IP response. The IP response of oil in seawater is at least 20 times stronger than any other mineral measured on land for an IP response (Wynn and Fleming, 2012).

Figure 1.

The Marine Induced Polarization array configured for deep ocean oil plume mapping.

Figure 1.

The Marine Induced Polarization array configured for deep ocean oil plume mapping.

Figure 2.

Marine IP streamer showing a titanium-wire transmitter dipole (center) and several Ag-AgCl non-polarizing receiver electrodes.

Figure 2.

Marine IP streamer showing a titanium-wire transmitter dipole (center) and several Ag-AgCl non-polarizing receiver electrodes.

Figure 3.

Oil in seawater induced polarization response, with a large (i.e., economic) land-based IP response for comparison, from Wynn and Fleming, 2012.

Figure 3.

Oil in seawater induced polarization response, with a large (i.e., economic) land-based IP response for comparison, from Wynn and Fleming, 2012.

Figure 4.

Sub-seafloor sequestered-oil marine IP mapping array.

Figure 4.

Sub-seafloor sequestered-oil marine IP mapping array.

## THE PROBLEM

There are more than 4,000 active oil platforms (up to 7,400 of these counting abandoned or removed platforms) in the Gulf of Mexico (National Geographic 2010). It is reasonable to assume that most former and existing platforms resulted in a well-head on the seafloor. In addition, there are at least 43,000 miles of active and inactive pipelines in the Gulf of Mexico (Edelstein, 2014), with one estimate ranging as high as 1,000,000 miles of pipeline from well-head to collection points on land (National Geographic, 2010). The large disparity may have to do with much of the pipeline network being poorly documented, and some of the pipelines extending from the sea to collection points on land, and not thus being counted as offshore pipe.

A 2003 National Research Council report groups oil releases into four categories: natural seeps, oil consumption, oil transportation, and oil extraction/production. The NRC estimate for natural seep volume ranges from 24 million to 71 million gallons worldwide. The “best estimate” available is 47 million gallons (National Academy Press, 2003). The U.S. Department of Energy estimates 1.3 million gallons (about 5 million liters) of petroleum spill into U.S. waters from vessels and pipelines in a typical year. A major spill could realistically double this number. Finally, the National Research Council estimates approximately 4,570,000 gallons of crude oil enter North American waters through natural seeps each year. These seeps contribute about 2/3rds of all oil released (National Academy Press, 2003). Worldwide, natural seeps contribute about 45% of all oil released into the environment. The rest of the oil releases come from extraction, transportation, but mainly from consumption spills of petroleum according to the National Academy Of Science (NAS, 2013).

To get a sense of the seriousness of the issue, some of the largest extraction disasters include:

• Ixtoc Well, Yucatan (1979): 140 million gallons of crude (NAS Ocean Studies Board, 2016). There are no reliable estimates of recovery costs for this disaster.

• Exxon Valdez (1989): 10.8 million gallons of crude. Estimates of recovery costs: $7 billion (NOAA, 1992; Lyon and Weiss, 2010). • Persian Gulf spill (1991): 6 million barrels/257 million gallons of crude. Estimates of recovery costs vary widely (Canby 1991; Baumann 2001). • Deepwater Horizon (2010): 4.2 million barrels/210 million gallons of crude. Estimates of recovery and liability costs:$61 billion (On Scene Coordinator, 2011).

• Kalamazoo River spill (2010): 1.1 million gallons of diluted bitumen from Athabasca). Estimated recovery costs: \$1.2 billion (EPA, 2010; 2016a).

With exceptions like these infrequent events, by far the largest bulk of oil released into international waters comes from oil seeps and oil consumption (NAS, 2016). An unknown amount leaks into the water from aging pipelines, and with marine IP we can now address this.

## AN EXAMPLE: THE WYCKOFF (EAGLE HARBOR) SUPERFUND SITE

The Wyckoff (Eagle Harbor) Superfund Site (fig. 5) is an abandoned creosote plant west of Seattle. It lies on the east side of Bainbridge Island in Central Puget Sound, Washington, close to a major ferry terminal. The EPA added Eagle Harbor to the Superfund site list in 1987, when environmental investigations revealed extensive contamination in soils (including a car-sized block of nearly pure naphthalene), 100 million gallons of polluted groundwater, and non-aqueous phase liquids (NAPLs) in the sediment on the bottom of Eagle Harbor. The list of contaminants of concern (COC’s) runs to 86 entries (EPA, 2016b; their figs 5 and 6), and also includes oils, naphthalene, and heavy metals. The proximity of the site to the terminal for the Bainbridge Island ferry system suggests that prop-wash is constantly remixing material deposited offshore of the Wyckoff plant, including disturbing the sediment cap that EPA initially placed northwest of Bill Point to contain it.

Figure 5.

Wyckoff site, showing NAPL detected using Tar GOST probes, from Moore et al., 2013.

Figure 5.

Wyckoff site, showing NAPL detected using Tar GOST probes, from Moore et al., 2013.

Figure 6.

A vertical log comparing TarGOST values to a geologic log for sample site 17A, NE of Bill Point, from Moore et al., 2013. This log was chosen to be closest to the marine IP profile of figures 7 and 8. Vertical scale on figure 6 is in inches below sediment top. Horizontal scale is a normalized fluorescence response. Colors represent different materials.

Figure 6.

A vertical log comparing TarGOST values to a geologic log for sample site 17A, NE of Bill Point, from Moore et al., 2013. This log was chosen to be closest to the marine IP profile of figures 7 and 8. Vertical scale on figure 6 is in inches below sediment top. Horizontal scale is a normalized fluorescence response. Colors represent different materials.

CH2M Hill conducted a TarGOST (Dakota Technologies’ Tar-Specific Green Optical Screening Tool, a laser-induced fluorescence probe) survey at the Wyckoff site for the EPA in 2012 (Moore et al., 2013). Figure 5 shows the location of the fluorescence response from this vertical-probe survey. Figure 6 shows a sample core-log from site 17A (Moore et al., 2013), chosen here as the closest to where we conducted a subsequent marine IP profile. Note that in Figure 7 the TarGOST map clearly misses most of the NAPL seen in the log of Figure 6. The CH2M Hill report makes it clear that there is tremendous heterogeneity in the site. Apparently for this reason the CH2M Hill report rates each log site at 10%, 50%, and 90% likelihood to host NAPLs. Figure 7 shows a fence diagram for the “50% Chance or Greater of Encountering NAPL” from Moore et al., 2013. Superimposed on this figure is the location of the marine IP profile we conducted.

Figure 7.

Wyckoff site - fence diagram (Moore et al, 2013) showing “50% Chance of NAPL detection” from TarGOST probes, from Moore et al., 2013.Orange line is the location of marine induced polarization line 5 profile in Figure 9.

Figure 7.

Wyckoff site - fence diagram (Moore et al, 2013) showing “50% Chance of NAPL detection” from TarGOST probes, from Moore et al., 2013.Orange line is the location of marine induced polarization line 5 profile in Figure 9.

Figure 7 gives a 3D sense of the distribution of TarGOST response to NAPLs offshore of Bill Point at the Wyckoff site (Moore et al., 2013). Figure 8 gives a perspective view of the entire marine IP profile around Bill Point, showing the track of the towed-streamer survey. Figure 9 shows the marine IP results where it crosses the TarGOST log in Figure 6. For reference, this segment of the marine IP profile is also shown as a red line in Figure 7. This entire survey was conducted in less than two hours of profile time, sufficient to examine all the known polluted areas around Bill Point. The eastern part of the survey was conducted in deep water in order to calibrate the marine IP array null response first. This calibration result (measuring IP phase-shifts over frequencies ranging from 4 Hz to 112 Hz) was then subtracted from all the subsequent shallow-water profile data seen in Figure 8. IP profile 5 (figs. 7 and 9) was chosen because it sampled locations of known sequestered NAPL pollution as well as areas with no NAPL pollution in the TarGOST survey.

Figure 8.

The Marine induced polarization calibration profile (right side) and test profiles(left side) – Wyckoff site, Eagle Harbor, WA. Seattle lies eastward, off the image to the right.

Figure 8.

The Marine induced polarization calibration profile (right side) and test profiles(left side) – Wyckoff site, Eagle Harbor, WA. Seattle lies eastward, off the image to the right.

Figure 9.

Wyckoff site, NE corner of Bill Point, marine induced polarization line 5 result (see Figure 7). Top is north. Colors indicate intensity of the induced polarization response above background noise level in milliradians of phase-shift. The blue color indicates background levels of IP response, while the red color indicates IP phase-shifts in excess of 30 milliradians above background. For reference, to the right is the frequency of the maximum phase-shift for each anomaly, ranging here from 28 to 36 Hz. In laboratory tests we can detect oil in seawater down to 2 ppm (Wynn et al., 2016), with peak phase shifts up to at least 124 Hz. The width of the colored bar indicates the approximate sampling reach of this array; depth of detection is approximately 5 meters for the 7.5-m dipole used in this survey. This profile was acquired in about 90 seconds while towing the marine IP streamer at about 3 knots. To acquire the same information with TarGOST probes would have required several days.

Figure 9.

Wyckoff site, NE corner of Bill Point, marine induced polarization line 5 result (see Figure 7). Top is north. Colors indicate intensity of the induced polarization response above background noise level in milliradians of phase-shift. The blue color indicates background levels of IP response, while the red color indicates IP phase-shifts in excess of 30 milliradians above background. For reference, to the right is the frequency of the maximum phase-shift for each anomaly, ranging here from 28 to 36 Hz. In laboratory tests we can detect oil in seawater down to 2 ppm (Wynn et al., 2016), with peak phase shifts up to at least 124 Hz. The width of the colored bar indicates the approximate sampling reach of this array; depth of detection is approximately 5 meters for the 7.5-m dipole used in this survey. This profile was acquired in about 90 seconds while towing the marine IP streamer at about 3 knots. To acquire the same information with TarGOST probes would have required several days.

Figure 9 (from WASSOC, 2016) represents the strength of the marine IP response (as phase-shift in milliradians) observed on a profile that took about 90 seconds to acquire at 3 knots. This profile is centered on the CH2M Hill vertical TarGOST log of figure 6. The dipole spacing here is 7.5 meters between the current transmitter pair and the first (non-polarizing Ag-AgCl) electrode receiver pair. By using more distant receiver pairs we can generate “heat map” plots of where the IP responses are, each one representing a different depth. We can thus build a 3D plot of reactive materials beneath the seafloor if we run multiple parallel profiles.

The greatest IP phase-shift in a given processed packet (collected from 4 Hz to 112 Hz) is the plotted value for that packet in Figure 9. Samples hand-acquired from the northwest side of Bill Point and tested in our laboratory have sample-specific peak phase-shift IP responses at 84 Hz and 116 Hz; chemical analyses were not complete by the deadline for this manuscript, but the samples were visibly distinct variations of creosote. The correlation of IP response to TarGOST response is quite good, though as noted (fig. 6) the TarGOST probe misses much of the NAPL that the Marine IP system detected.

Profile 5 (fig. 9) suggests that the marine IP system is sampling a different form of NAPL northeast of Bill Point from what was collected by hand in the embayment northwest of the plant site. Together, the distinct peak phase-shift responses we have observed here and in the laboratory suggest that we should not only be able to detect oil derivatives sequestered in sediment - but to also characterize them. We are currently assembling a library of IP phase-shift responses for different samples of NAPL, bunker oil, transported crudes (including Bakken and Macondo crudes), and dispersal agents (including CorExit).

## CONCLUSIONS

Marine IP technology can be extremely fast and efficient in mapping hydrocarbons in 3D drifting in the water column, or sequestered in sediment, as fast as a streamer array can be towed through or over them. We have shown (Wynn et al., 2016) that marine IP can work from lab samples (vibracore, clam-shell, grab-samples, etc.) to guide interpretation of electrical geophysical profiles acquired from a vessel-towed electrical geophysical streamer. In the laboratory we can detect oil down to 2ppm – in other words, to and below “actionable” levels. Marine IP technology has now been used in ~3,500 meter depths (Bismarck Sea), and in ~1 meter depths (Eagle Harbor, WA). In a single 2007 survey off the coast of South Africa, the marine IP technology successfully identified two large, previously-unknown titanium deposits buried beneath inert recent sediments to depths of 20 meters (Wynn et al., 2012; Wynn et al., 2016). Because the towed streamer can sample IP responses over a wide frequency range for 4 - 6 different depths every second, the South African survey acquired more IP measurements at sea in 25 days than had been acquired on land worldwide in the previous half-century (Wynn et al., 2012). Because of the conductivity of ocean and river water, electrodes do not have to be manually implanted in the sediment – the water provides the needed connectivity. Instead of acquiring ~100 typical IP measurements per day (on land) we can acquire up to six different depth-measurements of IP response every second as the streamer is towed (typically at 2 – 4 knots). This is nearly 40,000 IP measurements (this times six different depths simultaneously) in a 10-hour working day. Marine IP can also acquire far more detailed information in 90 seconds than a TarGOST probe survey can acquire in several days.

We have also observed that marine IP responds very strongly to buried (hidden) metallic debris, including under-seafloor pipelines, buried wrecks, and cables. This makes the marine IP technology a prime tool for precisely mapping the buried pipelines, leaking infrastructure, and natural seeps festooning the Gulf of Mexico. With marine IP technology we can map both pipelines and oil seeps (natural or man-caused) rapidly and efficiently.

Another possible use for marine IP is leak prevention. Most would agree that it is better to stop a leak before it develops and gets out of control. Marine IP could identify a hydrocarbon leak within seconds, and thus prevent a potential problem from becoming an expensive ecological disaster (Wynn and Fleming, 2012).

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