Title: Introduction Text: The use of electromagnetic (EM) surveys has been proposed for the delineation of non-aqueous phase liquids (NAPLs) in the subsurface. However, an EM survey should not be used as a stand alone site characterization tool because its usefulness for the detection and delineation of NAPLs is limited. The lessons learned presented in this Web Data Sheet describe these warnings and site-specific challenges in more detail. Case study results from NAVFAC sites are also discussed where EM surveys have been used both successfully and unsuccessfully to identify the extent of NAPL impacts.

Title: Types of EM Surveys (1 of 2) Text: There are two types of EM surveys including EM conductivity surveys and EM resistivity surveys as follows: EM conductivity surveys are carried out by a technician walking a hand-held instrument over the surface. The EM conductivity instrument has a transmitter and a receiver that measures changes in an electromagnetic signal to map terrain conductivity. Conductivity is a measure of how well the electromagnetic signal passes through a given material. The data can be used to locate changes in the conductivity caused by contaminants and/or buried metal objects in the subsurface. This technique is also referred to as terrain conductivity mapping. EM resistivity surveys are carried out with the use of a transmitter on the surface to send an electromagnetic signal and a receiver that is placed in a hole in the subsurface. The receiver is pulled up the hole by a technician to measure resistivity with depth in the subsurface. Resistivity is a measure of how well a material resists or opposes the movement of an electromagnetic signal. This technique is also referred to as electromagnetic offset logging (EOL).

Title: Types of EM Surveys (2 of 2) Text: The table at the left outlines the major differences between these two types of EM surveys. Electromagnetic conductivity employs the differences in the conductive capacity of native and non-native materials in the subsurface to delineate the extent of NAPL impact. The conductivity in NAPL-saturated zones has been observed to be lower than the conductivity of native soils and groundwater. Electromagnetic resistivity employs differences in the resistive capacity of the subsurface materials to delineate the extent of NAPL impact. The resistivity of NAPL-saturated zones has been observed to be higher than the resistivity of native soils and groundwater.

Title: EM Survey Applicability Text: EM surveys have a long history of use as geophysical techniques to characterize subsurface geology including soil types, fill material, presence of moisture, clay content, and porosity of the material. They can also be useful in the identification of man-made objects such as buried underground storage tanks. Their use for the identification of NAPL-impacted areas is less developed and subject to numerous interferences and interpretation errors. Below is a summary of site characteristics that may impact the effectiveness of this technology for NAPL investigations: Extent of NAPL Impact - EM surveys may not detect small NAPL quantities such as NAPL residual globules, ganglia, and small pools that are potentially out-of-range of the instrument’s spatial resolution. Depth of NAPL Impact - The depth limit for EM conductivity surveys depends on the transmitter-receiver spacing of the selected device, but meters can be selected with ranges from 3 ft up to 200 ft. The depth limit for EM resistivity surveys is 300 ft. Site Interferences - There can be site interferences due to traffic, electrical noise, or the presence of large metal objects very near the electromagnetic instrument; also, sometimes surface covering and buried man-made objects can create noise in the signal. Confirmatory Data is Required - EM surveys are not a stand alone technology and require some confirmatory boreholes or well information to verify results. At best, EM surveys provide qualitative data on the extent of NAPL impact. Contrary to some vendor claims, they are not very accurate at quantifying volumes and cannot at all identify the types or concentrations of chemical constituents present.

Title: EM Conductivity Surveys (1 of 2) Text: For EM conductivity surveys, a hand-held instrument is used with a transmitter at one end and a receiver at the other. The transmitter sends an electromagnetic signal into the subsurface and the receiver measures the strength of the returning signal from the subsurface. Different subsurface materials have different conductivities, so a contrast map can be produced based on these differences. The presence of a non-native material is indicated by a zone of contrasting conductivity that does not correspond to the conductivity of the surrounding native materials.

Title: EM Conductivity Surveys (2 of 2) Text: EM conductivity surveys are near-surface surveys based on the principle of electromagnetic induction. The instrument used for EM conductivity surveys transmits an alternating electromagnetic field into the ground. The magnetic component of this field induces eddy current flow into the ground, which produces a secondary magnetic field. The instrument receiver senses the secondary magnetic field strength which is a function of terrain conductivity. The instrument can also measure the ratio of the primary to secondary magnetic field. This is called the “in-phase” component and can be used to locate buried metal objects. The instrument measures the average conductivity of all the soil and fluids in the sensing radius. This average conductivity is generally referred to as apparent conductivity. The apparent conductivity measurement can be used to differentiate between different soil types and the presence of non-native materials such as NAPLs.

Title: Terrain Conductivity Mapping at NAS Alameda Text:

Title: Terrain Conductivity Mapping at NAS Alameda Text: Terrain conductivity mapping was evaluated as a tool to determine the presence and extent of a tarry refinery waste (TRW) in subsurface soils at CERCLA Site 13 at the Former Naval Air Station (NAS) Alameda in California. Terrain conductivity mapping was not successful in determining the extent of TRW contamination at this site. The site is 17.5 acres in size and located on land that was historically used by the Pacific Coast Oil Refinery, which operated from 1879 to 1903. The historic petroleum refinery operations included the distillation of crude oil to kerosene and fuel oil. One byproduct of the refinery process was the TRW which consists of a black, tarry/oily material containing in excess of 20,000 mg/kg of total petroleum hydrocarbons (TPH) and elevated levels of polycyclic aromatic hydrocarbons (PAHs) and lead.

Title: Technology Application Text: Two instruments were used at this site for terrain conductivity mapping including the Geonics EM-31 and EM-38 meters. The EM-31 meter is capable of taking measurements at depths less than 20 ft below ground surface (bgs) and the EM-38 meter is capable of taking measurements at depths less than 5 ft bgs. The mapping process began with the EM-31 meter and included collecting conductivity data every 5 feet along north to south grid lines. A portion of the site which contained underground utilities and a deactivated electrical line was resurveyed with the EM-38 meter. The total area mapped was approximately 300 ft by 300 ft.

Title: Confirmatory Samples Text: Six test pits were excavated for visual confirmation of the presence or absence of the black, tarry material and to collect samples for laboratory analyses of PAH, TPH, BTEX, and metals. The test pits were used to confirm the accuracy of the terrain conductivity mapping for identifying areas impacted by TRW. Click on the button to view the extent of TRW in Test Pit 1. The test pit observations were then compared with the terrain conductivity results. The terrain conductivity method would only be considered valid and useful if it predicted the presence or absence of the TRW in 70% or more of the test pit evaluations.

Title: Data Output Text: Data collected using the two meters was analyzed using specialized commercial software. Click on the map at the left to view color enhanced contour maps of conductivity created to display spatial trends. The colors show the relative conductivity of the soil in each area surveyed. In addition, the test pit results showing the presence or absence of visual observations of TRW are superimposed on the map. Based on these results, it was concluded that there was no correlation between the terrain conductivity readings and the presence of the TRW. It was also noted that buried utility lines and the metal trailer in the southern portion of the site interfered with some of the readings.

Title: EM Resistivity Surveys (1 of 2) Text: The EM resistivity survey is intrusive because it involves the use of a transmitter coil at the surface, while the receiver must be placed in a nearby drill hole or well. The EM resistivity survey allows 3-D imaging of highly resistive materials and fluids in the subsurface. NAPLs are highly resistive to electrical currents. For example, saturated sands will have a resistivity of 10 to 50 ohm-meters, while NAPLs typically exhibit resistivities on the order of 1 million ohm-meters. Direct push sensors are available to directly measure resistivity in a single borehole. However, the type of EM resistivity survey discussed here uses a surface transmitter to scan the resistivity of an entire volume of the subsurface. Click here for more information on the use of direct push technologies for site characterization.

Title: EM Resistivity Surveys (2 of 2) Text: The transmitter coil is moved along a grid in the study area and at each grid location an electrical signal is transmitted. The signal from the transmitter coil is then registered by the receiver which is mechanically pulled up the hole while measuring the primary and secondary electromagnetic fields produced from the given grid location. A large, long wavelength response is created representing the primary electromagnetic field. Superimposed on this primary field are responses related to the secondary electromagnetic fields caused by eddy currents moving around in the subsurface through materials with different resistivity characteristics. The raw data are later processed by computer software to remove the primary field and calculate and verify the secondary fields. The secondary fields are converted to an apparent resistivity value measured in ohm-meters. In general, the presence of NAPL can be inferred from zones represented by higher resistivity values. However, this relationship is not always linear due to underlying heterogeneity in the subsurface geology.

Title: EM Resistivity Survey at NAS North Island Text:

Title: EM Resistivity Survey at NAS North Island Text: Naval Air Station (NAS) North Island is located in San Diego, California. An EOL investigation was conducted to delineate the extent of a free product petroleum hydrocarbon release from former underground storage tanks near Building 379. All three of the buildings in the area (379, 391, and 397) were used for jet engine maintenance and testing. At one time, there were nine underground storage tanks (USTs) and four aboveground storage tanks (ASTs) at the site. The light non-aqueous phase liquid (LNAPL) extent determined by the EOL was similar to the confirmatory sampling results at the NAS North Island site.

Title: Background Text: At the time of the project, free product had been measured at a thickness of up to 5 ft due west of Building 379, but the full extent of the LNAPL impact had not been fully delineated. EOL was employed to investigate the likelihood of LNAPL presence under Building 379 and to minimize the need for additional boreholes at the northern and eastern sides of the building.

Title: Technology Application Text: The EOL survey was conducted over a 4-acre study area with a 30-ft grid that included points both outside and inside adjacent buildings. Two pre-existing 2-inch PVC wells were selected for use as the receiver points. These two receiver points were selected to provide subsurface information both under Building 379 and in the adjacent area north and east of the structure. At each grid point transmitter location, the first receiver was pulled up the well mechanically and resistivity measurements were logged until the receiver had reached the top of the casing. This process was continued for each grid point with an acceptable signal-to-noise ratio. The same procedure was carried out for the second receiver with a 20% overlap of grid points that were also used for the first receiver. The primary and secondary electromagnetic field data produced between the grid points and the receiver wells were recorded with a datalogger and processed by computer software into 3-D resistivity maps.

Title: Data Output Text: The resistivity data was mapped in a 3-D grid across the site. The technology vendor indicated that the resistivity data showed that the LNAPL had not migrated under Building 379. However, a small area with high resistivity was noted in the center part of Building 379, but was disconnected from the main LNAPL impacted area. This may represent a separate, secondary source of petroleum free product under the building. The data also indicated that the potential migratory pathway for the LNAPL was a buried and abandoned jet fuel pipeline trench on the northern side of Building 379. Although a formal statistical study was not conducted, the EOL results for the LNAPL impact did appear to correspond to the LNAPL extent indicated by the manual free product measurements from wells installed at the site. Click on the button to compare the results.

Title: EM Resistivity Case Study at NAS Alameda Text:

Title: EM Resistivity Case Study at NAS Alameda Text: NAVFAC also selected a site at former NAS Alameda for the demonstration of EOL for dense non-aqueous phase liquid (DNAPL) detection and delineation. This project was sponsored by the Environmental Security Technology Certification Program (ESTCP). The EOL technology would be considered successful if 90% of the predictions for DNAPL contamination could be verified based on physical and chemical analyses of samples taken from within the surveyed regions. However, the results showed a 0% accuracy rating and no ability to reliably detect DNAPL within the subsurface. The EM resistivity survey at NAS Alameda was located in and around Building 5, a former depot maintenance facility with underground tanks and sewers that previously contained industrial solvents and wastewaters. Chlorinated solvent DNAPL had been detected at the site and 1,1,1-trichloroethane (TCA) was present in groundwater at levels up to 790 parts per million (ppm). Although there was a significant amount of utilities and other features at this site, they did not adversely affect the EOL survey data.

Title: Data Output Text: The survey was conducted in a similar manner to the previous case study and the subsurface resistivity properties were measured and mapped within the study area. A horizontal slice from the 3-D site model is shown here corresponding to the resistivity contrasts found at 27 feet bgs. This image contains the most significant high resistivity anomalies found at this site. Based on this information, target points were selected to determine the technology's capability to identify the presence of DNAPL.

Title: EM Resistivity Performance Results Text: An overall review of study results at NAS Alameda indicated that there were no true positives, 20 true negatives, 18 false positives, and 1 false negative for DNAPL detection. This means that the EOL technology was not once able to predict and then confirm the presence of DNAPL in the subsurface. The summary table from the study shows that the technology had a 0% accuracy rating based on this demonstration project. The EOL imaging reported little to no DNAPL in specific zones at the site. However, later subsurface investigations using laser-induced fluorescence and video microscopic methods revealed significant quantities of DNAPL in these same zones. A possible source of error that may have led to these discrepancies was a result of the level of subsurface DNAPL being too diffuse to significantly alter the resistivity of the native soils. Due to the inconsistent results of this project, it was determined that this technology does not have the required performance capabilities to characterize DNAPL sites and does not compare favorably to more conventional approaches.

Title: Lessons Learned Text: 1. Unreliable for NAPL Detection Detection and delineation of NAPLs is limited by numerous interferences and potential interpretation errors. EM surveys are reliable in geophysical applications for imaging subsurface lithology. 2. Requires Confirmatory Sampling This technology is not a stand alone site characterization tool. It is interpretative and it requires confirmatory sampling to verify the presence of subsurface NAPL contamination. Confirmatory sampling may limit cost-effectiveness. 3. Affected by Metal Objects Site interferences like buried metal objects or nearby aboveground metal structures near the vicinity of the test site can be detrimental to the successful delineation efforts. 4. Unable to delineate type of contaminant These technologies only indicate differences in the conductive or resistive nature of the fluids and soils within the subsurface. EM surveys do not indicate the type or concentration of a given contaminant. 5. Dependent on physical properties of soil The conductive and resistive properties of the native materials also vary based on physical parameters such as soil type, moisture, porosity, and dissolved solids in groundwater.

Title: References Text: ESTCP, 2000. ESTCP Cost and Performance Report on Electromagnetic Surveys for 3-D Imaging of Subsurface Contaminants. PRC Environmental, 1996. Electromagnetic Offset Logging at Naval Aviation Depot Building 379 and 397, Naval Air Station, North Island, San Diego, California. Draft Technology Demonstration Report. Tetra Tech, 2003. Sampling and Analysis Plan, Pilot Test for Terrain Conductivity Mapping at Alameda Point, Alameda, California. Tetra Tech, 2004. Technical Memorandum, Results of a Pilot Test for Terrain Conductivity Mapping at CERCLA Site 13, Operable Unit 2A, Alameda Point, Alameda, California.

Title: Contact Text: For more information about EM survey methods, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil