Many active and former military installations have ranges and training areas that include adjacent water environments such as ponds, lakes, rivers, estuaries, and coastal ocean areas. In other sites, training and testing areas were deliberately situated in water environments. Disposal and accidents have also generated significant munitions contamination in the coastal and inland waters in the United States. On land, the Environmental Security Technology Certification Program (ESTCP) Classification Pilot Program has demonstrated that the advanced electromagnetic induction (EMI) sensor arrays emerging from research sponsored by the Strategic Environmental Research and Development Program (SERDP) and ESTCP can be used to reliably detect and classify buried unexploded ordnance (UXO) at real munitions response sites under operational conditions. The marine environment introduces complexities in the response of these sensor systems which could adversely affect performance. This project was a comprehensive research study of the factors affecting the performance of advanced EMI sensor arrays in the marine environment.
This project shows results from a series of measurements of target EMI responses in a large salt water tank, and background EMI responses in various salt water and sedimentary environments along the York River, VA estuary. The electronics and data acquisition system used in the tests was a standard transient EMI (TEM) package used for munitions detection and classification on land. For the tank tests, targets included simple aluminum and steel test items (spheres, spheroids and cylinders) as well as inert ordnance items. The basic test protocol involved carefully measuring and comparing target and background responses in air and in the water. On the whole the research team found nothing to indicate that a salt water environment per se compromises the utility of advanced TEM sensors for target classification.
Testing was done at the Naval Research Laboratory’s test facility at the Army Research Laboratory Blossom Point, MD facility. The basic test setup was a 10 ft diameter by 11 ft deep (3.05x3.35 m) 6000 gallon (22.7 kl) industrial plastic tank was partially buried and filled with a salt water solution. Salinity varied from 0 to 35%. During the course of the testing water temperatures ranged from less than 5°C to 25°C. At various times the electrical conductivity of the water was measured using a Hanna Instruments HI98304 DIST4 electrical conductivity sensor (using samples diluted with distilled water for conductivities greater than 2 S/m). At specific salinities, conductivity as a function of temperature was determined using standard tables and formulae. It varied from 0.04 to 5.6 S/m during the course of the testing, depending on salinity and temperature. Most of the data were collected at 35%, with conductivity in the range 3.2-4.5 S/m. The water in the tank was mixed with a removable pump each day before the start of testing to eliminate temperature and salinity stratification.
The research team observed relatively weak electric field contributions to the early time response for bare aluminum targets located off to the side of the transmit and receive coils. This was where the electric field was strongest relative to the primary magnetic field. Even for bare aluminum targets it was not a significant factor for most sensor-target geometries. With the exception of test setups specifically targeting electric field effects researchers found no significant differences between measured responses in air and in salt water for the different test items and test geometries.
The project plan included a Go/No-Go decision point following completion of the analysis of data from the tank tests. The results indicated that a salt water environment per se does not compromise the utility of advanced TEM sensors for target classification. Researchers therefore proceeded with field measurements of the effects of bottom sediments and water column variability on the EMI response in various types of marine environment. These comprised a series of cruises along the York River estuary during the summer of 2016.
Measurements of the background response vs. depth were made at field sites along the York River. The water salinity varied from fresh or slightly brackish (<2‰) to ~25‰, and bottom sediments ranged from mud to silty sands. Researchers found that a four layer model comprising a non-conducting half-space (air) over a conducting water layer, combined with a shallow conducting sediment layer overlaying a deeper, more resistive half-space provided a good match to the data. Calculations indicated that the background response and its variation were not small compared representative munitions signals at early times. However, when the background response varies smoothly it can be effectively dealt with by standard filtering techniques similar to those used in munitions response surveys on land.