The effects of corrosion and biofouling on the electromagnetic induction (EMI) and physical acoustic signatures of military munitions located at underwater munitions response sites are relatively unknown at this time. Munitions submerged in sea or fresh water for any length of time may become heavily rusted or encrusted with sea growth. At present, there is very little hard empirical data on the effects of corrosion and biofouling in marine environments on EMI and acoustic signature variability. Such data are needed for the success of unexploded ordnance (UXO) detection and classification in the underwater environment.
This project represents a combined effort and produced two final reports. Both reports can be viewed by downloading them from the corresponding products section.
This research systematically examined the effects of two primary types of corrosion and biofouling commonly found in the underwater environment and their effects on EMI and acoustic systems. The development of surrogates and corresponding testing was comprised of five elements: (1) accelerated corrosion and simulated biofouling of munitions surrogates in a controlled laboratory environment, (2) simultaneous environmental exposures of munitions items and surrogates, (3) EMI response signature measurements, (4) structural acoustic signature measurements, and (5) physical modeling to develop a framework for predicting the impact of corrosion and biofouling on buried UXO and clutter classification methodologies for use in munitions response activities.
The approach used combined laboratory tank tests and controlled exposure in real-world underwater environments. In a controlled laboratory environment, reliable methods for accelerated production of calcareous biofouling and magnetite corrosion layers were developed for our standard munitions surrogate. Several batches of the surrogate underwent environmental exposure for increasing biofouling layer development over time, including two inert munitions—a 155mm projectile and a 5-in rocket warhead. Further, the inert munitions, after cleaning, were exposed to an accelerated production of magnetite corrosion.
Advanced EMI response data were collected using the advanced EMI system developed for underwater UXO characterization in SERDP Project MR-2409. The underwater system is configured with a single transmitter coil and two triaxial receiver cubes. This system, much like our previous TEMTADS HandHeld (HH) sensor (ESTCP Project MR-200807), is not currently configured to simultaneously collect the same rich, bistatic data sets as typical advanced EMI arrays. As such, a dense, template-based data collection methodology was used for collecting classification-grade EMI data. The coils were driven with a new compact version (cDAQ) of the TEMTADS 2X2 electronics developed by G&G Sciences. The system is capable of generating a variety of bipolar transmit waveforms, as well as being flexible in stacking and time gating the measured transient decays. The standard TEMTADS pulse duration of 25 ms was used. Peak transmit currents were between 5 and 6 A. Raw transients sampled at 250 kHz were recorded for noise studies. For background and target response measurements roughly 200 decays were averaged and 121 logarithmically spaced time gates (5% gate width) were recorded, the last 100 or so of which (corresponding to decay times greater than about 0.1 ms) are typically used for target classification.
Testing was done at the Naval Research Laboratory’s EMI Underwater Test Facility at Blossom Point, MD. A 10-ft diameter by 11-ft deep (3.05 m x 3.35 m), 6000 gallon (22.7 kL) industrial plastic tank was partially buried and filled with a salt water solution. The salinity of the tank water was 35‰. During the course of the testing water temperatures ranged from less than 5⁰ C to 25⁰ C. The water in the tank was mixed with a removable pump each day before the start of testing to eliminate temperature and salinity stratification.
Sensors and targets were attached to a fiberglass grate which could be winched up above the tank or down into the water. The basic test protocol involved repeating measurements with and without a target in the water and then again in air with the assembly set out of the water, beside the tank. This allowed background response contributions to be removed from the target response measurements and allowed for direct in-water and in-air comparison. Data on corroded samples were also collected with a larger, 1-m x 0.5-m transmitter array constructed for MR-2409’s efforts and a single, triaxial receiver cube in a prototype water housing was used.
Structural Acoustics (SA):
The detailed frequency/angle structure in a measured acoustic color map provide effective “fingerprinting” features for the classification algorithm. The researchers measured these color maps for two UXO—a 5” rocket and a 155mm shell both filled with an epoxy resin—and then repeated the measurements after the targets have experienced biofouling and then corrosion. Further, the researchers attempted to determine how the various structural acoustic mechanisms that lead to these features were affected by the bio-fouling or corrosion. For example, the color map for a rigid rocket was a good approximation for the specular scattered component. The specular component was of interest since it was used to form an acoustic image providing direct classification information such as size and shape. The researcher team attempted to determine how the various mechanisms including circumferential and axial elastic or creeping waves travelling in the shell casing were affected. Finally, the researchers assessed the effect of the biofouling and corrosion on the performance of the typical Relevance Vector Machine (RVM) classification algorithm.
The acoustic scattering measurements were carried out in Naval Research Laboratory’s (NRL) state-of-the-art Laboratory for Structural Acoustics Facility. The measurements were made over the broad frequency band from 2 kilohertz (kHz) to 160 kHz and over a full 360-degrees in steps of one-half or one degree. The band was covered by using two facility configurations: a high-frequency arrangement that utilizes a relatively small source and receiver, and a low-frequency system that deploys a large near-field source array to project a plane wave on the nearby target. After baseline scattering measurements were carried out on the “clean” targets, the UXO were exposed for 16 weeks underwater at an at-sea fouling location in Englewood, FL, that contained an aggressive fouling community with a high percentage of hard fouling species, so that significant biofouling occurred during this relatively short time. After completing acoustic measurements on the bio-fouled UXO, the shells were cleaned and readied for accelerated corrosion in order to produce magnetite/Hermatite corrosion layers. To accomplish this, the munitions were anodically polarized in a bath of 0.05M sodium chloride (NaCl) for one week, such that the corrosion rate was accelerated, simulating approximately three months of seawater exposure. After the corrosion process was completed, the acoustic measurements were repeated once more.
The biofouled and magnetite-corroded surrogates showed no measurable difference in the EMI response when measured in artificial sea water and when measured in air. No measurable difference was seen between bare surrogates and those with the corrosion layers applied. A small difference was observed for laboratory-generated calcareous deposits for one EMI response axis only. This result was potentially corroborated by contact—electrochemical measurements made on samples continuously exposed long-term to seawater.
Structural Acoustics Results:
The baseline measurements were analyzed in terms of basic echo mechanisms, which included specular scattering from shell ends and cylindrical surfaces, circumferential Rayleigh wave and compressional wave ring resonances, axial compressional and creeping wave interference with specular scattering, elastic wave generation in filler due to phase matching, and other mechanisms. Low frequency (structural acoustic domain) acoustic scattering measurements made on the two biofouled shells together with subsequent analysis demonstrated the following: Overall, the biofouling has affected the scattering levels a small amount but not the overall acoustic color frequency-angle spectra. The level changes, amounting to on average <2 decibels (dB) for the 5-inch shell and about 5dB for the 155mm shell, resulted in a corresponding drop in signal-to-noise and in turn the related detection ranges. However, the robustness of the acoustic color spectra indicated for the performance of the RVM classification algorithms. In this regard, both the acoustic color features and certain pressure magnitude features maintained good UXO classification performance against the set of six non-UXO targets. In particular, pressure magnitude features were shown to separate 4 of 6 false targets from biofouled UXO, whereas acoustic color features separated all 6 of 6 false targets from biofouled UXO.
Further, researchers found that biofouling tends to attenuate some axial creeping (~1.2–1.7 kHz) and circumferential elastic waves (~6 kHz–12 kHz) reducing fine structure. This was especially true for the 155mm shell. Reduced levels of fine structure were expected to lead to sharper low-frequency images since these mechanisms lead to echo time elongation, which corrupts the time-delay beamforming processing used in imaging. Regarding the small amount of corrosion achieved in the laboratory which attempts to accelerate the corrosion process, except for the apparently anomalous change for the echo from the front and back of both shells at the lowest frequencies and the significant effect on the circumferential Rayleigh wave for the 5-inch shell, there was as expected, no change of any consequence caused by this thin corrosion layer at the low structural acoustic frequencies.
The research teams concluded without specifically demonstrating it that there would be little impact on acoustic detection ranges, classification performance, or maximum burial depth caused by this thin corrosion layer. Regarding the measurements made over the high frequency (conventional imaging regime), the specular echo was affected only in a minor way by biofouling, and much of the fine structure due to creeping or elastic axial/circumferential waves was washed out (less true for 155mm shell). Finally, the thin corrosion layer had no noticeable effect on acoustic response over almost the entire band.
A number of Strategic Environmental Research and Development Program (SERDP) projects have been exploring SA-based sonar (SAS) and EMI for detection and classification of underwater UXO. Until now, there has been little or no information collected regarding the effect of seawater biofouling or corrosion on the acoustic color maps used to generate the SA features and images or the EMI signatures of these items. This project has established that significant biofouling and/or thin corrosion layers have little impact on acoustic color or on the related classification approaches, including EMI.