Two Ways to Detect and Assess Munitions in Underwater Environments
The Munitions Response Program Area is addressing munitions response for a wide variety of aquatic environments such as ponds, lakes, rivers, estuaries, and coastal areas. Cost-effective technologies are needed to allow for rapid assessment of underwater areas in order to identify the extent and concentration of munitions and areas free from munitions in particularly shallow water areas (water depths less than 5 meters).
A research team lead by Dr. Gregory Schultz at White River Technologies is focusing on demonstrating unmanned aerial system (UAS) magnetometers and supporting airframe sensors in challenging
unexploded ordnance (UXO) contaminated nearshore and lacustrine environments ( Project Overview). The combination of new miniature atomic total field magnetometers, drone-integrated tow bird systems, mission planning and electro-optical imaging, and streamlined data processing methods enables a direct path to new UXO wide area surveying technology for delineating risk amongst sites. However, there are challenges to overcome; such as controlled surveying at low altitudes, limited flight durations, launch and recovery with a towed payload, and mitigating electromagnetic noise to make the most of acquired data.
In June 2020, the research team, in collaboration with Dr. Martin Helmke of West Chester University, completed flight tests of an integrated sensor and UAS system flown over a test area with surrogate UXO targets. The test was conducted over both dry land and shallow water areas using a new radar altimeter-based terrain following methodology that enables automated controlled surveys over land and water and around obstacles with similar navigation precision as that found in established ground-based geophysical surveying. Follow-on testing in July exercised and demonstrated the potential of UAS-based sensors to effectively map over water and land and to generate geo-registered magnetic anomaly maps that can be correlated with aerial photographs for both areal delineation of munitions concentrations and detection and location of individual munition items. Continued work on well-controlled flight to minimize motion-related sensor noise and to optimize survey design strategies on multiple airframes is now underway. One of the primary goals for upcoming full-scale demonstrations is to validate the combination of UAS surveying, aerial magnetic sensing, and electro-optical mapping from multiple types of drone airframes over shallow water environments.
Low frequency synthetic aperture sonar (LF-SAS) is a key technique for the detection and characterization of UXO in the underwater environment. The performance of LF-SAS against buried UXO objects, however, heavily depends on the characteristics of the objects of interest and of the environment. As illustrated in the data examples below, proud objects can be observed within the entire footprint, whereas the detectability of the same object buried at 0.3 m heavily varies with range. As a consequence, it is not straightforward to answer the question whether a buried UXO object, if present at a certain position, would have been detected or not. This question, however, needs to be addressed to evaluate UXO surveys.
The objective of this SERDP project, “ Area and Depth Coverage Assessment for Acoustic Unexploded Ordnance Detection Surveys,” led by Dr. Robbert van Vossen (TNO) is to investigate how such a range- and depth coverage assessment capability can be obtained. The envisaged approach includes the following steps: (i) environmental assessment to obtain the necessary information on the environment, (ii) target-in-environment-response modelling, (iii) merge simulated targets with measured data, and (iv) range- and depth coverage assessment using the simulated targets that are embedded in the measured data.
Ideally, information on the environment is obtained through the sensor using the UXO survey system itself. Then, relevant information is acquired in the entire survey area. An interesting result has been obtained in an environment with a soft seabed sediment layer (mud) with sand underneath (figure below). Interferometric height estimates for a high-frequency sonar are interpreted to correspond to the water-mud interface, whereas interferometric height estimates for LF-SAS originate from interface scattering between the mud/sand interface or volume scattering within the sand layer. The low and high frequency height estimates are identical for a proud object. This example illustrates that a multi-band interferometric sonar can be used to map a mud layer thickness in a survey area. The project team will be presenting at the International Conference on Underwater Acoustics on September 9, 2020.