Area and Depth Coverage Assessment for Acoustic Unexploded Ordnance Detection Surveys
Robbert van Vossen | TNO (Netherlands Organisation for Applied Scientific Research)
Low frequency synthetic aperture sonar (LF-SAS) is a key technique for the detection and characterization of unexploded ordnance (UXO) in the underwater environment. It is applicable to wide area surveys with a high area coverage rate and it is able to provide detailed information on sites where the presence of UXO already has been confirmed. For buried UXO, however, it is rather difficult to determine the range and depth up to which the detection of UXO is feasible. This critically depends on the detection system properties, environmental properties such as water depth and sediment type, target properties, and the actual burial depth up to which UXO needs to be detected. For the evaluation of UXO detection surveys, it is therefore not only relevant to analyze the acquired data to determine the presence or absence of UXO, but also to assess which area actually has been covered by the survey, and up to which depth coverage has been obtained. This assessment relies on the availability of tooling to estimate the achieved area and depth coverage. The objective of the project is to develop and validate a methodology for coverage assessment.
To determine the actual coverage achieved by a side looking LF-SAS system, the detectability of UXO will have to be assessed for different ranges and burial depths. The project coverage assessment approach will be a hybrid approach, combining measured sensor data with model predictions for the target signal, also referred to as the target-in-environment response (TIER). To gain insight in the relevant environmental properties needed to predict the target signal, a through-the-sensor approach will be developed, relying on measurements acquired by a wideband LF-SAS system, supported by measurements of a high-frequency side scan sonar. These measurements can be used to identify the dominant scattering mechanisms, for example, and indicators can be derived from these observations up to which depth sound penetrates into the seabed. To make inferences on the detectability of UXO objects, the coverage assessment also needs to take into account the variations in TIER with aspect angle. It is intended to develop the coverage assessment methodology for a side looking LF-SAS and to validate the methodology using measured data acquired with TNO’s Low-Frequency Synthetic Aperture Sonar System. Available data sets contain test objects at various ranges and different burial depths and are available in different environments.
Perhaps the most important measure of effectiveness for UXO clearance operations, in common with mine-hunting and mine-sweeping operations, is not the number of devices remediated but the probability that any device remains in an area after the end of operations, and the corresponding risk that these objects impose. The calculation of this measure is inherently difficult because it involves the assessment of the significance of an absence of evidence. The amount of UXO remediated can definitively be measured but the likelihood of any remaining must be predicted. The project coverage assessment is a key ingredient for the risk assessment. Information on achieved coverage by an LF-SAS system provides essential information for developing concepts of operation for UXO surveys, it provides information on strengths and weaknesses of LF-SAS, and consequently how to best use this technique in combination with other techniques such as magnetic anomaly detection.