Measurements and Modeling of Acoustic Scattering from Unexploded Ordnance (UXO) in Shallow Water

Introduction: The gradual development of coastal areas throughout the world in proximity to military training ranges and areas of past conflict has created a need to locate underwater unexploded ordnance (UXO). These ordnance, which may have veered off course and failed to detonate when they were initially released, may still pose a risk of explosion if they are subject to handling. This complicates the process of converting areas formerly restricted to military training, for instance, into areas with general undersea and surface activities such as fishing, boating, or diving. Searches for UXO can be costly and time consuming due to the large areas of water that need to be covered and the possibility that UXO are buried in the seafloor. Probing the ocean bottom with sound waves offers an effective means of searching for UXO because sound pulses can propagate relatively long distances underwater and penetrate the seafloor. The use of low-frequency sound, which for this application is 2 to 30 kHz, has distinct advantages for this application. Sound waves in the lower end of this spectrum can easily penetrate the seafloor and inside the UXO, causing it to respond with structural acoustic scattering signatures different from other objects. This is in contrast to high-frequency sonar, which is useful for creating highly resolved images of an object lying on the ocean bottom but is less effective if the object is buried. These structural acoustic clues help in classifying whether an echo from an object corresponds to UXO or to an object that poses no risk. We are using controlled laboratory tank experiments and computer modeling to gain a fundamental understanding of how low-frequency sound waves interact with UXO at various burial depths and in the presence of seafloor roughness. An example UXO we have examined is a 5 in. diameter rocket warhead (Fig. 1). It consists of a thickwalled steel shell with interior compartments located at its ends and an inert filler material throughout most of its body.