Construction of a Large Underwater Acoustic Array

Abstract Recently installed in Lake Penal Oreille, Idaho, the acoustic arrays for the Intermediate Scale Measurement System involved an extensive effort in construction planning, Composed of many inter-connected structural lines, electrical cables, anchors, buoys, sheaves, hydrophones and acoustic transducers, the array was assembled underwater one component at a time. Because each component had to be placed in a precise position, the installation effort required a technically advanced positioning system, a specially built project barge, and unique deployment methods. Many of these installation procedures are applicable to installation of other types of deep-water structures. Project Background Situated in Lake Penal Oreille, Idaho, the Intermediate Scale Measurement System (ISMS) provides high quality measurements of the structural acoustic response of large-scale submarine models in a controlled environment 1. Fig. 1 shows the Near-Field Array (NFA) of the ISMS. The NFA consists of the following system components, located in the 1,200-feet (366- meters) deep lake:Transmit Array (TA): A vertical plane of projectors for transmitting an acoustic signal.Target Model Haul-Down (TMH); A mechanism for handling a buoyant submarine model.Receive Array (RA): A horizontal ring of hydrophones for measuring the acoustic signature.l N-Array (NA): A small set of hydrophones for measuring the incident acoustic signal. All acoustic components and the submarine model are in a horizontal plane at approximately mid-depth in the lake. The TA projectors hang below a rigid buoyant truss structure, Two anchor lines at each end of the TA truss hold the truss approximately 64 feet (20 meters) below the lake surface. Located on shore approximately 2.5 miles (4 km) from the NFA site, winches haul the buoyant submarine model underwater. A network of underwater electrical and fiber-optic cables connects the acoustic arrays to shore. Of the NFA components, the RA provided the most challenging installation effort. Thus in this paper, we shall focus on the RA structure. Receive Array (RA). The receive array consists of 12 segments of hydrophones, totaling 158 hydrophones. When connected end to end, these segments form an oblong ring surrounding the submarine model. The longest hydrophone segment is 282 feet (86 meters) long. A structural node at each end of each segment holds each hydrophone segment in place. Each node is 602 feet (183 meters) above the lake bottom. Electrical cables carry acoustic information from each end of each RA segment to the TMH and then on to shore. A buoy near the water surface and three anchors at the lake bottom hold each node in place. Each node buoy is a 6-foot (1.8-meter) diameter syntactic foam sphere. Each anchor is an 8-foot (2.4-meter) square steel 'cookie-cutter,' A 0.5-inch (1 .25- cm) diameter parallel-fiber aramid (Kevlar) line connects each anchor to each node. End sections of wire rope provide for precise final adjustments in length given as-installed anchor locations.