In-situ far-field calibration of multibeam sonar arrays for precise backscatter imagery

Calibration of large hull-mounted sonar transducer arrays is usually done by measuring the performance of individual components in a test tank. Multiple transducer element beam patterns are superimposed and combined with window attenuation and other partial measurements to give an approximation of the total beam pattern. This process often ignores or fails to accurately model factors such as mounting hardware, array misalignment, reflections from the hull structure and other real world absorbers, reflectors and attenuators which can have significant effect on the actual beam pattern. This paper describes an application of a novel technique for in-situ measurement of the far-field beam pattern of an entire installed hydrophone array using an ROV to carry a calibrated reference transducer. The sonar array is part of a Simrad EM-121 multibeam sonar system installed aboard the Navy's new survey ship the USNS SUMNER TAGS-61. Test tank measurements of individual hydrophone transducer packages were available and a composite array beam pattern was computed from this data. Direct at-sea measurement of the far-field beam pattern was then made by placing a reference transducer on an ROV and navigating it through an are in the far field of the shipboard array. These two sets of beam patterns are compared. Backscattered signals in a multibeam sonar system can be the basis for characterization of seafloor morphology in terms of mud, clay, silt, sand, gravel, rocks, etc. The degree of differentiation between bottom types, or the number of bottom types which can be differentiated, depends on how accurately the signal strength is calibrated. Factors which affect this measurement include the sonar transmitter electrical output, projector efficiency, projector array beam pattern, spreading loss and attenuation in the water column, losses at the bottom reflection, hydrophone array beam pattern, hydrophone sensitivity and receiver gains. The degree of improvement in seafloor characterization resulting from the in-situ hydrophone array calibration are examined.