Buried Object Scanning Sonar for AUVs

A 252 channel FM sonar is developed to generate images of objects buried in sediments using reflection tomography. An omnidirectional source, transmitting FM pulses over the band of 2 to 12 kHz, illuminates buried targets. The backscattered signals are measured with 252 hydrophones and processed with a digital matched filter. Coherent nearfield focusing generates a 3D map of acoustic intensity for each transmission event. As the sonar approaches and passes buried targets, the processor generates 3D matrices of acoustic intensity, overlapping in space and referenced to a coordinate system fixed to the seabed. The co-located pixels are added incoherently to generate a multi-aspect image of the target. The change in vehicle position between transmissions is measured with a DVL(doppler velocity log) and IMU (inertial measurement unit). The resulting reflection tomographic images provide target shape information useful for target classification. The sonar can be mounted on small AUVs by replacing the 252 channel, 1.5 m diameter array with one meter long line arrays mounted as wings. The receiver aperture is generated synthetically using the near field focusing processor which performs time delay focusing based on the positions of the hydrophones in the line array and vehicle motion data. A comparison between synthetic swath and tomographic images of a rigid spherical target in water shows the improvement provided by reflection tomography.