Synthetic aperture sonar 3-D imaging of targets in air using multiple, non-parallel shot lines

Subsurface borehole radar imaging is an attractive technology in a number of applications. In order to test algorithm development beyond pure simulation, acoustics radar (sonar) offers valuable insights into the practical performance of an algorithm. A sonar system operating at 40 kHz in air has been developed to allow the capture of acoustic image data in a laboratory environment. A deconvolution filter is generated for range compression, via a calibration technique, in which the system response is measured by pointing the transmitting transducer directly at the receiving transducer. The focused image is obtained by time domain synthetic aperture focusing/migration techniques. The practical drilling of boreholes results in non-parallel holes which can, however, be logged in three dimensions. A 3D reconstructed algorithm has been developed and implemented to reconstruct the image of the scene by multiplying the magnitude images acquired from multiple, non-parallel shot lines. The paper is structured as follows: firstly the inverse synthetic aperture sonar (ISAS) imaging experiment is described, followed by the signal processing techniques. Thereafter, the 3D imaging technique using focused magnitude images from non-parallel shot lines and the reconstruction grid spacing are described.