Circular synthetic aperture ultrasonic acoustic imaging using a 2.25 MHz underwater sonar

Experiments were performed in the USNA physical acoustics lab to demonstrate underwater sound imaging using circular synthetic aperture acoustic C-SAA imaging in two dimensions. The apparatus includes a single 2.25 MHz submersion transducer acting as both transmitter and receiver (2.54 cm diam with a manufacture's specified 4 cycle pulse shape) located 15 cm from the target's center. A structure of thin perforated disks (7.6 cm diameter) comprise a step-motor controlled rotational platform that supports several slender stainless steel cylindrical rod vertical targets (2 mm diameter, 10 cm long). The apparatus and supporting electronics serves in part as a teaching tool for imaging targets in a laboratory environment. This setup might be called inverse C-SAA considering a medical imaging tomography arrangement where the transmitter-receiver is fixed while the targets rotate at small angular intervals. Here, the platform “stops” for an echo versus time measurement and then rotates (or “hops”) to the next location for the following measurement. The transmitted pulse is generated by a rectangular pulse (duration T = 0.5/2.25 MHz) or utilizing a short tone burst. Correlation pulse compression can be used if a chirp is transmitted. A back-projection algorithm performed on Mathematica® was used to generate a 2-D reflectance image—for several targets.