A side scan sonar is a sonar device that can look sideways. This can be compared with a radar, but uses ultrasound echoes instead of electromagnetic pulses. The sound pulses are usually on frequencies between 100 and 500 kHz. A higher frequency gives better resolution but less range.
This technique was developed by Professor Harold Edgerton and others in the 1960s. It is commonly used for wreck search in underwater archaeology.