This study introduces an algorithm for geometric distortion corrections in sonar images. The proposed algorithm mainly consists of two stages. At the first stage, the raw images are processed with median filter and Frost filter for noise reduction and intensity enhancement. In the last stage, the geometric distortion correction is conducted using angular information given by a gyro sensor. The algorithm was successfully applied on raw sonar data collected on a pier survey.

Damages due to typhoon and floods are increasing as a result of global warming and abnormal climate phenomenon over the world. Flood and scour is one of the major causes for bridge collapse. However, maintenance and inspection techniques still need improvements especially for underwater structures like bridge piers in rivers. Conventionally, manual inspection by divers has been carried out for underwater structures of bridges. This has difficulties in gathering accurate and reliable field data under severe environmental conditions. Therefore, in this study, underwater inspection of bridges has been carried out using the side scan sonar (SSS), one of unmanned underwater inspection equipment. Compared to the conventional manual inspection, it has been found that the SSS can be efficiently utilized to underwater inspection of bridges especially with bad visibility or large-scale inspection area.