Sonar used in search for missing SA family

Police in South Australia will resume searching for a family who never returned from a boat trip with the help of sonar technology.

Len and Annette Vandepeer, both 56, and their 27-year-old son Doug set out on their boat from Cape Jaffa in the state's southeast on December 12 and failed to return in the evening.

An extensive air, sea and land search for the missing trio and their seven-metre boat was called off on December 19, with the family conceding their loved ones had likely died.

The SA police water unit will on Wednesday begin a new search for the Vandepeers.

Police say they will use side-scan sonar to search the sea floor for the boat in a four-day operation.