Jasper Schaer, U.S. Army Corps of Engineers Galveston District, Hydro Survey Team, prepares a shipping channel map prior to beginning a post dredge survey. Single beam sonar sound waves are measured and recorded aboard the survey vessel, Tanner II. Single beam sonar was used for this survey because this version uses lower frequency sound than multi-beam sonar. The lower frequency gives better measurements through the fluff or fine sediment at the bottom of the Houston Shipping Channel.