Jasper Schaer, U.S. Army Corps of Engineers Galveston District, Hydro Survey Team, observes a Houston Shipping Channel map after completing a post dredge survey aboard the Tanner II survey vessel. 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.