Jasper Schaer, U.S. Army Corps of Engineers Galveston District, Hydro Survey Team and technical lead, retrieves a GPS sounding tool prior to beginning a post dredge survey. Calibrating sonar instruments to local water quality is an important part of taking accurate measurements. Sounding measures the speed at which the single beam sonar waves pass through the water. When a large ship passes, it disturbs fine sediment at the bottom of the channel, known as fluff, which slows down the sonar signal. With the heavy traffic in the Houston Shipping Channel, Schaer performs frequent sounding checks to ensure precise measurement of the channel floor.