By Jason Chaytor (USGS)
Prior to the late 1970’s, seafloor depths were measured using single-beam echosounders. These single-beam systems measure the water depth directly beneath the research vessel. The hull-mounted transceiver transmits a high-frequency acoustic pulse in a beam directly downward into the water column. Acoustic energy is reflected off the seafloor beneath the vessel and received at the transducer (underwater speaker). Seafloor depth is determined by the time it takes for the pulse to leave and return to the transducer (corrected for the speed at which sound travels through water — 1500 m/s).
The primary tools currently used to map the seafloor are multibeam sonars (short for SOund NAvigation and Ranging). A multibeam sonar system sends out an array of sound pulses (a ‘ping’) in a fan shape with the sound bouncing off the seafloor at different angles (Figure 2). Because the sound bounces back at different angles, it is received by the ship at slightly different times, allowing seafloor depths to be determined from underneath the ship and from either side as well. This is sometimes referred to as swath bathymetry as it produces a swath of depth information along the path of the ship. Unlike single-beam echosounders which provide only a single depth reading per pulse, the multibeam sonar system (Kongsberg Simrad EM 1002) on the Nancy Foster provides as many as 111 soundings per ping. All of the soundings collected during a survey are merged together to produce a bathymetric (seafloor elevation) map.
Multibeam sonar systems like the one on the Nancy Foster not only provide information about the depth of the seafloor, they can also be used to learn about the composition of the seafloor, for example, if it is rock, mud, or sand. The intensity (or strength) of the sound that reflects off the seafloor is different for each type of material; sound reflects more intensely off harder material such as rock, while some of the sound is absorbed by softer materials such as sand and mud. Just as with the seafloor depth information, these intensity measurements can be merged together to produce a map of seafloor characteristics.