4-m Image of the Northern Half of the Backscatter Intensity Mosaic of the Sea Floor off Northeastern Cape Cod from USGS Cruise 98015 (CAPENORTHMOS_GEO4M_WGS84.TIF, Geographic, WGS84)

This data set includes backscatter intensity of the sea floor offshore of northern Cape Cod, Massachusetts. The data were collected with a multibeam sea floor mapping system during USGS survey 98015, conducted November 9 - 25, 1998. The surveys were conducted using a Simrad EM 1000 multibeam echo sounder mounted aboard the Canadian Coast Guard vessel Frederick G. Creed. This multibeam system utilizes 60 electronically aimed receive beams spaced at intervals of 2.5 degrees that insonify a strip of sea floor up to 7.5 times the water depth (swath width of 100 to 200 m within the survey area). The horizontal resolution of the beam on the sea floor is approximately 10% of the water depth. Vertical resolution is approximately 1 percent of the water depth.
With backscatter intensity, the intensity of the acoustic return from the sea floor from the multibeam system, is a function of the properties of the surficial sediments and of the bottom roughness. Generally, a strong return (light gray tones) is associated with rock or coarse-grained sediment, and a weak return (dark gray tones) with fine-grained sediments. However, the micro-topography, such as ripples, burrows, and benthic populations also affect the reflectivity of the sea floor. Direct observations, using bottom photography or video, and surface samples, are needed to verify interpretations of the backscatter intensity data. The backscatter data have a weak striping that runs parallel to the ship's track. Some of the striping is the result of poor data return at nadir that appears as evenly-spaced thin speckled lines. Some striping is also due to critical angle effects, where the intensity of return varies as a function of the angle of incidence of the incoming sound on the seafloor (Hughes-Clark and others, 1997).

本数据集涵盖美国马萨诸塞州科德角北部近海海底的反向散射强度（backscatter intensity）数据。该数据采集于1998年11月9日至25日开展的美国地质调查局（USGS）98015号勘测任务，采用多波束海底测绘系统（multibeam sea floor mapping system）完成。本次勘测使用搭载于加拿大海岸警卫队“弗雷德里克·G·克里德”号（Frederick G. Creed）船舶的Simrad EM 1000多波束测深仪（multibeam echo sounder）开展作业。该多波束系统配备60个电子定向接收波束，波束间距为2.5度，可覆盖宽度达7.5倍水深的海底条带（本次勘测区域内条带宽度为100至200米）。海底波束的水平分辨率约为水深的10%，垂直分辨率约为水深的1%。

反向散射强度即多波束系统接收的海底声学回波强度，其大小与表层沉积物（surficial sediments）特性及海底粗糙度（bottom roughness）密切相关。通常而言，强回波（浅灰色色调）对应岩石或粗粒沉积物，弱回波（深灰色色调）则对应细粒沉积物。但微地形（micro-topography，如沙波、洞穴及底栖生物群落（benthic populations））同样会影响海底反射特性。需借助海底摄影、视频记录及表层沉积物采样等直接观测手段，对反向散射强度数据的解译结果进行验证。

该反向散射数据存在与船舶航迹平行的微弱条带噪声。部分条带源于天底点（nadir）处的数据回波质量不佳，表现为间距均匀的细碎斑点状线条。部分条带则由临界角效应（critical angle effects）所致：当入射声波在海底的入射角变化时，回波强度也会随之改变（Hughes-Clark等，1997）。