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。该数据由美国地质调查局（USGS）编号为98015的勘测项目于1998年11月9日至25日期间采集，所用设备为多波束海底测绘系统。本次勘测使用安装于加拿大海岸警卫队"弗雷德里克·G·克里德"号船舶上的Simrad EM 1000型多波束回声测深仪（multibeam echo sounder）。该多波束系统配备60个电子定向接收波束，波束间隔为2.5度，可覆盖最大达水深7.5倍的海底条带，本次勘测区域内条带宽度为100米至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采集到的海底声学回波强度，是表层沉积物属性与海底粗糙度的函数。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）对应细粒沉积物。不过，微地形（如沙波、掘穴以及底栖生物群落）同样会影响海底反射特性。若要验证反向散射强度数据的解译结果，需借助海底摄影、视频观测以及表层沉积物取样等直接观测手段。

该反向散射数据存在与船舶航迹平行的微弱条带噪声。部分条带噪声源于天底处的数据回波质量不佳，表现为均匀分布的细斑点线条；另有部分条带噪声则由临界角效应引发：当入射声波在海底的入射角发生变化时，回波强度也会随之改变（Hughes-Clark等，1997）。