Backscatter B [USGS]--Offshore of Santa Barbara, California

This part of SIM 3281 presents data for part of the acoustic-backscatter map (see sheet 3, SIM 3281) of the Offshore of Santa Barbara map area, California. The raster data file is included in "BackscatterB_USGS_OffshoreSantaBarbara.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSantaBarbara/data_catalog_OffshoreSantaBarbara.html.
The acoustic-backscatter map of the Offshore of Santa Barbara map area, California, was generated from backscatter data collected by California State University, Monterey Bay, Seafloor Mapping Lab (CSUMB) and by the U.S. Geological Survey (USGS). These metadata describe the acoustic-backscatter data collected by the USGS. See "BackscatterA_CSUMB_OffshoreSantaBarbara_metadata.txt" metadata for a description of the acoustic-backscatter data collected by CSUMB. Small areas in the far-east nearshore, as well as further offshore to the west and in the southeast outer shelf area, were mapped by the USGS in 2005 and 2006, using a combination of 468-kHz (2005) and 117-kHz (2006) SEA (AP) Ltd. SWATHplus-M phase-differencing sidescan sonars. Within the acoustic-backscatter imagery, brighter tones indicate higher backscatter intensity, and darker tones indicate lower backscatter intensity. The intensity represents a complex interaction between the acoustic pulse and the seafloor, as well as characteristics within the shallow subsurface, providing a general indication of seafloor texture and sediment type. Backscatter intensity depends on the acoustic source level; the frequency used to image the seafloor; the grazing angle; the composition and character of the seafloor, including grain size, water content, bulk density, and seafloor roughness; and some biological cover. Harder and rougher bottom types such as rocky outcrops or coarse sediment typically return stronger intensities (high backscatter, lighter tones), whereas softer bottom types such as fine sediment return weaker intensities (low backscatter, darker tones).

本部分内容隶属于SIM 3281数据集，提供了加利福尼亚州圣巴巴拉近海制图区部分声反向散射图 (acoustic-backscatter map，参见第3幅图，SIM 3281) 的相关数据。该栅格数据文件包含于"BackscatterB_USGS_OffshoreSantaBarbara.zip"压缩包中，可通过http://pubs.usgs.gov/ds/781/OffshoreSantaBarbara/data_catalog_OffshoreSantaBarbara.html获取。
加利福尼亚州圣巴巴拉近海制图区的声反向散射图 (acoustic-backscatter map)，由蒙特雷湾加州州立大学海底测绘实验室 (California State University, Monterey Bay, Seafloor Mapping Lab，简称CSUMB) 与美国地质调查局 (U.S. Geological Survey，简称USGS) 采集的反向散射数据生成。本元数据仅用于说明USGS采集的声反向散射数据；关于CSUMB采集的声反向散射数据的详细描述，请参阅"BackscatterA_CSUMB_OffshoreSantaBarbara_metadata.txt"元数据文件。
USGS于2005年和2006年对远东部近岸小片区域、西部远海区域以及东南部外陆架区域开展了测绘工作，所用设备为SEA (AP) Ltd.生产的SWATHplus-M相位差分侧扫声呐 (phase-differencing sidescan sonars)，其中2005年使用的工作频率为468 kHz，2006年为117 kHz。在声反向散射影像中，色调越亮代表反向散射强度 (backscatter intensity) 越高，色调越暗则代表反向散射强度越低。该强度反映了声脉冲与海底之间的复杂相互作用，以及浅表层地下的特征，可用于大致指示海底质地与沉积物类型。反向散射强度受多种因素影响：声源级、海底成像所用的声波频率、掠射角 (grazing angle)、海底的组成与特征（包括粒度、含水量、体积密度 (bulk density) 及海底粗糙度），以及部分生物覆盖物。通常而言，硬质粗糙底质（如岩石露头或粗颗粒沉积物）会返回更强的信号（高反向散射，色调偏亮），而软质底质（如细颗粒沉积物）则返回较弱的信号（低反向散射，色调偏暗）。