Backscatter B [USGS]--Offshore of Coal Oil Point, California

This part of SIM 3302 presents data for part of the acoustic-backscatter map (see sheet 3, SIM 3302) of the Offshore of Coal Oil Point map area, California. The raster data file is included in "BackscatterB_USGS_OffshoreCoalOilPoint.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreCoalOilPoint/data_catalog_OffshoreCoalOilPoint.html.
The acoustic-backscatter map of the Offshore of Coal Oil Point map area, California, was generated from backscatter data collected by California State University, Monterey Bay, Seafloor Mapping Lab (CSUMB), by the U.S. Geological Survey (USGS), and by Fugro Pelagos. These metadata describe the acoustic-backscatter data collected by the USGS. See "BackscatterA_CSUMB_OffshoreCoalOilPoint_metadata.txt" metadata for a description of the acoustic-backscatter data collected by CSUMB, and see "BackscatterC_Fugro_OffshoreCoalOilPoint_metadata.txt" metadata for a description of the acoustic-backscatter data collected by Fugro Pelagos. Most of the nearshore and shelf areas in the Offshore of Coal Oil Point map area were mapped by the USGS in the summers of 2006, 2007, and 2008, using a combination of 117-kHz and 234.5-kHz 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).

本数据集属于SIM 3302系列，包含加州煤油角近海制图区部分区域的声反向散射图（acoustic-backscatter map，参见第3幅图，SIM 3302）相关数据。栅格数据文件收录于"BackscatterB_USGS_OffshoreCoalOilPoint.zip"压缩包，可通过http://pubs.usgs.gov/ds/781/OffshoreCoalOilPoint/data_catalog_OffshoreCoalOilPoint.html获取。

加州煤油角近海制图区的声反向散射图由加州州立大学蒙特雷湾分校海底测绘实验室（CSUMB）、美国地质调查局（USGS）以及富格罗佩拉戈斯（Fugro Pelagos）采集的反向散射数据生成。本元数据（metadata）仅针对USGS采集的声反向散射数据进行说明：如需了解CSUMB采集的声反向散射数据详情，请参阅"BackscatterA_CSUMB_OffshoreCoalOilPoint_metadata.txt"元数据文件；如需了解Fugro Pelagos采集的声反向散射数据详情，请参阅"BackscatterC_Fugro_OffshoreCoalOilPoint_metadata.txt"元数据文件。

USGS于2006、2007及2008年夏季，结合使用117 kHz与234.5 kHz的SEA (AP) Ltd. SWATHplus-M相位差分式侧扫声呐（sidescan sonars），完成了该制图区内大部分近岸与陆架区域的测绘。在声反向散射影像中，色调越亮代表反向散射强度越高，色调越暗则代表反向散射强度越低。该强度值反映了声脉冲（acoustic pulse）与海底之间的复杂相互作用，以及浅表层（shallow subsurface）的特征，可用于大致表征海底纹理与沉积物类型。

反向散射强度受多个因素影响：声源级、海底成像所用频率、掠射角（grazing angle）、海底组成与特征（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。坚硬且粗糙的底质（如岩石露头或粗粒沉积物）通常会返回更强的信号（高反向散射，对应亮色调），而松软底质（如细粒沉积物）则会返回较弱的信号（低反向散射，对应暗色调）。