02031 - Geophysical Surveys of Bear Lake, Utah-Idaho, September 2002 - Sidescan-sonar Tracklines (SSS_TRK)

Bear Lake is a tectonic lake that has existed for at least several hundred thousand years. The lake basin is a relatively simple half graben, a spoon-shaped depression tilted toward the main fault on the east side of the lake. The U.S. Geological Survey, in cooperation with researchers from several universities, has been studying the sediments of Bear Lake since 1996. The general purpose of this effort is to reconstruct past limnological conditions and regional climate on a range of timescales, from hundreds of years to hundreds of thousands of years. This research relates to a variety of human concerns, including water usage in the Bear River basin. Past work has included several coring operations, a seismic-reflection survey, sediment-trap deployments, a barge-mounted drilling operation with the GLAD800 drill rig, and a variety of other studies. The objectives of the September, 2002 operations, preliminarily reported here, were (1) to compile a detailed bathymetric map of the lake using swath-mapping techniques, in order to provide baseline data for a variety of applications and studies, and (2) to complete a sidescan-sonar survey of the lake, providing a nearly complete acoustic image of the lake floor. Limited amounts of subbottom acoustic-reflection data (chirp) were also collected, along with samples of lake-floor sediments representative of different kinds of backscatter patterns. These surveys followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).贝尔湖（Bear Lake）为一处构造湖（tectonic lake），其存续历史至少已达数十万年。该湖盆属于相对简单的半地堑（half graben）构造，为朝湖东侧主断层倾斜的勺状凹陷。美国地质调查局（U.S. Geological Survey）与多所高校的研究人员合作，自1996年起启动贝尔湖沉积物的系统性研究。本研究的总体目标是重建数百年至数十万年时间尺度下的古湖沼学环境与区域气候格局。此项研究关联多个人类现实关切议题，其中涵盖贝尔河流域（Bear River basin）的水资源利用相关问题。过往研究已开展多次沉积物取芯作业、地震反射勘探、沉积物捕集器布放、搭载GLAD800钻机的驳船钻探作业，以及其他多项相关研究工作。 2002年9月的野外作业目标（本文在此初步报道）包含两点：（1）采用条带测深技术（swath-mapping）绘制贝尔湖高精度水深地形图，为各类应用与研究提供基准数据集；（2）完成贝尔湖全域侧扫声呐（sidescan-sonar）调查，获取近乎全覆盖的湖底声学影像。本次作业还同步采集了少量亚底声学反射（subbottom acoustic-reflection）数据（即啁啾脉冲（chirp）系统数据），并采集了对应不同反向散射模式的湖底沉积物样本。此次调查接续1997年开展的亚底声学反射调查，当时使用的是震源与3.5 kHz探测系统（S. M. Colman，未发表数据）。