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followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).贝尔湖（Bear Lake）是一处至少存在数十万年的构造湖（tectonic lake）。其湖盆为结构相对简单的半地堑（half graben），是一处朝湖东侧主断裂倾斜的勺状凹陷。美国地质调查局（U.S. Geological Survey）联合多所高校的研究人员，自1996年起持续开展贝尔湖沉积物研究，核心目标是重建数百年至数十万年时间尺度下的古湖沼学状况与区域气候。该研究关联诸多人类关切议题，其中包括贝尔河流域的水资源利用问题。过往研究已开展多项取芯作业（coring operations）、地震反射勘探（seismic-reflection survey）、沉积物捕集器布设（sediment-trap deployments）、搭载GLAD800钻机（GLAD800 drill rig）的驳船式钻探作业，以及其他各类相关研究。 本文初步报告的2002年9月野外作业目标有二：其一，采用条带测绘技术（swath-mapping techniques）绘制贝尔湖高精度水深图（bathymetric map），为各类应用与研究提供基础数据；其二，完成全湖侧扫声呐勘探（sidescan-sonar survey），获取近乎完整的湖床（lake floor）声学影像。本次作业还同步采集了少量水底声学反射（chirp）数据，以及代表不同反向散射模式（backscatter patterns）的湖床沉积物样本。上述勘测工作接续了1997年开展的水底声学反射勘探，当时采用了电火花震源（boomer）与3.5 kHz系统（3.5 kHz systems）（S. M. 科尔曼，未发表数据）。