Geophysical Surveys of Bear Lake, Utah-Idaho, September 2002 - Bathymetric Grid (BATHYGRD.TIF)

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 (chrip) 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followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).贝尔湖是一处至少存在数十万年的构造湖（tectonic lake）。其湖盆为相对简单的半地堑（half graben）构造，系一处朝湖东侧主断层倾斜的勺状凹陷。美国地质调查局（U.S. Geological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）联合多所高校的研究人员自1996年起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尺度下的古湖沼环境与区域气候演化历史。本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。既往研究已开展多次取芯钻探作业、地震反射调查、沉积物捕集器布设、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月的作业目标（本文在此初步报道）为：(1) 采用多波束条带测深技术（swath-mapping techniques）绘制高精度湖泊水深图，为各类应用与研究提供基准数据；(2) 完成全湖侧扫声呐（sidescan-sonar）调查，获取近乎完整的湖床声学影像。本次调查还同步采集了少量啁啾式亚底声学反射数据，并获取了对应不同反向散射特征的湖床沉积物样本。本次调查延续了1997年开展的亚底声学反射调查工作，当时采用了电火花震源（boomer）与3.5 kHz声波系统（S. M. 科尔曼，未发表数据）。