02031 - Geophysical Surveys of Bear Lake, Utah-Idaho, September, 2002 - Bathymetry Tracklines (BATHY_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 (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followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).贝尔湖（Bear Lake）是一处至少存续了数十万年的构造湖（tectonic lake）。其湖盆为相对简单的半地堑（half graben），是一个朝湖东侧主断裂倾斜的勺状凹陷。美国地质调查局（U.S. Geological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年起开展贝尔湖沉积物研究，该工作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。此项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钻探作业、地震反射勘探、沉积物捕集器部署、搭载GLAD800钻机的驳船钻探作业，以及其他多项相关研究。 本文初步报道的2002年9月野外作业目标有二：（1）采用多波束测深（swath-mapping）技术绘制高精度湖盆水深图，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）勘测，获取近乎完整的湖床声学影像。本次作业还同步采集了少量亚底声学反射（chrip）数据，以及代表不同反向散射模式的湖床沉积物样本。本次勘测接续了1997年采用boomer与3.5 kHz系统开展的亚底声学反射勘测（S. M. Colman，未发表数据）。