02031 - Geophysical Surveys of Bear Lake, Utah-Idaho, September 2002 - Shot Point Navigation 500 shot interval (CHRP_500)

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followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).

贝尔湖是一处至少形成于数十万年之前的构造湖。其湖盆为结构相对简单的半地堑构造，呈勺状凹陷，朝湖东侧主断裂倾斜。美国地质调查局（U.S. Geological Survey）自1996年起联合多所高校研究人员，持续开展贝尔湖沉积物研究工作。此项研究的核心目标是重建数百年至数十万年时间尺度下的古湖沼环境与区域气候。该研究关联诸多人类关切议题，其中涵盖贝尔河流域的水资源利用问题。既往研究工作已开展多次岩芯钻探、地震反射勘探、沉积物捕集器布设、搭载GLAD800钻机的驳船钻探作业及其他多项相关研究。本文初步报道了2002年9月野外作业的两大目标：其一，采用条带测深技术编制湖泊高精度水深地形图，为各类应用与研究提供基准数据；其二，完成全湖侧扫声呐勘测，获取近乎全覆盖的湖底声学影像。本次作业还采集了少量亚底声学反射（chirp）数据，并同步采集了对应不同反向散射特征的湖底沉积物样本。此次勘测接续了1997年开展的亚底声学反射勘测工作，当时采用了电火花震源（boomer）与3.5kHz声呐系统（S. M. Colman，未发表数据）。