Continuous resistivity profiling data processed with multiple water conductivity values from Indian River Bay, Delaware, during April 2010 on U.S. Geological Survey Field Activity 2010-006-FA

A geophysical survey to delineate the fresh-saline groundwater interface and associated sub-bottom sedimentary structures beneath Indian River Bay, Delaware, was carried out in April 2010. This included surveying at higher spatial resolution in the vicinity of a study site at Holts Landing, where intensive onshore and offshore studies were subsequently completed. The total length of continuous resistivity profiling (CRP) survey lines was 145 kilometers (km), with 36 km of chirp seismic lines surveyed around the perimeter of the bay. Medium-resolution CRP surveying was performed using a 50-meter streamer in a bay-wide grid. Results of the surveying and data inversion showed the presence of many buried paleochannels beneath Indian River Bay that generally extended perpendicular from the shoreline in areas of modern tributaries, tidal creeks, and marshes. An especially wide and deep paleochannel system was imaged in the southeastern part of the bay near White Creek. Many paleochannels also had high-resistivity anomalies corresponding to low-salinity groundwater plumes associated with them, likely due to the presence of fine-grained estuarine mud and peats in the channel fills that act as submarine confining units. Where present, these units allow plumes of low-salinity groundwater that was recharged onshore to move beyond the shoreline, creating a complex fresh-saline groundwater interface in the subsurface. The properties of this interface are important considerations in construction of accurate coastal groundwater flow models. These models are required to help predict how nutrient-rich groundwater, recharged in agricultural watersheds such as this one, makes its way into coastal bays and impacts surface water quality and estuarine ecosystems. For more information on the survey conducted for this project, see https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA.

2010年4月，针对美国特拉华州印第安河湾（Indian River Bay）下方的淡水-咸水地下水界面及其伴生的海底沉积构造，研究团队开展了一项地球物理勘探工作。本次勘探在霍尔特兰丁（Holts Landing）研究站点周边采用了更高空间分辨率的测量方案，该区域后续完成了密集的陆上与近海研究。连续电阻率剖面法（continuous resistivity profiling, CRP）测线总长度达145千米，同时在海湾周边布设了36千米的啁啾地震测线（chirp seismic lines）进行测量；全海湾网格状的中分辨率CRP测量则采用50米长的拖缆（streamer）开展作业。勘探与数据反演（data inversion）结果显示，印第安河湾下方埋藏有大量古河道（paleochannels），在现代支流、潮汐溪流与沼泽分布区域，这些古河道大致垂直于海岸线延伸；在海湾东南部靠近怀特溪（White Creek）的区域，成像到了一处规模格外宽大深邃的古河道系统。多数古河道伴随有高电阻率异常，对应着与其相关的低盐地下水羽流（low-salinity groundwater plumes），这一现象大概率源于河道充填物中赋存的细粒河口泥质与泥炭（estuarine mud and peats），这类物质可作为海底隔水层（submarine confining units）；当此类隔水层存在时，陆上补给的低盐地下水羽流便可越过海岸线运移，在地下形成复杂的淡水-咸水地下水界面。该界面的相关属性是构建精准沿海地下水流动模型（coastal groundwater flow models）的核心考量因素，此类模型可用于预测在类似农业流域（agricultural watersheds）中补给的富营养地下水如何进入沿海海湾，并对地表水水质与河口生态系统造成影响。如需了解本次勘探项目的更多信息，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。