Ship tracklines along which continuous resistivity profiling data were collected in the Indian River Bay, Delaware, on April 13, 2010, on U.S. Geological Survey Field Activity 2010-006-FA (JD103GPS_LINES_SPLITS.SHP, Geographic, WGS 84)

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，美国特拉华州印第安河湾（Indian River Bay）开展了一项地球物理勘探工作，旨在划定该区域下方的淡水-咸水地下水界面及其相关的海底次表层沉积构造。本次勘探在霍尔特登陆点（Holts Landing）的研究区域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调查。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）。海湾全域采用50米拖缆（streamer）开展中分辨率连续电阻率剖面法测量。勘探与数据反演结果显示，印第安河湾下方存在大量埋藏古河道（paleochannels）；在现代支流、潮汐溪与沼泽分布区域，这些古河道大致垂直于岸线延伸。在海湾东南部靠近怀特溪（White Creek）的区域，成像到了一套规模尤为宽阔深邃的古河道系统。多数古河道伴随有高电阻率异常，对应与其相关的低盐度地下水羽流，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，这类物质可作为海底隔水层（submarine confining units）。此类海底隔水层可使陆上补给的低盐度地下水羽流越过岸线扩散，在地下形成复杂的淡水-咸水地下水界面。该界面的相关物性参数，是构建精准海岸地下水流模型的核心考量因素。此类模型可用于预测在类似的农业流域（agricultural watersheds）中补给的富营养地下水如何汇入海岸海湾并影响地表水水质与河口生态系统（estuarine ecosystems）。如需了解本次项目勘探的更多信息，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。