Raw continuous resistivity profiling data collected in the Indian River Bay, Delaware, on April 12, 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，美国特拉华州印第安河湾（Indian River Bay）下方开展了一项地球物理勘测，用于划定淡水-咸水地下水界面及其相关的海底沉积构造。本次勘测在霍尔特兰丁（Holts Landing）研究站点周边采用更高空间分辨率进行测量，该站点后续完成了密集的陆上与近海研究。连续电阻率剖面（Continuous Resistivity Profiling, CRP）测线总长度达145千米，环绕海湾周边采集的啁啾地震（chirp seismic）测线总长36千米。全域网格的中分辨率CRP勘测采用50米拖缆（streamer）开展。勘测结果与数据反演结果显示，印第安河湾下方埋藏有大量古河道（paleochannel）：在现代支流、潮汐溪与沼泽区域，这些古河道大体垂直于海岸线延伸；在海湾东南部靠近怀特溪（White Creek）的区域，成像到了一处格外宽阔深邃的古河道系统。许多古河道还伴随有高电阻率异常，对应着与之相关的低矿化度地下水羽流（groundwater plumes），这大概率是因为河道充填物中含有细粒河口泥质与泥炭，这些物质可作为海底隔水单元（confining units）。当这类隔水单元存在时，陆上补给的低矿化度地下水羽流便可越过海岸线，在地下形成复杂的淡水-咸水地下水界面。该界面的特性是构建精准海岸地下水流模型（coastal groundwater flow models）的重要考量因素。这类模型可用于辅助预测：在此类农业流域中补给的富营养地下水如何进入海湾，进而影响地表水水质与河口生态系统。如需了解本次项目勘测的更多信息，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。