Point shapefile of navigation and best depth values at ship positions during continuous resistivity profiling data collection in the Indian River Bay, Delaware, on April 14, 2010, on U.S. Geological Survey Field Activity 2010-006-FA (JD104GPS_BESTDEPTH.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，旨在厘清该区域下方的淡水-咸水地下水界面及相关的海底沉积构造。本次勘探在霍茨兰丁（Holts Landing）研究站点的周边区域采用了更高的空间分辨率测量方案，该站点后续完成了密集的陆上与近海研究工作。本次连续电阻率剖面（continuous resistivity profiling, CRP）测线总长达145千米，另在海湾周边布设了36千米的啁啾地震测线。中分辨率CRP测量采用50米拖缆（streamer），在全海湾范围内构建测量网格开展作业。勘探与数据反演（data inversion）结果显示，印第安河湾下方发育有大量埋藏古河道；在现代支流、潮汐水道及沼泽分布区域，这些古河道大致垂直于岸线延伸。其中在海湾东南部紧邻怀特溪（White Creek）的区域，成像到了一套尤为宽阔深邃的古河道系统。多数古河道还伴随有高电阻率异常，对应与之相关的低矿化度地下水羽流，该现象大概率是由于河道充填物中赋存细粒河口泥质与泥炭，这些物质充当了海底隔水层（submarine confining units）。当这类隔水层存在时，陆上补给的低矿化度地下水羽流便可突破岸线限制，在地下形成复杂的淡水-咸水地下水界面。该界面的特征是构建精准海岸地下水流模型的核心考量因素之一。此类模型可用于辅助预测：如本次研究所在的农业流域中，经补给形成的富营养地下水如何汇入近岸海湾，并对地表水水质与河口生态系统造成影响。如需了解本次项目勘探的更多详情，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。