Point shapefile (100 shot interval) of navigation for chirp seismic data collected in the Indian River Bay, Delaware, on April 13, 2010, on U.S. Geological Survey Field Activity 2010-006-FA (IR_100SHOT_SORT.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，针对特拉华州印第安河湾（Indian River Bay）下方的淡水-咸水地下水界面及相关海底沉积构造，研究团队开展了地球物理勘探工作。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，其中环绕海湾周边的啁啾地震测线（chirp seismic lines）长度为36千米。全域海湾网格下的中分辨率CRP测量采用50米拖缆（streamer）完成。勘探与数据反演结果显示，印第安河湾下方埋藏有大量古河道（paleochannels），在现代支流、潮汐水道及沼泽区域，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（high-resistivity anomalies），对应与其相关的低矿化度地下水羽流（low-salinity groundwater plumes），这大概率是因为河道充填物中赋存细粒河口泥质与泥炭（estuarine mud and peats），它们可作为海底隔水层（submarine confining units）发挥作用。这类隔水层可使陆上补给的低矿化度地下水羽流越过海岸线运移，在地下形成复杂的淡水-咸水地下水界面。该界面的特性是构建精准海岸地下水流动模型的重要考量因素。此类模型可用于预测：在该类农业流域中补给的富营养地下水如何涌入沿海海湾，并对地表水水质及河口生态系统造成影响。如需了解本项目本次勘探的更多信息，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。