Unique shot point navigation for chirp seismic data collected in Indian River Bay, Delaware, April 13, 2010, on U.S. Geological Survey Field Activity 2010-006-FA (IR_SEISNAV.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，研究团队于美国特拉华州印第安河湾（Indian River Bay）开展了一项地球物理勘测工作，旨在明晰该区域的淡水-咸水地下水界面，以及湾下相关的海底下伏沉积构造。本次勘测在霍茨登陆点（Holts Landing）附近的研究区域采用了更高的空间分辨率，后续在此区域完成了密集的陆上与近海研究。本次连续电阻率剖面（continuous resistivity profiling, CRP）测线总长度达145千米，另在海湾周缘布设了36千米的啁啾地震（chirp seismic）测线。研究团队采用50米长的拖缆（streamer），在全海湾范围内开展网格状中分辨率CRP勘测。 勘测结果与数据反演结果表明，印第安河湾下方埋藏有多条古河道；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，还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，对应与之伴生的低矿化度地下水羽流；这一特征大概率源于河道充填物中细粒河口泥质与泥炭的存在——这类物质可作为海底隔水单元。若此类隔水单元存在，陆上补给的低矿化度地下水羽流便可突破岸线边界向外扩散，进而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，是构建高精度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：在类似本次研究的农业流域中补给的富营养地下水，如何运移进入近岸海湾，并对地表水水质与河口生态系统造成影响。如需了解本次勘测项目的更多细节，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。