Continuous resistivity profiling data processed with multiple water conductivity values from Indian River Bay, Delaware, during April 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 http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2010-006-FA.2010年4月，针对美国特拉华州（Delaware）印第安河湾（Indian River Bay）下方的淡水-咸水地下水界面及其相关海底下部沉积构造的圈定工作，开展了一项地球物理勘探。本次勘探在霍茨兰丁（Holts Landing）研究站点周边采用了更高空间分辨率的测量方案，该区域后续已完成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145千米，同时在海湾周边完成了36千米的啁啾地震（chirp seismic）测线测量。全海湾网格尺度的中等分辨率CRP测量采用50米拖缆（streamer）开展。 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，在现代支流、潮汐溪与沼泽分布区域，这些古河道大体垂直于岸线延伸。在海湾东南部靠近怀特溪（White Creek）的区域，成像到一处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，对应着与其相关的低矿化度地下水羽流，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，它们充当了海底隔水单元（submarine confining units）。当此类隔水单元存在时，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。 该界面的特征对于构建精准的海岸地下水流模型至关重要。此类模型可用于预测，在这类农业流域中补给的富营养地下水如何进入近岸海湾，并对地表水水质与河口生态系统造成影响。 如需了解本项目勘探工作的更多详细信息，请访问：http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2010-006-FA。