RES2DINV format continuous resistivity profiling data collected in the Indian River Bay, Delaware, on April 13, 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次沉积构造的圈定工作，开展了地球物理勘探。本次勘探涵盖了在霍尔特登陆点（Holts Landing）研究区域周边进行的高空间分辨率测量，该区域后续完成了密集的陆上与海上研究工作。连续电阻率剖面法（continuous resistivity profiling, CRP）测线总长度达145千米，海湾周边布设的chirp地震测线总长36千米。本次勘探采用50米拖缆在全海湾网格中开展了中分辨率CRP测量。经数据反演后的勘探结果显示，印第安河湾下方发育多条埋藏古河道（paleochannels），这些古河道在现代支流、潮汐水道与沼泽区域大致垂直于海岸线延伸。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大深邃的古河道系统。多数古河道还伴随有高电阻率异常，对应与其相关的低盐地下水羽流（low-salinity groundwater plumes），这一现象大概率源于河道充填物中含有细粒河口泥质与泥炭（estuarine mud and peats），这些物质可作为海底隔水层（submarine confining units）发挥作用。若存在此类隔水层，陆上补给的低盐地下水羽流便可越过海岸线运移，进而在地下形成复杂的淡水-咸水地下水界面。该界面的相关特性是构建精准海岸地下水流模型（coastal groundwater flow models）时的重要考量因素。此类模型可用于预测在类似的农业流域（agricultural watersheds）中补给的富营养地下水如何流入海湾，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信息，请访问：http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2010-006-FA。