Study on the high density resistivity method in oyster reef exploration
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The Oyster reef in Tianjin is an important geological relic remaining at Tianjin coastal plain since Holocene and records evidence of the change of sea and land and is of scientific value for induction of formation of continent of Tianjin coastal plain.The new method,high density resistivity method was tested to detect oyster reef distribution at the south bank of Zengkou River in Qilihai town,Ninghe County.Seven holes were drilled to verify the method.The drilling result is coincident with that of the high density resistivity detection thus the method is economic,quick and effective.Cite
Abstract Fresh ground water is widely distributed in subsurface sediments below the coastal bays of the Delmarva Peninsula (Delaware, Maryland, and Virginia). These conditions were revealed by nearly 300 km of streamer resistivity surveys, utilizing a towed multichannel cable system. Zones of high resistivity displayed by inversion modeling were confirmed by vibradrilling investigations to correspond to fresh ground water occurrences. Fresh water lenses extended from a few hundred meters up to 2 km from shore. Along the western margins of coastal bays in areas associated with fine‐grained surficial sediments, high‐resistivity layers were widespread and were especially pronounced near tidal creeks. Fresh ground water layers were less common along the eastern barrier‐bar margins of the bays, where sediments were typically sandy. Mid‐bay areas in Chincoteague Bay, Maryland, did not show evidence of fresh water. Indian River Bay, Delaware, showed complex subsurface salinity relationships, including an area with possible hypersaline brines. The new streamer resistivity system paired with vibradrilling in these investigations provides a powerful approach to recovering information required for extension of hydrologic modeling of shallow coastal aquifer systems into offshore areas.
Peninsula
Submarine groundwater discharge
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Geomorphological survey are conducted and drilling samples are collected in the Daqing River coastal plain of the Liaodong peninsula.Totally 113 sedimentary samples from the corresponding depths of the 7 drilling cores are obtained to measure conductivity of pore water in the sediments.The regularity of the land-sea changes in the plain during Holocene can be deduced from both the environmental alternations between the terrestrial sedimentation and the marine sedimentation along the drilling profile,and the characteristics of the conductivity curves.It is shown in the results that there are 2 peaks in each of the conductivity curves respectively for the g1 and g2 drill holes.It means that a complete transgression has occurred in the studied area and the transgrassive sea level almost approached to the location of the g3 drill hole.In addition,a lower peak can be found in the curve for the g3 drill hole which is the most landward,and it corresponds to the 2.1 m above the contemporary sea level.Therefore it is deduced that the sea level reached there and the conductivity became higher.It is believed that the ancient sea level i.e.the ancient mean high water had its maximnm corresponding to the 2.1 m above the contemporary sea level.The ancient sea level was 1.0 m below the contemporary one.In other words,the studied area has become uplifted in tectonics since Holocene.
Marine transgression
Peninsula
Coastal plain
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The need for disposing materials dredged from ship channels is a common problem in bays and lagoons. This study is aimed at investigating the suitability of scour features produced by dredging oyster shell deposits in Mobile Bay, Alabama, to dispose excavated channel material. A study area approximately 740 by 280 m lying about 5 km east of Gaillard Island was surveyed using underwater electrical resistivity tomography (UWERT) and continuous electrical resistivity profiling (CERP) tools. The geophysical survey was conducted with the intent to map scour features created by oyster shell dredging activities in the bay between 1947 and 1982. The geoelectrical surveys show that oyster beds are characterized by high resistivity values greater than 1.1 ohm.m while infilled dredge cuts show lower resistivity, generally from 0.6 to 1.1 ohm.m. The difference in resistivity mainly reflects the lithology and the consolidation of the shallow sediments: consolidated silty clay and sandy sediments rich in oyster shell deposits (with less clay content) overlying unconsolidated clayey materials infilling the scours. Results show that most of the infilled dredge cuts are mostly distributed in the north-south direction. Considering that the scours are generally up to 6 m deep across the survey location, it is estimated that about 0.8 million cubic meters of oyster shells and overlying strata were dredged from the survey location.
Dredging
Lithology
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