The marine differential electric dipole (DED) is applied for the first time to study a subseafloor groundwater body in the coastal region of Bat Yam, Israel. Previous marine long-offset transient electromagnetic applications detected this freshwater body underneath the Mediterranean seafloor. We have applied the novel DED method for the first time in the marine environment to further investigate this natural phenomenon. The main objectives are to locate the freshwater-seawater interface at the western aquifer edge and to identify the mechanism controlling this freshwater occurrence beneath the seafloor. The acquired step-on signals allow one to detect the freshwater body in the vicinity of the Israeli coastline at a depth of approximately 70 m beneath the seafloor. However, aquifer thickness is only poorly determined and may vary between 40 and 100 m. A lateral resistivity contrast is observable between adjacent 1D inversion models and also apparent in data profile curves that constrain the seaward extent of the detected resistive body to a distance of less than 4 km from the coastline. A subsequent 2.5D forward-modeling study aims to find a subseafloor resistivity distribution that adequately explains all measured DED data simultaneously. The results further constrain the lateral extent of the resistive aquifer to approximately 3.6–3.7 km from the Israeli coast. Furthermore, the data indicate that the aquifer system may be susceptible to seawater intrusion, as a superior data fit is achieved if a brackish water zone of approximately [Formula: see text] with a lateral extent of less than 300 m is located at the head of the freshwater body.

The existence of aquifers extending from land beneath the sea floor up to a distance of several kilometres has been observed and examined all over the world. The coastal aquifer of Israel is a heavily used groundwater reservoir which has to be constantly monitored to ensure the drinking water supply. Former land‐based electromagnetic measurements show that it is, in several places, blocked to seawater intrusion and is consequently a candidate for submarine extension. Multicomponent long‐offset transient electromagnetic measurements were carried out offshore on the coast of Israel. We deployed a 400‐m‐long grounded dipole as transmitter and several electric and magnetic receivers on the sea floor up to a distance of 4.8 km from the coast. Altogether, we deployed 8 transmitter positions and received data sets at 14 receiver stations onshore and offshore, with offsets of mostly 400 and 800 m. In this paper, we present the survey and 1D Occam and Marquardt inversions of the offshore horizontal electric components in the broadside and inline configuration. In addition, the vertical magnetic component in the broadside position is also considered. Only single inversions, both single offset and single component, were used to detect the aquifer under sea sediments. We prove the submarine existence of the Israeli coastal aquifer up to a distance to the coast of approximately 3.2 to 3.6 km using all measured long‐offset transient electromagnetic components. In addition, we present modelling studies with synthetic data derived from a subsurface model adjusted to our survey area with very shallow water from 10 to 50 m. As part of the planning before the survey, a parameter study of the expected subsurface, the examination of the airwave phenomenon and the justification for our 1D inversion strategy are shown. More detailed eigen parameter analyses are added to explain the measured data.

Abstract. Ground water rising to within 6 m (20 feet) of average ground surface elevations in Louisville, Kentucky caused concern to municipal officials and building owners in the central urban area. An average rise of more than 11 m (35 feet) occurred between 1969 and 1980. An evaluation of foundation conditions and structural configurations in central Louisville indicated rising ground water could create: 1. slight but significant possibilities of structural settlement problems; 2. high possibilities of damage to basement floors and walls; and 3. very high possibilities for disruption of utility conduits. Efforts to determine the cause of this rise in ground‐water level have focused on the historical relationships between ground‐water levels, pumpage rates and precipitation values. Historical data indicated that ground‐water levels in a system undisturbed by man could reach ground surface elevations in central Louisville. Preliminary studies indicated a strong relation between average ground‐water levels and changes in pumping rates and incident precipitation. A further detailed study showed extremely high correlation (R = 0.995) between average ground‐water levels in 1966–1980 and cumulative departures in precipitation and pumping rates from 1950–1965 average precipitation and pumping rates. A study of the feasibility of lowering ground‐water levels while simultaneously storing energy in the aquifer system was begun but was interrupted by devastating explosions of hexane in the sewers beneath south‐central Louisville on February 13, 1981. Although a dry year in 1980 and no change in pumping rate have slowed the rise in ground‐water level temporarily, long‐term solutions to this problem need to be developed.

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