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Electrical geophysical methods to outline ground water rising in urban areas

Hydrological conditions in Delta Volga, RussiaThe groundwater table rises steadily in the delta Volga, where Astrakhan’ city is located because of irrigation and rising of the Caspian Sea level. The highly saline groundwater enhances secondary salinity in the area. The groundwater caused visible destruction of more than 20% of the buildings in Astrakhan’ city. Natural hazardous groundwater condition in delta Volga was further aggravated in the urban areas by the uncontrolled leakage from the canals and plumbing pipes.

The methods of vertical electrical sounding (VES) and non-contact electromagnetic profiling (NEP) were tested in 1995 for detail outlining of the groundwater table within the representative part of Astrakhan’ city. The study area was located in the center of Astrakhan’ with a large change of elevation, which induced a high variation of groundwater table within the of geophysical survey in Astrakhan City

Profiles of alluvial soils in delta Volga consist of thin layers of silt, clay, and sand. However, only water and salt content distributions within the soil profile cause considerable differentiation of the electrical resistivity in these soils. The soil profile can be generally divided into the top unsaturated layer with high resistivity and the bottom layer saturated by saline groundwater with low resistivity. Considering high distinction in electrical resistivity between unsaturated and saturated zones, the VES method was applied for detection of groundwater table. With the 1-D computer interpretation of the VES data the transition between top layer with high resistivity and bottom layer with low resistivity (i.e. groundwater table) was determined accurately. Compared with the groundwater tables measured in wells, the relative errors of the VES estimation were from 3 to 13%.

The VES measurements indicated that the depth of groundwater table decreased downhill for the investigated area in Astrakhan’ city. The groundwater tables were 0.4, 0.75, 3.1, and 5 m for locations 1, 3, 4, and 6, respectively (Astrakhan' map). Although the determination of the groundwater table by the VES method is much faster than by conventional drilling methods, it still requires considerable time to cover an extent area and can be applied in urban areas only in places with open ground, such as parks and lawns. In practice, to outline the area where the groundwater table or salinity is higher than safe level is, sometimes, more important than to determine the exact groundwater tables at individual locations. The NEP method can be used to outline areas where groundwater table is higher than a threshold level.
NEP electrical profiling in Astrakhan

Fig. 2 Profiles of electrical resistivity measured by the NEP along the slope in Astrakhan’, Russia: profiling with (A) 9-m distance between aerials and with (B) 16-m distance between aerials. Vertical lines show location of crossroads. The single exclamation mark indicates local increase of resistivity near the Dramatic Theatre. The double exclamation marks indicate increase of resistivity at the crossroads in the low part of the hill.

The principles of estimating groundwater table and soil salinity developed in agricultural and rural areas in delta Volga were applied to characterize NEP profile measured in Astrakhan’ city. General decrease of the resistivity downhill was revealed by the NEP at Soviet Street (Figure above). Profile A indicated that the saline groundwater have risen higher than 1 m to the surface at the bottom of the hill, whereas at the top, near the Kremlin, groundwater table was deeper than 5 m, as indicated by the VES measurement. Low electrical resistivity (3-30 ohm m) of the soil at the bottom of the hill as measured by 9-m array shows, that top 1 m of soil is non saline or low saline. The drop of electrical resistivity to almost 1 ohm as measured with 16-m array m at the bottom of the hill indicated that there highly saline groundwater is within 2.2 m from soil surface. Notably, the NEP profiles designated even local fluctuations of the groundwater table. For example, the area near the Dramatic Theatre was drained by the specially constructed active drainage as shown by the resistivity increase. Local increases in electrical resistivity were observed on the crossroads. These increases appeared, probably, due to draining effect of sand and cloth isolations of the pipes gathering under the crossroads.

Such detailed outline of the subsurface electrical resistivity was obtained by the NEP method in less than 30 minutes and corresponded with VES measurements and data from the stationary wells located nearby. Noteworthy, VES application as well as conventional boring for evaluation of groundwater table is restricted in cities to the areas with open soil surface. Supplemental NEP profiles provide continuous and detailed hydrological information about the soil subsurface even through the concrete pavement.

This research was published in journal Urban Water, UK; registered users can download reprints of this and other papers related to usage of geophysics in civil engineering and urban hydrology (attached to this page). The research was reported on ASSA-CSSA-SSSA meeting in 1999 and was featured in Salt Lake City Tribune.

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Larisa Golovko (Pozdnyakova), Ph.D. - President of Landviser, LLC