Low-depth vertical electrical sounding

      Electrical sounding with the short supply and receiving lines (the spacing of the supply electrodes A, B up to 30-40 m) generally applyed to determine the electrical resistance of the soil at depths up to 10-12 m for determining the optimal depths of the grounding electrodes. When the soundings used a symmetric Schlumberger spread line.
An example of this type of work is the determination of the resistance of the topsoil to a depth of 6 m at the site of construction of the CHPP-2 power unit in Riga.

The measurements were performed in an area the size 250х180 m on network points in free places from warehouses of construction materials. At each point, the measurements were carried out on two mutually perpendicular lines at four fixed distances between the supply and the measuring electrodes: 1.5, 3.0, 4.5, 6.0 m. Maximum difference between the supply electrodes of the symmetric arrangement was thus 18 m. Measurements were performed at fixed values of stabilized supply current: 5, 10, 20 mA. In single cases, measurements were made at a supply current of 50 mA.

The averaged values of electrical resistivity for all four receiving and supply electrode spacing were then calculated for each measurement point. Measurements obtained at different values of the supply current were used for averaging. Then the average resistances were calculated for each spacing value along the two perpendicular electrode arrangements. According to the calculated average values, the specific resistance distribution maps are constructed on the site, respectively, for each difference. According to existing standards, it is believed that the maps characterize the distribution of the specific resistance at the depth levels from the surface, equal to half the difference between the electrodes.

 Map of the electrical resistivity  distribution in soil massif at a depth of 1.5 m from the ground surface at the site of construction of CHPP-2 unit

 Map of the electrical resistivity  distribution in soil massif at a depth of 3.0 m from the ground surface at the site of construction of CHPP-2 unit

 Map of the electrical resistivity  distribution in soil massif at a depth of 4.5 m from the ground surface at the site of construction of CHPP-2 unit