Seismic prospecting, GPR and natural electric field surveys

Work on the site of right bank interchange of designed South bridge over the Daugava river in Riga is made by seismic methods of reflected and refracted waves, GPR and resistivity sounding method of natural field (SP). To interpretation of geophysical data the results of the geotechnical drilling of JSC “Ceļuprojekts” were involved. As a result of the performed investigations in places of a number statement of the three-level traffic intersection support the manifestations of the buried karst in the bedrock Dolomites lying under a layer of dense moraine loams are revealed.

Manifestations of palaeokarst on the GPR and seismic profiles on one of the interchange lines

Karsted dolomites in the construction site occur at a depth of about 6 m and overlapped by alluvial sediments and bulk soil with a capacity of 4-5 m, underlain by a layer of thick moraine loams with a capacity of up to 1.5 m. Figure shows a typical manifestation of karst areas on seismic and GPR sections.

The interpretation of the manifestations of palaeokarst on the seismic time sections

Map of the potential of the natural electric field and possible places of manifestation of the buried karst

Manifestations of palaeokarst at depths of 6-8 m and 10-12 m identified in the band width of 30-40 m, which extends across the construction site in the meridional direction. At the same time, one of the distinct anomalies shown in the figure on right correspond to the installation site of one of the highest supports of the second-level overpass. Further conducted natural electric field potential survey revealed in the band of palaeokarst manifestations elongated positive anomaly values of the gradient, the axis of which is directed towards the lake in the floodplain of the river. The anomaly is interpreted as related to underground seepage flow at the level of the bedrock surface. According to historical data, in this place there was a small tributary of the Daugava river, now overlapped by bulk soils of urban development.