A.I. Konkov* (Geodevice), A.N. Oshkin (Moscow State University), N.A. Ragozin (Institute Orgenergostroy), V.I. Ignatev (Geodevice), R.Y. Ermakov (Moscow State University)
In 2016 it was carried out the crosshole seismic testing (CST) on pressure and shear waves at the site of proposed construction of the NPP “Paks-2” (Hungary). The works were performed in four sections between the central well (with depth of 100 m) where excitation was performed and four observation wells (with depth of 70 m). Observation wells were located with “envelope” layout at distances varying from 24 to 35 m from the central. Geological section at the location of wells to a depth of 26 m is represented by sandy-gravel deposits of the Quaternary age: in the roof – by silty sands, in the bottom – by gravel and gravel-pebble sands. Below underlay sediments of the Pannonia formation – very dense silty sands with interlayers of coarser grained sands and clays. When preparing wells for the production of a multiwave CST, it becomes necessary to ensure a good mechanical contact of the casing with the surrounding rock. The most reliable way to ensure this contact is the cementation of annular space, the process of which is described in sufficient detail in ASTM D 4428 / D 4428M standard . In Figure 1 it is schematically shown the design of wells used during operations on the background of the geological section. It can be seen from it that the cementation was carried out only in the lower part of the section (related to the Pannonia rocks) and the upper part of the casing was simply covered with sand.