Field works. Example 12.
Permafrost mapping by geophysical methods
Exploration area of building new railway “Salekhard-Nadym” is situated on the territory of Ural’s and Nadym’s districts in the the Yamalo-Nenets Autonomous District (The Tyumen Region).
The main target was to determine the subsurface structure, the rock iceness and the permafrost top using engineering geophysical methods.
To solve this problem were carried out electrotomography and seismic tomography. As an inversion result, were obtained geological-geophysical models. Layer in the base of the model have velocity values about 2.5 km/sec and associates with sand. The top of the sand layer is extremely rough. Higher there is a thin alternation with rough and running top and with velocity values from 1.6 to 1.9 km/sec, which, probably, corresponds to the loam and sandy loam. Form a depth 15-16 m, there is sand with velocity values up to 1.4 km/sec. The upper layer is folded by loam with low velocity values. The condition of this layer is close to flow. Because of resistivity increasing (to 1000 Ohm m and more) in the right part of the model, sand and sandy loam below 3.5 m is traced throughout the model.
Geophysical investigation by electrotomography and seismotomography methods allows to select permafrost top and lithological boundaries between boreholes.
Interpretation seismotomography and electrotomography were processed with the use of ZondST2d and ZondRes2d.