An application of magnetic and ecletromagnetic methods for environmental research

In 1941 when the German armies invaded Sevastopol (Crimea), the Soviet sappers blew up the arsenals of the Black Sea fleet stored in the limy massif galleries known as the Inkerman galleries. However, when undermining only small part of the arsenal has been detonated, the massif was strongly destroyed and the basic part of the arsenal has remained under blockages galleries. According archive data the area of the galleries at the moment of explosion was more than 1 hectare, there were from 10000 up to 30000 tons of ammunition, mainly large aerial bombs, sea mines, torpedoes, charges for ship artillery of the big calibers, etc. At present, there are some crevices in the destroyed limy massif through which it was possible to penetrate into the blockages of the arsenal and to find kept ammunition on the area about 40 m2. The find state of the ammunition is quite satisfactory, that allows using the mine clearing and the further recycling technologies. At the moment of explosion the vaults of the galleries were destroyed and sank on 10-15 m. The rock massif has intensively cracked and turned to a congestion of limestone blocks in the volume up to 1000-5000 m3. Cracks are filled with the clay formations and water supplied. Intensively progressing karstic processes in cracked limestone create danger of explosive objects of the arsenal corrosion, which is situated within the city boundaries. On territory of Inkerman galleries SINECO Company carried out measurements of a full vector of magnetic field on a uniform mesh 5m×5m. Several powerful sign-variable anomalies were revealed there, however their locality and huge gradients (up to 200-400 nT/m) testified to the small size and small depth of bedding (not more than 1-3 m). As the goal of the magnetic survey was the detailed mapping of the ammunition’s value, the decision to involve in researches TEM-FAST sounding technology was accepted. This technology is used successfully to solve the engineering, geological, hydro-geological and environmental problems. Primary goals of the research were the following: 1) Mapping of the ammunition on the depth of 10-30 m from a surface; 2) Three-dimensional mapping of sluggies in the limestone massif. For the solution of these tasks TEM-FAST 48 HPC (AEMR ltd.) system with 25m×25m antenna has been used. This system defined transient characteristics of the media in a range of times from 2-4 μs up to 16 ms, both in one-loop configuration, and in configuration loop-in-loop. Preliminary full-scale test has shown that well conducting layers of limestone screen metal objects even of 5 ton aerial bomb size which practically are not visible at depths more than 10 meters. However the super paramagnetic effect (SPM), caused frequency dependence of a magnetic susceptibility of metals χ(ω), is confidently registered even for much big depths. When measurements of transient characteristics of an electromagnetic field the voltage on receiving antenna U(t) in case of SPM-effect [1] is inversely to time U(t) ~1/t1+δ (|δ|<<1). In regulation E (t) = t*U (t)/I the response of the media at t > 100 μs weakly depends on time E(t) ~ t−δ. At the same time the induction signal caused by attenuation of the curl currents in rock, attenuates much faster: E(t) ~ 1/tn, where n ~ 1-3. Examples of the transients for two sites: P-38 and P-44 in the above regulation are given in the fig. 1.