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In 1987 the Shirshov Institute of Oceanology conducted a large scale experiment "MEGAPOLYGON" in the North Pacific, north of the Kuroshio. A cluster of 177 moored buoys was set for two months in the region between 38/spl deg/-42/spl deg/ N and 152/spl deg/-158/spl deg/ E. The moorings were set at a distance of 24 miles. The current meters were set at 120 m and 1200 m levels. The time sampling was 30 minutes. The period of the measurements in August September, 1987 is characterized by the influence of powerful meteorological typhoons which strongly affected the surface floats of the moorings. This led to the displacement and drift of several moorings and to the failure of the records in the instruments. This region is also an area of intensive fishery, and unfortunately some of the moorings were entangled in the fishing nets, which caused displacement and losses of the moorings. The main objective of the experiment was to study synoptic and mesoscale eddies and rings of the Kuroshio. At the same time, the cluster provides good opportunities to study tidal internal waves with a spatial scale of the order of hundred kilometers. The spatial location of moorings with current meters in the cluster forms a good antenna to study tidal internal waves and their variability. The main source for the generation of tidal internal waves in the North Pacific is the Emperor Ridge. Tidal internal waves are generated due to the interaction of the barotropic tide with the bottom topography over the slopes of the ridge d.

Strong semidiurnal internal waves are generated over the Emperor Ridge in the North Pacific. The waves propagate in both directions east and west of the ridge. The propagation and dissipation of the waves west of the ridge is studied on the basis of data obtained during a Russian large scale moored experiment "MEGAPOLYGON". The current meter measurements were performed from 170 moored buoys set for two months in the northwest Pacific. Amplitudes, wavelengths, direction and variability of tidal internal waves, and their variability were studied on the basis of these data. The cluster of moorings serves as an antenna to measure the wave length and the directions of the waves. The amplitudes of the waves are calculated from the temperature oscillations registered at the moored stations divided by the mean vertical temperature gradient. East of the ridge the same parameters were studied using the data of the US moorings.

In the course of seafloor EM survey the topographical changes of the seafloor can have influence on the EM components. These topographical inhomogeneities are 3-D ones. However, they are frequently elongated in one direction and can be treated as 2-D inhornogeneties. The aim of this model study is to present a numerical method for frequency domain marine controlled-source electromagnetic measurements using horizontal electric dipole sources on the seafloor and illustrate how the EM responses are affected by a 2-D trench. The horizontal electric dipole source is parallel and perpendicular to the struetural strike and the EM field components are computed along an in-line horizontal seafloor profile perpendicular to the axis of the trench at two frequencies.

We have analyzed the neutron fluxes correlated with thunderstorm activity recently measured at mountain altitudes by Tien-Shan, Tibet and Aragats groups. We perform simulations of the photonuclear reactions of gamma rays born in the electron-gamma ray avalanches in the thunderstorm atmosphere and calculate expected count rates of the neutron counters used by 3 groups. Our analysis supported the Tibet group conclusion on the photonuclear nature of thunderstorm-correlated neutrons. The photonuclear reactions of the gamma rays born in the electron-photon avalanches in the thunderstorm atmospheres interacting in the lead producer of a Neutron monitor can provide neutron yield compatible with additional count of NM at least for the largest Thunderstorm Ground Enhancements (TGEs).

GEANT4 simulations of the propagation of electrons in the thunderstorm atmospheres were performed to explain the Thunderstorm Ground Enhancements (TGEs), detected by the particle detectors of the Aragats Space Environmental Center (ASEC) operating at altitude 3200 m on slopes of Mt. Aragats in Armenia. The charged particle propagation and multiplication processes were simulated in the uniform electric fields of different strengths and elongation. The Gamma ray, electron and neutron energy spectra were obtained on the exit of the electrical field and beneath. Simulation results prove existence of 2 mechanisms of particle enhancements, first connected with electron – gamma ray avalanche process; and the second – with modification of energy spectra of particles in the strong electrical fields of the thunderclouds. The avalanche process can multiply number of electrons and gamma rays by several orders of magnitude well above the cosmic ray background in the energy range up to 40MeV; the energy spectra modifications lead to a few percent enhancements in the energy range up to 100MeV, as well as to depletion of the high-energy muon flux. Consequently the energy spectra at low energy are better fitted by exponential law; at high energies – by the power law.

Far from relatively narrow conducting anomalies (graphite shist belts, fracture zones, etc.) the background electrical resistivity of the upper part of crystaline Earth crost reaches 105 -106 Ohm.m. It coincides with laboratory measurements if taken into account porosity value and water in pores. One could expect an increase of the resistivity with depth due to closing of pores. But unexpectedly resistivity is decreasing at dep th of 10-20 km practically for all investigated areas of Europe. Magnetotelluric soundings of the last decade showed that the conductivity value of the lower crost is similar to that of the sediment cover. This fact is in a sharp disagreement with high resistivity of the dry lower crost rocks derived from laboratory measurements.

A number of authors have used the drill bit as a seismic source. In one type of study (Rector et al, 1989), an accelerometer is placed on the drill string, giving an estimate of the signature of the drill bit source. Crosscorrelating this estimate with the data recorded by one or more geophones enhances drill-bit energy in the recorded traces and gives results whose quality is comparable to VSP data.

An important result of recent years is the normal resistivity profile. It was obtained by interpretation of the combined apparent resistivity curve (magnetotelluric sounding and geomagnetic deep sounding) for the East European platform. This profile has no highly conducting layer and resistivity is greater than 100 ohm-m at asthenospheric depths. It corresponds well with geothermal indications of the absence of partial melting beneath the Precambrian plates. Nearly the same profiles have been obtained for the Canadian shield, and the Siberian and Australian platforms. Investigations carried out in many “hot” regions confirm the existence of a well-developed low-resistivity asthenosphere. Partially molten zones have conductances of about several thousand Siemens in the Eastern Pacific, Iceland and in the North American rift zone. Magnetotelluric soundings show that in many continental areas the lower part of the crust has low resistivity, in the range 10–20 ohm-m. Usually this crustal conductive layer is observed in regions of recent activity. Its total conductivity changes from several hundred to several thousand S. Many investigators propose that the most natural explanation of this conductivity is water solutions. It is necessary to note the distorting role of near-surface inhomogeneities. Local distortions can be eliminated by simple averaging of the experimental data. These average apparent resistivity curves are the starting point for the construction of deep geoelectrical models.

The distortions in the low-frequency plane wave electric field due to variations of the conductance values of the sedimentary cover are considered for the Rheingraben area. The results suggest that the main part of the observed low-frequency magnetotelluric anomaly can be explained by these distortions.

Three groups of electromagnetic data have been considered in order to construct a preliminary geoelectrical depth model of the old basement of the Baltic shield: (i) audiomagnetotelluric data in the period range 1 3700 − 1 8 s; (ii) magnetotelluric data in the period range 25–1000 s; and (iii) global magnetovariation-sounding data in the period range >6 h . These data sets fit together on a common apparent-resistivity curve, indicating the existence of a unified geoelectrical model. This model does not support the existence of conducting crustal or asthenospheric layers.

Theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the Baltic Shield. The Earth model includes the undistorted East European Platform resistivity-versus-depth model, and crustal and asthenospheric conducting layers. The results show that close to the source region, for periods > 15 min, the magnetic field depends only weakly on the Earth's conductivity. Accordingly, the resolution of apparent resistivity curves obtained by the horizontal spatial gradient (HSG) technique becomes poor at longer periods close to the source region. A comparison with experimental HSG apparent resistivity from Kiruna and Kevo in northern Scandinavia supports the existence of high conductivity in the lower crust. The sharp decrease of apparent resistivity at periods > 1 h may—within the limits of experimental error—be a result of the non-uniform source rather than an indication of a good conductor in the upper mantle.

Distortion effects of surface geoelectrical inhomogeneities on deep MHD soundings must be taken into account in the interpretation of the measurements. Computer programs which calculate these effects for the steady-state part of the MHD pulses using the thin sheet approximation and which present the results graphically are used to study the distribution of apparent resistivity for various kinds of models in which the MHD generator is located off the coast of the Kola Peninsula.

The experimental MT data and theoretical data calculated by the thin sheet technique for a low-frequency telluric field are analysed and compared for the Kamchatka Peninsula. The comparison shows a good agreement between the experimental and theoretical data and results in a conclusion that the DC-type distortions of the telluric field play the most important role in the structure of the MT field. The component of the telluric field which is parallel to the general direction of the Kamchatka Peninsula is less distorted and should be used for the estimation of deep geoelectric structures.

A thin-sheet model is considered which is equivalent to the gradient resistivity structure of stable regions. For this model a numerical algorithm is developed using the FEM-technique. Results of the calculations can be applied to the interpretation of deep DC-soundings.

The results of geoelectrical studies in the Baikal region have been reconsidered. A new geoelectrical model of the Baikal rift zone has been constructed using a simplified statistical approach, with a normal cross-section as the background. The interpretation proved a success due to several long-period reference magnetotelluric soundings. The most trustworthy element of the model is the highly-conducting layer which is supposed to consist of two horizons. The first horizon belongs to the Earth's crust and is likely to be of hydrothermal nature; the second one, which has been found in Trans-Baikal, overlaps the upper mantle and is connected with partial melting of rocks. The asthenosphere of the Baikal region is believed to be of moderate type.

Block forward modeling and smooth Backus-Gilbert linearized inversion have independently been used to two-dimensionally model two east-west magnetotelluric profiles in the central Rio Grande rift. A crustal conductive zone of at least 1500 S has been detected at about 10 km depth along the northern line. However, along a line 40 km to the south where shallow, active magma is inferred and microseismicity is highest, no high conductivity zone is found. The conductive zone beneath the northern line is hypothesized to occur where water is trapped beneath an undisturbed ductile cap. Under the southern line, magma injection through the cap may release the water, thereby resulting in a relatively more resistive crust with no conductive, midcrustal layer. The resulting paradox is that where there is active magma injection into the upper crust of the Rio Grande rift, the crust is more resistive rather than more conductive.