Geological faculty, Department of geophysics

The statistical and correlation characteristics for a forecast of secular variation (SV) in the Earth's main magnetic field were derived for the territory of Russia and the adjacent water areas in comparison with other regions of the terrestrial globe. The spatial structure of the SV forecast was found to correlate best with the SV of the preceding epoch. SV is forecasted with the largest error for the Russian territory and the adjoin ing equatorial area. The existence of small size SV anomalies that vary over time in the spatial structure is characteristic for both regions. The presence of a large anomaly, such as the Brazilian Geomagnetic Anomaly, influences the error level in forecasting the mean and relative standard deviation. The absolute values of the forecast error in this case are comparable in magnitude with the values obtained for the other regions. The spatial structure of the SV forecast error points to the integral nature of SV anomalies and the independent change in the parameters of different scale flows in the liquid core that determine them. When the methods of coefficient extrapolation are used for the SV forecast, its accuracy can be improved only by updating fore cast coefficients with an interval of no more than two years on the basis of data from magnetic observatories and repeat stations.

Presented are the results of the study of the processes of interaction between the river and sea water in the estuary of the Keret' River flowing into the White Sea. The studies were carried out using the georadar method which enables one to register the boundary between fresh and salt water with the high spatial and temporal resolution. The use of this method helped to determine the boundaries of the area of distribution of fresh water in the estuary during different phases of the tidal cycle and to reveal the typical fluctuations of the boundary between fresh and salt water manifested at different values of river runoff and weather conditions. The study of dynamic, thermohaline, and biochemical processes in the estuaries of rivers flowing into the tidal sea is one of the fundamental problems of modern hydrology and coastal oceanography [3, 5, 6]. The importance of studying such areas is caused by the complex interaction between river water runoff and the tidal dynamics of sea water area. Such poorly explored phenomena and processes as the tidal bore, hydrofront, and natural marginal filter are also observed in the estuaries. The study of the mentioned phenomena needs the comprehensive approach combining standard in situ hydrological, oceanographic, and meteorological observations, noncontact methods of studying the features of phenomena in the water column in the near-real-time mode, and the numerical hydrodynamic modeling of corresponding control processes. From this point of view, the tidal estuary of the Keret' River and Gulf of Keret' being a part of the Gulf of Chupa of the Kandalaksha Bay of the White Sea is the almost ideal testing area for studying the interaction between river and sea water (Fig. 1). To study the in ter ac tion be tween salt sea wa ter and fresh river wa ter, it is rea son able to use the method of subsurface radiolocation (or georadiolocation) [2, 8]. The method is based on the study of the re flec tion of short elec tro mag netic pulses sent to the lower half-space from the bound aries in side the me dium sep a rating ar eas with dif fer ent elec tric prop er ties. Since sea wa ter and river wa ter have dif fer ent con duc tiv ity, there are good pre req ui sites for trac ing the bound ary be tween them us ing the georadar sound ing. In the limits of the es tu ary, fresh river wa ter be ing trans par ent to the elec tro mag netic ra di a tion prop a gates in the surface layer, as a rule, in ter acts and is partly mixed with the sea wa ter. In many es tu ary zones of large rivers, there is rel a tively sta ble in ter face be tween fresh and salt wa ter called halocline; its position is stud ied ex ten-sively [4]. How ever, small-scale pro cesses of wa ter mix ing in tidal es tu ar ies of small rivers have been explored much less. Such re searches re quire ob ser va tions with high spatiotemporal res o lu tion which can be pro vided only us ing georadiolocation. The use of the georadiolocation method en ables one to ob tain the large vol ume of de tailed in for ma tion; how ever, it can hardly be in ter preted with out stan dard hy dro log i cal and ocean o graphic meth ods of ob ser-va tions. At the same time, the car ry ing out of the full set of mea sure ments en ables one to study the ef fects that the dy nam ics of the mix ing of fresh and salt wa ter have on the course of bi otic pro cesses in the es tu ary zones of rivers. In the pres ent pa per, the re sults are given of the study of the Keret' River es tu ary us ing the georadar. The field tests were car ried out by the spe cial ists of the St. Petersburg State Uni ver sity in the summer of 2007–2012 at the Marine Biological Station (SPbSU MBS).

The permafrost has a complex structure, associated with different ice formations, such as ice-wedge networks and cryopegs. One of the most important problems connected with an engineering and geotechnical works in permafrost terrains is the near-surface active-layer soil condition monitoring. This problem is also important during the estimation of the solifluxion danger.
This work represents the results of GPR researches, performed during the expedition “Yamal-Arctic 2013” in August-September 2013. GPR was one of the set of geological and geophysical works along with VES and drilling.
The main objectives of GPR were:
1. studies of the structure of ice-wedge polygons and other features of the frozen active layer and permafrost;
2. study of inhomogeneities (swells and ice wedges) in the near-surface permafrost;
3. evaluation of the possibility of using GPR for determining the statistical and correlation properties of polygonal structures.
The ground penetrating radar “OKO2” with the dipole shielded antenna (400 MHz) was used for the near-surface permafrost investigations. The touch probe and drilling were used to determine active-layer soil thickness. This parameter was also calculated from hyperbolic reflections and GPR sounding. The spectral and correlation analysis of GPR signals, received over the polygonal structures, were also obtained. The paper also shows an example of a possible assessment of moisture distribution in the active-layer soil.

There are several reasons why permafrost studies in large territories require special approaches. These studies often take place in hard conditions -in wetlands and in impassable areas like taiga with dense vegetation cover. Permafrost structure is very sensitive to various mechanical and thermal influences. Thus, methods of research in such conditions should be non-invasive, possibly remote, quick and effective. Geophysical research of frozen ground is sparing to environment and soils itself. Airborne research is a promising method, but the interpretation of images cannot give complete information about ground conditions of the study area. The complex of geological and geophysical works on the key sites should be carried out and further combined with images interpretation results. Such works allow extrapolating geophysical data from these specific sites to the nearest area and significantly increase the accuracy of satellite image interpretation.