The studies of a depth dynamics of seasonally thawing and freezing are of a considerable practical importance. Now the temperature bore hole measurements are the most-used method for solution of this problem. The depth of thawing can be determined indeed sufficiently exactly by 0o C isotherm disposition. However, during an active layer freezing this method have the difficulties in the phase boundary deteminations owing to a zero curtain. Also, the temperature method is practically inapplicable for saline soils if its freezing (thawing) - point depression is not known beforehand.
Therefore, the special field experiments have been carried out to elaborate the bore hole electrometrical techniques free from the temperature method limitations. The electrical pa and SP micrologging as well as the measurements of inter electrodes voltage /\ Ue and resistance Re have been tested. These studies were realized by use the "electrical plaits", contained thermal and electric equally spaced (0.1 or 0.2 m) sensors and buried in the holes. The thickness of active layer in the site of studies (Western Yakutia) is 1.5-1.8 m. with the seasons of thawing from May to September and - of freezing from October to December.
The best results have been obtained for /\ Ue and Re techniques. Thus, during a thawing, when a phase boundary crosses an upper electrode of a measuring couple, the magnitude of /\ Ue sharply increase five-several ten times and clear positive anomaly are formed. When a lower (in this couple) electrode is crossed by a thawing front the /\ Ue magnitude sharply decrease up to some value that remains during a whole time of active layer unfrozen state at a correspouding depth. Freezing (from above) is caracterized by similar /\ Ue clear changes but with reverse sign, i.e. in this case a negative anomaly are arised. Examples of these positive and negative anomalies are presented and discussed. The accuracy of a phase boundary location is about a half distance between measuring electrrodes.
The experimental data obtained in annual observation cycles permit to recommend a joint temperature and /\ Ue technique for detailed and reliable monitoring of seasonally thawing and freezing.
Sergei A. Boykov Vilui Permafrost Research Station Permafrost Institute, Siberian Branch Russian Academy of Sciences Republic Saha (Yakutia), Russia. Anatoly M. Snegirev Vilui Permafrost Research Station Permafrost Institute, Siberian Branch Russian Academy of Sciences Republic Saha (Yakutia), Russia. Anatoly D. Frolov United Scientific Council on Earth Cryology Russian Academy of Sciences Moscow, Russia. Telephone: (095) 930-2634 Fax: (095) 253-1192 E-mail: kvb@kvb.msk.ru (For A.Frolov)