Studies on detection of permafrost structure

Field observations and laboratory experiments were carried out in order to examine the applicability of geoelectrical sounding methods to the detection of the permafrost base. In the laboratory, electrical resistivity values of frozen soils were measured under different temperature and soil moisture conditions in order to develop a reliable model, which was applied to field observation. The experimental results show that the resistivity values of Fujinomori clay, silty clay and clayey silt changed gradually near 0 ℃ with temperature changes due to the presence of unfrozen water. In these soils, the resistivity difference between frozen and unfrozen soils with low water content was small. Conversely, as Toyoura sand has no unfrozen water near 0 ℃, the resistivity values changed drastically between frozen and unfrozen states. Based on the measured resistivity values, the resistivity model as a function of temperature was proposed. The model calculation validated that the resistivity of frozen soils was mainly affected by both the amount and resistivity ofunfrozen water. The direct current (DC) electrical sounding result carried out at Teller shows that the permafrost base could be detected by the DC method, as the resistivity difference between frozen and unfrozen sand was large. While the soil type at the observation sites was silty clay at Moskuslagoon and clayey silt in Caribou-Poker Creeks, the differences of resistivity values between frozen and unfrozen states at both sites were small. Therefore, it was difficult to detect the permafrost base by the DC electrical sounding investigation. TEM survey conducted at Caribou-Poker Creeks shows that the estimated permafrost thickness by the DC method was less than that yielded by the TEM method. The measured borehole temperature profiles supported the resistivity structure suggested by the TEM method. Thus, the TEM method is applicable to obtain the subsurface structure. The estimated permafrost base at Nalaikh in Mongolia was in good agreement with the results obtained by the ground temperature profile, even though the resistivity difference between frozen and unfrozen soils is small. In continuous permafrost area at Neleger in Siberia, it is noted that the permafrost base was detected at a depth of 400 m. An intermediate conductive layer observed between the upper and lower resistive layers represents an unfrozen layer. The vertical resistivity distribution is useful for reconstructing the history ofpermafrost aggradation. However, the observation at Khatgal in Mongolia indicates that shallow bedrock may exist and the permafrost base could not be detected because the resistivity difference between frozen and unfrozen rocks is very small. A new method to detect the permafrost base, based on the ratio of resistivity values between frozen and unfrozen layers, was proposed. The resistivity ratios had a value larger than 1.5 from the observational resuits. Applying the experimental results, the ratio of the resistivity values between at 0 ℃ and at -1 ℃ exceeded the value of 1.5 in all the samples used in this study. It is suggested that the resistivity ratio of more than 1.5 represents the difference between frozen and unfrozen states. This value of the ratio coincides with the field measurements at various locations.