In August 1980, observations of seismic waves generated by 35 explosions, one on land and the others at sea, were conducted at about 60 sites on land and four sites at sea bottom in a profile from Hatoyama to off Izu Peninsula to obtain the crustal structure in the northern part of the Philippine Sea plate. The profile, approximately parallel to the Suruga trough, crosses the Kan'nawa fault area, Hakone volcanoes, Izu Peninsula, and the Zenisu ridge.By the time term analysis, a velocity of 5.9 km/s was obtained for the granitic layer and a velocity of 6.8 km/s for the basaltic layer.The Pn velocity of 7.7 km/s was assumed for the time term analysis on the basis of travel time data in this experiment as well as previous results. The crustal structure thus obtained shows an interesting transition from the continental to oceanic type towards the Shikoku basin. It is possible that there are offsets in the upper boundaries of the granitic and the basaltic layers and in the Moho. Since these offsets are located in a narrow zone, this zone might be the boundary between the Philippine Sea plate and the Eurasian plate. Comparison of hypocenter distribution along the profile with the crustal structure supports the above idea since the pattern of hypocenter distribution between the two sides of the offset zone differs. This comparison also shows that earthquakes take place in the granitic layer beneath the Izu Peninsula. The distribution of observed Bouguer gravity anomaly along the profile is consistent with that expected from the crustal structure.

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A transient electromagnetic (TEM) survey was performed for permafrost mapping in Central Alaska, where permafrost is discontinuously distributed. The most stable data were obtained from the measurement of an outside loop with a single‐turn loop of 60×60 m. Electrical sounding was also conducted to evaluate the potential of TEM for permafrost mapping. In permafrost areas, the one‐dimensional inversion results typically show three‐layer geoelectrical sections of resistive subsurface and basement layers separated by a thin conductive layer. The resistive layers of subsurface and basement correspond to permafrost and the top of bedrock, respectively. The second conductive layer indicates an unfrozen state. The ground temperature profiles support these postulated resistivity structures. In permafrost‐free areas, two layers are resolved. The projected thickness of permafrost from the electrical sounding is thinner than the estimates derived from the TEM method. In permafrost‐free areas, results obtained by two methods coincide with each other. We conducted a numerical experiment using synthetic data calculated from a four‐layer permafrost model with appropriate noise for two methods. The inversion procedures show that TEM is better used more precisely for determining the permafrost thickness. Copyright © 2000 John Wiley & Sons, Ltd.

In this paper, we present a signal processing method suitable for estimating the position of a planar interface and its inclination using a directional borehole radar. The receiving directive antenna in a radar system is a coaxial‐fed circular dipole array antenna in a borehole (CFCAB), which we have proposed previously. The method combines least‐square fitting with a model of plane‐wave incidence and uses an inverse boundary scattering transform. This approach makes it possible to estimate parameters from measurements collected at only two narrowly separated depths of a radar sonde. We give a numerical example to verify the proposed method. In this simulation, the array data were generated with the Method of Moments modified for a borehole radar. We conducted field experiments in the Nakatatsu mine in Japan, where there is a fault in the skarn. We constructed a 3D image of the fault with a directional borehole radar and the signal processing method. The image agrees well with the information obtained from boring core samples and observations in the gallery.

Deep resistivity structures of permafrost in Siberia were investigated using a transient electromagnetic (TEM) method. The data were compared with temperature profiles. The high and low resistivity layers corresponded to permafrost and a talik, respectively, and the boundary between high and low resistivity was in good agreement with the temperature profile. In TEM surveys conducted from an alas to the taiga forest, the permafrost base was detected at a depth of more than 400 m. This corresponds to the known permafrost depth in this area. A talik was also found to exist at a depth of 100-200 m. Numerical studies indicate that the talik could have been produced by a thermokarst lake. The estimated period after initiation of alas formation agrees with radiocarbon dating results. In combination with the numerical analysis, the geo-electrical information provides a basic model for reconstructing the history of permafrost.