In 3-D resistivity inversion, the number of the model parameters to be inverted is very large and hence the computing time for solving a large system of equations can be larger than that for forward modeling. The subspace method is investigated to circumvent this computational difficulty. The major benefit of using the subspace method is that the number of unknowns is reduced to the number of the basis vectors selected. The basis vectors chosen are the gradient of the segmented data misfit function multiplied by the inverse of the model roughness (Laplacian) operator. It is shown that the use of the subspace method allows significant reduction in computing time without serious degradation of inversion results.

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 We can find many symmetrical gravity anomalies such as center line symmetry in gravity map series (1 : 200,000) published by Geological Survey of Japan, AIST. To analyze these gravity anomalies, the authors reviewed the computing equations of vertical gradient of gravity based on the model bodies of center line symmetry (rectangular parallelopiped, terraced graben, parabolic graben, elliptical graben, and inverted pyramidical graben). Some characteristics are described in the maps drawn by these computing equations.

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 We have been developing a software system for helicopter-borne magnetic survey using an equivalent source technique. As one of the functions of this system, we are investigating the procedure of generalized mis-tie control. In a helicopter survey of mountainous area, even at the intersection of horizontal trajectories of traverse and tie lines, the altitudes of both lines differ considerably in practice. This means that the conventional crossover analysis cannot be applied. As the position fix is accurate by the use of GPS, only magnetic level shifts between lines are considered. An algorithm of estimating the magnetic level shifts was developed as an extension of the data reduction procedure (underdetermined inverse problem) of using an equivalent source analysis of survey line data. In solving for unknown parameters of different kinds of quantity, adequate scaling for parameters is essential for the underdetermined problem. In the case of allowing variable level shifts along survey lines, improper scaling may remove some components of actual magnetic anomalies. The method with parameter scaling was applied to actual survey data, which revealed a capability of removing the erroneous magnetic level shifts.
There is another application of this technique in detecting the magnetic anomaly change by repeated surveys. Repeated aeromagnetic surveys over an active volcano are one of the promising ways of examining volcanic activity. Since the track lines of different surveys cannot be the same, the inspection for the repeatability of a magnetic anomaly pattern is quite important to acquire genuine information of the activity. It was proven that the use of our method of generalized mis-tie control could eliminate the spatial alias effect, even in cases of insufficient survey line spacing.

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 Excitation loads must be properly formulated for a 3D numerical tool to simulate wayside vibrations induced by running trains. We classify the loads, in our preceding paper, into two types of vehicle-synchronized loads and position-synchronized ones, and insist that (a) in a homogeneous medium responses by the former loads are extremely small but ones by the latter have reasonable amplitudes and that (b) the responses by the former are contaminated visibly by waves radiated from a starting point and an ending one of the moving route. However, the insistences are not necessarily proved in a strict sense since they are simply based on some examples illustrated by numerical simulations. This paper presents further supports for their validity through two kinds of analytical methods. The first method applies an acoustic Green function to the propagator of our system, which is a strong assumption but enables us an exactly analytical treatment. It reconstructs the phenomena of (a) and (b) suggested in the preceding paper and also outlines some general behaviors of our system. The second is to use a Green function representing surface wave propagation, which is a weak assumption but makes a rigorous treatment impossible. However, a far-field approximation also suggests occurrence of the phenomena similar to (a). It is emphasized that the first method is a convenient tool to obtain some perspective for behaviors of numerical simulation results.

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 A 3D basin model around the fault area of the 2004 Mid Niigata prefecture earthquake (M_J6.8) was constructed using filtered gravity data and S-wave profiles derived from microtremor array observations. We validate the appropriateness of the 3D model using the simulation of earthquake ground motions from aftershocks events with M_J of 4.3 and 6.1. The major features of the ground motions from aftershocks are well simulated in the synthetic motion. Then, strong ground motions from the main shock are calculated using the basin model and irregular fault rupture model. Furthermore, we estimate distributions of the peak ground velocities for the main shock.

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 MCS surveys targeting deeper reflection image are increasing these days, and longer streamer cables or two ships for shooting and recording are used to try acquiring the deeper image. However, the survey with longer streamer is difficult to conduct in strong currents and ship traffic congestion. To obtain deeper reflection image, we have developed the integrated imaging method of OBS and MCS data. In shallow part, conventional MCS data is used. For deep image, wide-angle reflection OBS data is used. Moreover, the verification of the imaging method has been confirmed with synthetic and field data, and parameter tests for effective imaging have been conducted. Consequently, the reflection of plate boundary is well imaged from shallow to deep continuously, down to 20-km depth. We believe this method is an effective way to discover the new scientific information.

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 Yatsushiro Sea is one of the high tidal fluctuation areas in Japan, and the water's edge retreats about 200 meters at the low tide. Also there are some submarine fresh groundwater discharge points on the tidal flat of Yatsushiro Sea. VLF-MT survey was carried out to detect some freshwater seepage point on the shore side and the tidal flat of Yatsushiro Sea. In the tidal flat, a lot of observed apparent resistivity and phase values are very low except in the western side. This feature suggests that the deeper part has higher resistivity. The low phase may be caused by fresh water zone. In the western area, observed data show high resistivity, and high phase of more than 35 degrees. Because the basement (tuff breccia) crops out, this characteristic may be caused by this basement. The observed time changes are smaller than detected spatial variation. In addition, two anomalous areas showing high resistivity and low phase are detected in the tidal flat. An anomalous area near the shore side exists on the east side of a well with visible freshwater discharge. Around another 100 m offshore anomalous area, a fresh water discharge was observed by the seepage meter. Therefore, these anomalies are considered to relate to the local fresh water discharges.

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