BUTSURI-TANSA(Geophysical Exploration)
Online ISSN : 1881-4824
Print ISSN : 0912-7984
ISSN-L : 0912-7984
Volume 59, Issue 6
Displaying 1-9 of 9 articles from this issue
Special Issue: Leading edge in application of exploration geophysics I
Review Articles
  • Tatsuya Ishiyama, Hiroshi Sato
    2006 Volume 59 Issue 6 Pages 515-524
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    Shallow seismic reflection profiling across active faults reveals details of their subsurface structures. Especially in case of active thrusts, integration of multi-disciplinary dataset including seismic reflection data coupled with tectonic geomorphology and structural geology may be used to define coseismic behavior and kinematic evolution of active fold and thrust belts. Future research fields of active faults shed light by exploration geophysics may include (1) three dimensional structural models of active faults, (2) subsurface structures of segment boundaries between en-echelon active faults that may reveal their coseismic behavior, and (3) development of high-resolution seismic exploration to resolve upper Pleistocene to Holocene sediments, and deep seismic reflection and refraction experiments. Integration of these multi-scale sections across active faults may define their otherwise inaccessible structural and kinematic evolution at millennial timescales.
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  • Junzo Kasahara, Kayoko Tsuruga, Hitoshi Mikada, Koshun Yamaoka, Naoyuk ...
    2006 Volume 59 Issue 6 Pages 525-537
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    The EARS (Exploration of Asperities-Reflectors System) is intended to integrate research components needed for the estimation of on-going, active plate subduction. Major component of EARS are: mapping of Earth's seismic structure, time lapse (active and continuous) monitoring, real time data telemetry and study of physical properties. In addition to ordinary 2D and/or 3D MCS surveys, combination of refraction and wide-angle reflection survey method using OBS and air guns is essential technology to map the seismological nature of the subducting plate boundary. The second component is time-lapse measurement (active and continuous monitoring). The ACROSS (Accurately and Routinely Operated Signal System) is a unique survey method that can use either seismic or electromagnetic, active and continuous signal sources. It can also be used for time-lapse measurements of oil reservoirs. Using an ACROSS seismic source and temporary seismic array across Central Japan, we carried out a feasibility study to characterize seismic arrivals through a slow slip region where a strong reflection has been observed in the region NE of Lake Hamana and large aseismic slip has also been detected by GPS. By comparing synthetic seismograms and observed records, we can interpret the major phases observed. Although, the observational offset was not large enough to identify the reflected phase from the plate boundary, the results showed high potential to use ACROSS for time-lapse and EARS studies.
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  • Tsuneomi Kagiyama
    2006 Volume 59 Issue 6 Pages 539-548
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    Research on prediction of volcanic eruptions has achieved a lot of results so far. For instance, it succeeds in detecting many precursory phenomena such as increase of seismic activity, deformation of volcanic body suggesting the accumulation and the movement of magma, geomagnetic changes caused by the rise of the temperature and anomalous change of volcanic gases. Knowing the structure of the volcano also enables us to understand the meaning of these anomalous phenomena. However, the research on prediction of volcanic eruption has left a lot of problems. For instance, eruption has not occurred in Iwate Volcano, though many precursory phenomena such as deformation of volcanic body and increase of seismic activity indicating magma intrusion were detected. In recent Asama Volcano, the correspondence of the seismic swarm and the eruption has changed to be unclear and the prediction of the eruption turned to be difficult. These problems originate in the difficulty of forecast of magma ascent. This paper proposes three aspects on these problems. It is firstly important to know the depth that magma has stopped. High accuracy determination of hypocenters led by accurate velocity structure is required. High accuracy determination of deformation source is also requested. Secondarily, it is important to know the density difference with magma and surrounding mediums. Magma will stop rising after it reaches a low density medium, while it continues to rise in a high density medium. Therefore, future experiments for the structure of volcano are requested to reach estimation of not only seismic velocity but even also density. Thirdly, the observation that can discuss the degassing from the magma should be required. According to the TDEM survey in Unzen Volcano, a high conductance zone trending W-E direction in the shallow part is found along the high seismic zone from the magma reservoir toward the summit. This evidence indicates the emission of volcanic gases into the shallow water saturated layer through the fault system in the western part of Unzen during the magma migration of the latest eruption. The research on prediction of volcanic eruptions is requested to answer the above mentioned problems, and new technique for geophysical survey is also requested.
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Original Paper
  • Hiroaki Yamanaka, Nobuyuki Yamada
    2006 Volume 59 Issue 6 Pages 549-560
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    A three-dimensional S-wave velocity model for the Kanto basin, Japan, is established using phase velocity of Rayleigh waves obtained in microtremor array explorations. First, we collected the phase velocity data obtained in previous microtremor explorations in the area. We also conducted microtremor array explorations in the northern and eastern parts of the basin where many explorations have not been conducted before. All the phase velocities at about 240 sites are inverted to one-dimensional S-wave profiles using genetic inversions with the same assumptions. The 1D profiles are integrated to construct a 3D S-wave velocity model of the basin. We, next validate appropriateness of the 3D model by comparing observed ground motion during a moderate earthquake near the Izu-Ohshima with the synthetic ones for the basin model. The synthetic ground motions were calculated using finite difference simulation of wave propagation in the 3D basin model. The comparison shows a good agreement between the long-period observed and synthetic ground velocities indicating a high performance of the 3D basin model in estimation of strong ground motion.
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Technical Note
  • Tadashi Takano, Ken-ichiro Maki, Eriko Soma, Shigeo Chiba, Takashi Mae ...
    2006 Volume 59 Issue 6 Pages 561-573
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    Microwave emission was found when materials were destroyed by a hypervelocity impact or by a static pressure. This paper describes the experimental setup to observe the phenomena, the obtained terms and results, and the possibility to apply the phenomena to geophysical explorations. As this field may not be familiar to most readers and the special techniques to receive and measure impulsive microwaves are required, the overall understanding is pursued instead of detailed description. In the receiving system, microwave signal is first amplified by a low noise amplifier, digitized in a sampling frequency high enough for the observed frequency, and then stored as data. The observed frequency was 22GHz, 2GHz, 300MHz and 1MHz. If the data storage capacity is too small to keep the data, namely at 22GHz and 2GHz, the signal is converted to a lower frequency by a heterodyne receiver and then processed to data.
    In the impact experiment, the velocity is 7 km/sec at maximum. Target material was selected from metal such as aluminum or iron, ceramic, brick or rubber. In the destruction experiment due to a static pressure, four kinds of rocks were pressed with a compressor. The observed microwave is intermittent quite narrow pulses in every destruction mode. In the rock destruction due to a static pressure, 22 GHz was detected only from quartzite. As the waveforms thus obtained are almost sinusoidal in shape, we can calibrate the power through the receiving system. As a result, the average emitted power at 2 GHz was 2.7x10-5mW and 2.7x10-8mW in the hypervelocity and static pressure experiments, respectively. The cause of microwave emission is inferred to be the dissociation of atoms or molecules, but is not yet completely confirmed. Currently, the phenomena are expected to be applied to geophysical explorations in the following fields:
    (1) Research of material characteristics: celestial body impacts, material science, space debris issues.
    (2) Change of the underground structure: rock crush.
    (3) Earthquake detection.
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Tutorials
  • Tadashi Nishitani, Tomoki Tsutsui, Shin'ya Sakanaka
    2006 Volume 59 Issue 6 Pages 575-583
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    Although it is necessary to develop new theories and techniques to progress the geophysical exploration, it is also important to develop human resources. Young people who do not stick a past are necessary in order to produce techniques with new idea and epoch-making geophysical explorations. At present, young people tend to keep their distance from the science and technology. It is an important problem to foster the human resources who supports future geophysical exploration, so that advertising the interest of geophysical exploration to young people, especially schoolchildren, is our immediate need. In Akita University, the hands-on learning has been carried out in order to cultivate young people. Understanding the principles is not a last goal in our hands-on learning; the purpose is to demonstrate how actually the equipment they made reacts to the target. The hands-on learning in Akita University clearly showed that the simple exploration devices were effective for the training of the geophysical exploration.
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  • Takafumi Kasaya, Tada-nori Goto, Ryo Takagi
    2006 Volume 59 Issue 6 Pages 585-594
    Published: 2006
    Released on J-STAGE: June 18, 2010
    JOURNAL FREE ACCESS
    Natural magnetic field is attenuated by electrically conductive water. Therefore, the marine magnetotelluric survey has been used at long period range(1000 to 100000 sec), and investigated deep mantle structure. To obtain the crustal structure, however, electromagnetic data at the periods between 1 and 1000s is very important, for example the investigation for mega-thrust earthquake zone. Because there is no suitable ocean bottom electro-magnetometer (OBEM), we developed small sized OBEM and OBE system. On the other hand, controlled-source survey techniques are needed to obtain a shallower detail structure than conventional MT method. The DC resistivity survey is powerful tool for a shallower survey. Our marine DC resistivity sounding system has a 150m long electrode cable, and this cable attached the Deep-tow system. In this article, we describe the mechanism of developed OBE(M) and DC resistivity survey system, and show some results of field tests
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