BUTSURI-TANSA(Geophysical Exploration)
Online ISSN : 1881-4824
Print ISSN : 0912-7984
ISSN-L : 0912-7984
Volume 58, Issue 1
Displaying 1-16 of 16 articles from this issue
Collaboratively Published by Exploration Geophysics(Australia) and Mulli-Tamsa(Korea)
Geophysics In The Western Pacific Environment: II High Precision Geophysical Methods
  • Atsushi Tanaka, Yoshinori Sanada, Yasuhiro Yamada, Toshifumi Matsuoka, ...
    2005 Volume 58 Issue 1 Pages 1-6
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Analogue physical modelling using granular materials (i.e., sandbox experiments) has been applied with great success to a number of geological problems at various scales. Such physical experiments can also be simulated numerically with the Discrete Element Method (DEM). In this study, we apply the DEM simulation to the collision between the Indian subcontinent and the Eurasian Plate, one of the most significant current tectonic processes in the Earth.
    DEM simulation has been applied to various kinds of dynamic modelling, not only in structural geology but also in soil mechanics, rock mechanics, and the like. As the target of the investigation is assumed to be an assembly of many tiny particles, DEM simulation makes it possible to treat an object with large and discontinuous deformations. However, in DEM simulations, we often encounter difficulties when we examine the validity of the input parameters, since little is known about the relationship between the input parameters for each particle and the properties of the whole assembly. Therefore, in our previous studies (Yamada et al., 2002a, 2002b, 2002c), we were obliged to tune the input parameters by trial and error.
    To overcome these difficulties, we introduce a numerical biaxial test with the DEM simulation. Using the results of this numerical test, we examine the validity of the input parameters used in the collision model. The resulting collision model is quite similar to the real deformation observed in eastern Asia, and compares well with GPS data and in-situ stress data in eastern Asia.
    Download PDF (1797K)
  • Derecke Palmer, Ramin Nikrouz, Andrew Spyrou
    2005 Volume 58 Issue 1 Pages 7-17
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    The determination of seismic velocities in refractors for near-surface seismic refraction investigations is an ill-posed problem. Small variations in the computed time parameters can result in quite large lateral variations in the derived velocities, which are often artefacts of the inversion algorithms. Such artefacts are usually not recognized or corrected with forward modelling. Therefore, if detailed refractor models are sought with model-based inversion, then detailed starting models are required.
    The usual source of artefacts in seismic velocities is irregular refractors. Under most circumstances, the variable migration of the generalized reciprocal method (GRM) is able to accommodate irregular interfaces and generate detailed starting models of the refractor. However, where the very-near-surface environment of the Earth is also irregular, the efficacy of the GRM is reduced, and weathering corrections can be necessary.
    Standard methods for correcting for surface irregularities are usually not practical where the very-near-surface irregularities are of limited lateral extent. In such circumstances, the GRM smoothing statics method (SSM) is a simple and robust approach, which can facilitate more-accurate estimates of refractor velocities.
    The GRM SSM generates a smoothing “statics” correction by subtracting an average of the time-depths computed with a range of XY values from the time-depths computed with a zero XY value (where the XY value is the separation between the receivers used to compute the time-depth). The time-depths to the deeper target refractors do not vary greatly with varying XY values, and therefore an average is much the same as the optimum value. However, the time-depths for the very-near-surface irregularities migrate laterally with increasing XY values and they are substantially reduced with the averaging process. As a result, the time-depth profile averaged over a range of XY values is effectively corrected for the near-surface irregularities. In addition, the time-depths computed with a zero XY value are the sum of both the near-surface effects and the time-depths to the target refractor. Therefore, their subtraction generates an approximate “statics” correction, which in turn, is subtracted from the traveltimes.
    The GRM SSM is essentially a smoothing procedure, rather than a deterministic weathering correction approach, and it is most effective with near-surface irregularities of quite limited lateral extent. Model and case studies demonstrate that the GRM SSM substantially improves the reliability in determining detailed seismic velocities in irregular refractors.
    Download PDF (5191K)
  • Derecke Palmer, Leonie Jones
    2005 Volume 58 Issue 1 Pages 18-25
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    We derive refraction statics for seismic data recorded in a hard rock terrain, in which there are large and rapid variations in the depth of weathering. The statics corrections range from less than 10 ms to more than 70 ms, often over distances as short as 12 receiver intervals. This study is another demonstration of the importance in obtaining accurate initial refraction models of the weathering in hard rock terrains in which automatic residual statics may fail.
    We show that the statics values computed with a simple model of the weathering using the Generalized Reciprocal Method (GRM) and the Refraction Convolution Section (RCS) are comparable in accuracy to those computed with a more complex model of the weathering, using least-mean-squares inversion with the conjugate gradient algorithm (Taner et al., 1998). The differences in statics values between the GRM model and that of Taner et al. (1998) systematically vary from an average of 2 ms to 4 ms over a distance of 8.8 km. The differences between these two refraction models and the final statics model, which includes the automatic residual values, are generally less than 5 ms. The residuals for the GRM model are frequently less than those for the model of Taner et al. (1998). The RCS statics are picked approximately 10 ms later, but their relative accuracy is comparable to that of the GRM statics.
    The residual statics values show a general correlation with the refraction statics values, and they can be reduced in magnitude by using a lower average seismic velocity in the weathering. These results suggest that inaccurate average seismic velocities in the weathered layer may often be a source of short-wavelength statics, rather than any shortcomings with the inversion algorithms in determining averaged delay times from the traveltimes.
    Download PDF (1604K)
  • Shaokong Feng, Takeshi Sugiyama, Hiroaki Yamanaka
    2005 Volume 58 Issue 1 Pages 26-33
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Inversion of multi-mode surface-wave phase velocity for shallow engineering site investigation has received much attention in recent years. A sensitivity analysis and inversion of both synthetic and field data demonstrates the greater effectiveness of this method over employing the fundamental mode alone. Perturbation of thickness and shear-wave velocity parameters in multi-modal Rayleigh wave phase velocities revealed that the sensitivities of higher modes: (a) concentrate in different frequency bands, and (b) are greater than the fundamental mode for deeper parameters. These observations suggest that multi-mode phase velocity inversion can provide better parameter discrimination and imaging of deep structure, especially with a velocity reversal, than can inversion of fundamental mode data alone.
    An inversion of the theoretical phase velocities in a model with a low velocity layer at 20 m depth can only image the soft layer when the first higher mode is incorporated. This is especially important when the lowest measurable frequency is only 6 Hz.
    Field tests were conducted at sites surveyed by borehole and PS logging. At the first site, an array microtremor survey, often used for deep geological surveying in Japan, was used to survey the soil down to 35 m depth. At the second site, linear multichannel spreads with a sledgehammer source were recorded, for an investigation down to 12 m depth. The f-k power spectrum method was applied for dispersion analysis, and velocities up to the second higher mode were observed in each test. The multi-mode inversion results agree well with PS logs, but models estimated from the fundamental mode alone show a large underestimation of the depth to shallow soft layers below artificial fill.
    Download PDF (1983K)
  • James Roberts, Michael Asten
    2005 Volume 58 Issue 1 Pages 34-40
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    We have used the microtremor method, with arrays of up to 96 m diameter, to carry out non-invasive estimation of shear-wave velocity profiles to a depth of 30 to 50 m in unconsolidated Quaternary Yarra Delta sediments. Two silt units (Coode Island Silt, and Fishermans Bend Silt) dominate our interpretation; the method yields shear velocities for these units with precision of 5%, and differentiates between the former, softer unit (Vs = 130 m/sec) and the latter, firmer unit (Vs = 235 m/sec). Below these silts, the method resolves a firm unit correlating with known gravels (Vs 500 to 650 m/sec).
    Using surface traverses with the single-station H/V spectral ratio method, we show that the variation in thickness of the softer silt can be mapped rapidly but only qualitatively. The complexity of the geological section requires that array methods be used when quantitative shear-wave velocity profiles are desired.
    Download PDF (1493K)
  • Rie Kamei, Masami Hato, Toshifumi Matsuoka
    2005 Volume 58 Issue 1 Pages 41-49
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing zones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data.
    This model is applied to two simulations of elastic wave propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering.
    We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing zones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.
    Download PDF (2196K)
  • Hyoung-Seok Kwon, Jung-Ho Kim, Hee-Yoon Ahn, Jin-Sung Yoon, Ki-Seog Ki ...
    2005 Volume 58 Issue 1 Pages 50-58
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Recently, the imaging of geological structures beneath water-covered areas has been in great demand because of numerous tunnel and bridge construction projects on river or lake sites. An electrical resistivity survey can be effective in such a situation because it provides a subsurface image of faults or weak zones beneath the water layer. Even though conventional resistivity surveys in water-covered areas, in which electrodes are installed on the water bottom, do give high-resolution subsurface images, much time and effort is required to install electrodes. Therefore, an easier and more convenient method is sought to find the strike direction of the main zones of weakness, especially for reconnaissance surveys.
    In this paper, we investigate the applicability of the streamer resistivity survey method, which uses electrodes in a streamer cable towed by ship or boat, for delineating a fault zone. We do this through numerical experiments with models of water-covered areas. We demonstrate that the fault zone can be imaged, not only by installing electrodes on the water bottom, but also by using floating electrodes, when the depth of water is less than twice the electrode spacing. In addition, we compare the signal-to-noise ratio and resolving power of four kinds of electrode arrays that can be adapted to the streamer resistivity method.
    Following this numerical study, we carried out both conventional and streamer resistivity surveys for the planned tunnel construction site located at the Han River in Seoul, Korea. To obtain high-resolution resistivity images we used the conventional method, and installed electrodes on the water bottom along the planned route of the tunnel beneath the river. Applying a two-dimensional inversion scheme to the measured data, we found three distinctive low-resistivity anomalies, which we interpreted as associated with fault zones. To determine the strike direction of these three fault zones, we used the quick and convenient streamer resistivity survey on additional grid-style survey lines. In this way, we could delineate the strike direction of faults beneath the riverbed very efficiently.
    Download PDF (5643K)
  • Kyosuke Onishi, Toshiyuki Yokota, Satoshi Maekawa, Tetsuma Toshioka, S ...
    2005 Volume 58 Issue 1 Pages 59-66
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    The main purpose of this paper is to describe a highly efficient common mid-point (CMP) data acquisition method for ground-penetrating radar (GPR) surveying, which is intended to widen the application of GPR. The most important innovation to increase the efficiency of CMP data acquisition is continuous monitoring of the GPR antenna positions, using a real-time kinematic Global Positioning System (RTK-GPS). Survey time efficiency is improved because the automatic antenna locating system that we propose frees us from the most time-consuming process — deployment of the antenna at specified positions. Numerical experiments predicted that the data density and the CMP fold would be increased by the increased efficiency of data acquisition, which results in improved signal-to-noise ratios in the resulting data.
    A field experiment confirmed this hypothesis. The proposed method makes GPR surveys using CMP method more practical and popular. Furthermore, the method has the potential to supply detailed groundwater information. This is because we can convert the spatially dense dielectric constant distribution, obtained by using the CMP method we describe, into a dense physical value distribution that is closely related to such groundwater properties as water saturation.
    Download PDF (5695K)
  • Myeong-Jong Yi, Jung-Ho Kim, Sam-Gyu Park, Jeong-Sul Son
    2005 Volume 58 Issue 1 Pages 67-72
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    To investigate the feasibility of a new concept of storing Liquefied Natural Gas (LNG) in a lined hard rock cavern, and to develop essential technologies for constructing underground LNG storage facilities, a small pilot plant storing liquid nitrogen (LN2) has been constructed at the Korea Institute of Geoscience and Mineral Resources (KIGAM). The LN2 stored in the cavern will subject the host rock around the cavern to very low temperatures, which is expected to cause the development of an ice ring and the change of ground condition around the storage cavern. To investigate and monitor changes in ground conditions at this pilot plant site, geophysical, hydrogeological, and rock mechanical investigations were carried out. In particular, geophysical methods including borehole radar and three-dimensional (3D) resistivity surveys were used to identify and monitor the development of an ice ring, and other possible changes in ground conditions resulting from the very low temperature of LN2 in the storage tank. We acquired 3D resistivity data before and after storing the LN2, and the results were compared. From the 3D images obtained during the three phases of the resistivity monitoring survey, we delineated zones of distinct resistivity changes that are closely related to the storage of LN2. In these results, we observed a decrease in resistivity at the eastern part of the storage cavern. Comparing the hydrogeological data and joint patterns around the storage cavern, we interpret this change in resistivity to result from changes in the groundwater flow pattern. Freezing of the host rock by the very low temperature of LN2 causes a drastic change in the hydrogeological conditions and groundwater flow patterns in this pilot plant.
    Download PDF (6068K)
  • Don Hunter, Anton Kepic
    2005 Volume 58 Issue 1 Pages 73-77
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    To correctly invert and interpret Surface Nuclear Magnetic Resonance (SNMR) data collected in conductive terrains, an accurate estimate of subsurface conductivity structure is required. Given such an estimate, it would be useful to determine, before conducting an SNMR sounding, whether or not the conductivity structure would prevent groundwater being detected. Using SNMR forward modelling, we describe a method of determining the depth range from which most of the SNMR signal originates, given a model of subsurface conductivity structure.
    We use the method to estimate SNMR depth penetration in a range of halfspace models and show that for conductive halfspaces (<10Ω.m) the depth of penetration is less than 50 m. It is also shown that for these halfspaces, increasing coincident loop size does not significantly improve depth penetration. The results can be used with halfspace approximations of more complicated 1D conductivity structures to give a reasonable estimate of the depth range over which signal is obtainable in conductive terrains.
    Download PDF (589K)
  • Kate Wilkinson, Tessa Chamberlain, Mike Grundy
    2005 Volume 58 Issue 1 Pages 78-85
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    This study, combining geophysical and environmental approaches, was undertaken to investigate the causes of secondary salinity in the Goondoola basin, in southwestern Queensland.
    Airborne radiometric, electromagnetic and ground electromagnetic datasets were acquired, along with data on soils and subsurface materials and groundwater. Relationships established between radiometric, elevation data, and measured material properties allowed us to generate predictive maps of surface materials and recharge potential. Greatest recharge to the groundwater is predicted to occur on the weathered bedrock rises surrounding the basin. Electromagnetic data (airborne, ground, and downhole), used in conjunction with soil and drillhole measurements, were used to quantify regolith salt store and to define the subsurface architecture. Conductivity measurements reflect soil salt distribution. However, deeper in the regolith, where the salt content is relatively constant, the AEM signal is influenced by changes in porosity or material type. This allowed the lateral distribution of bedrock weathering zones to be mapped.
    Salinisation in this area occurs because of local- and intermediate-scale processes, controlled strongly by regolith architecture. The present surface outbreak is the result of evaporative concentration above shallow saline groundwater, discharging at break of slope. The integration of surficial and subsurface datasets allowed the identification of similar landscape settings that are most at risk of developing salinity with groundwater rise.
    This information is now being used by local land managers to refine management choices that prevent excess recharge and further salt mobilisation.
    Download PDF (10544K)
  • Sam-Gyu Park, Shiho Asano, Sumio Matsuura, Takashi Okamoto, Jung-Ho Ki ...
    2005 Volume 58 Issue 1 Pages 86-91
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    We present the results of electrical resistivity surveys carried out to estimate the seasonal variation of the water table level in a large-scale landslide area of Tertiary geology in Japan. One long profile, trending NE-SW, was established perpendicular to the main regional geology of the region. Three boreholes are located very close to the profile. The profile was surveyed twice, once before snowfall and once after snow had melted.
    The relationship between resistivity and water saturation of pyroclastic materials was clarified through laboratory tests. We did this in order to estimate the water content of the pyroclastic layer from the observed resistivity distribution in the landslide area. The resistivity of the saturated pyroclastic deposit calculated using an empirical formula was found to be 570 Ω.m. Based on this computed resistivity, the groundwater level was deduced by assuming that the pyroclastic deposits were fully saturated beneath the water table. We show that the estimated water table before snowfall is lower than that inferred after snow has melted, by about 1.1 to 4.7 m. This suggests that the water table in the upper part of the pyroclastic layer in the landslide area fluctuates greatly, compared to the lower part. This seasonal groundwater fluctuation is possibly caused by the infiltration of water into the subsurface after snowmelt.
    Download PDF (2714K)
  • Sung-Ho Song, Yoonho Song, Byung-Doo Kwon
    2005 Volume 58 Issue 1 Pages 92-96
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Comprehensive field surveys, including various hydrogeological and geophysical methods, were carried out to appraise the applicability of those methods to a leakage problem at the Sandong earth fill dam in southwestern Korea. The methods applied in the field site were tracer tests, monitoring of drawdown and leakage with discharge of reservoir water, electrical resistivity surveys using the dipole-dipole array, self-potential (SP), and temperature logging methods. The leakage pattern in the reservoir wall was demonstrated by hydrogeological methods and was further clarified by the geophysical surveys. Leakage turned out to be through the right abutment of the reservoir wall. In this study, we confirmed that the electrical resistivity method is effective in detecting the zones favorable to leakage, and SP methods are useful for delineating the leakage pathways themselves, because leaks generate strong streaming-potential anomalies.
    Download PDF (2246K)
  • Ahmed Salem, Toshio Hamada, Joseph Kiyoshi Asahina, Keisuke Ushijima
    2005 Volume 58 Issue 1 Pages 97-103
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Recent development of marine magnetic gradient systems, using arrays of sensors, has made it possible to survey large contaminated areas very quickly. However, underwater Unexploded Ordnances (UXO) can be moved by water currents. Because of this mobility, the cleanup process in such situations becomes dynamic rather than static. This implies that detection should occur in near real-time for successful remediation. Therefore, there is a need for a fast interpretation method to rapidly detect signatures of underwater objects in marine magnetic data.
    In this paper, we present a fast method for location and characterization of underwater UXOs. The approach utilises gradient interpretation techniques (analytic signal and Euler methods) to locate the objects precisely. Then, using an iterative linear least-squares technique, we obtain the magnetization characteristics of the sources. The approach was applied to a theoretical marine magnetic anomaly, with random errors, over a known source. We demonstrate the practical utility of the method using marine magnetic gradient data from Japan.
    Download PDF (1270K)
  • Hai Soo Yoo, Su Jeong Kim, Dong Won Park
    2005 Volume 58 Issue 1 Pages 104-111
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Dmitri Donskoi, the Russian cruiser launched in 1883, is known to have sunk near Ulleung Island (East Sea, Korea) on May 29, 1905, while it was participating in the Russo-Japanese War. In order to find this ship, information about its possible location was obtained from Russian and Japanese maritime historical records. The supposed location of the ship was identified, and we conducted a five-year geophysical survey from 1999 to 2003. A reconnaissance three-dimensional topographic survey of the sea floor was carried out using multi-beam echo sounder, marine magnetometer, and side-scan sonar. An anomalous body identified through the initial reconnaissance survey was identified by a detailed survey using a remotely operated vehicle, deep-sea camera, and the mini-submarine Pathfinder. Interpretation of the acquired data showed that the ship is hanging on the side of a channel, at the bottom of the sea 400 m below sea level. The location is about 2 km from Port Jeodong, Ulleung Island. We discovered 152 mm naval guns and other war materiel still attached to the hull of the ship. In addition, the remnants of the steering gear and other machinery that were burnt during the final action were found near the hull.
    Strong magnetic fields, resulting from the presence of volcanic rocks in the survey area, affected the resolution of the magnetic data gathered; as a result, we could not locate the ship reliably using the magnetic method. Severe sea floor topography in the gully around the hull gave rise to diffuse reflections in the side-scan sonar data, and this prevented us from identifying the anomalous body with the side-scan sonar technique. However, the sea-floor image obtained from the multi-beam echo sounder was very useful in verifying the location of the ship.
    Download PDF (14538K)
  • Peter Hatherly, Terry Medhurst, Renate Sliwa, Roland Turner
    2005 Volume 58 Issue 1 Pages 112-117
    Published: 2005
    Released on J-STAGE: November 02, 2007
    JOURNAL FREE ACCESS
    Geophysical logging is routinely undertaken as part of most coal mine exploration programs. Currently, the main application for the logs is to determine coal seam depth and to qualitatively estimate coal quality, lithology, and rock strength. However, further information can be obtained, if quantitative log interpretation is made.
    To assist in the uptake of quantitative interpretation, we discuss log responses in terms of the mineralogy of the clastic sedimentary rocks frequently found in the Australian black coal mining areas of the Sydney and Bowen Basins. We find that the log responses can be tied to the mineralogy with reasonable confidence. Ambiguities in the interpretation will be better resolved if a full suite of logs is run. A method for checking for internal consistency, by comparing calculated and observed velocities, is also described.
    A key driver for quantitative interpretation is geotechnical characterisation. We propose a classification system for clastic rocks that takes into consideration physical rock properties that can be inferred from geophysical logs.
    Download PDF (2479K)
feedback
Top