The 14th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2010)

Convolution of seismic wavelet with reflectivity of the earth yields seismic trace which also involves additive noise. Sparse spike deconvolution can remove wavelet effect from seismic noisy data and recover impulse response of the earth. The problem of deconvolution can be viewed as an inversion problem which consists of two main steps. The first stage is to find locations of spikes and the second one is to calculate amplitudes of them. Indeed, desired solution is a set of spikes with appropriate amplitudes such that when convolved with known seismic wavelet fit the data within a predetermined misfit error. Detection of spikes time lags is a nonlinear optimization problem that can be solved using Adaptive Simulated Annealing (ASA). ASA is one of the global stochastic optimization methods which can find the global minimum of complex cost functions with good accuracy. Here for a given seismic trace we locate spikes one by one and after achieving the optimized set of spikes, amplitudes are evaluated using linear least squares.

In Japan, in the high economic growth period in 1960’s, a great number of slopes were formed to construct many roads. Now, the slopes have been aging, it is important to estimate the health of the aging slope and maintain slopes effectually. So, in situs, we usually carry out seismic wave method, surface wave method, electric method, electromagnetic wave method, frequency domain electromagnetic method and so on. However, there is not the technique to compound and interpret the result of each geophysical exploration in a numerical formula of the engineering now. Therefore, we notice to self-organizing maps (SOM) used widely in a field of the information processing engineering, and tried to interpret multidimensional data by integrating. In this paper, we classified the ground property by self-organizing maps. The classification result is relatively conformal with boring data. Therefore, it is recognized that it can be used to improve the interpretative accuracy of compound geophysical explorations. And, it can be shown that this technique is effective to estimate of the ground property of the aging slope.

A computational tool so called the “EKP-postprocessor” has been developed to calculate space/time distributions of self-potentials caused by electrokinetic coupling resulting from histories of underground conditions (pressure, temperature, vapor saturation, concentration of dissolved species, etc.) computed by unsteady multi-dimensional geothermal reservoir simulations (Ishido and Pritchett, 1999). This forward calculation technique enables us to incorporate self-potential monitoring data into history-matching studies to improve mathematical reservoir models. In addition to history-matching studies, the EKP-postprocessor can be applied to various problems related to subsurface fluid flow in volcanic areas, sedimentary basins, etc. and characterization of the transport properties of reservoir-forming rocks. In this paper, the results of numerical simulation studies using the EKP-postprocessor to appraise the utility of a combination of pressure and self-potential transient data to characterize fractured reservoirs are presented.

A coda-wave is formed by superposed signals caused by scatterers such as cracks and medium inhomogeneities in the underground materials. For an inhomogeneous medium, it is natural to deal with stochastic methodologies to interpret seismic data. In the past, it is revealed that coda-Q, which is derived from the coda-wave has relationship with frequency of earthquakes1). If the frequency of earthquakes is reflected the change of loaded stress, there is the relationship between the loaded stress and coda-Q. Our purpose is to reveal that relationship. We employ 2-D finite difference method to simulate scatter waves which is generated by cracks. When the stress is loaded to a simulation model, anisotropy of velocity is occurred along direction of the stress, and formation of the scatter waves is changed. Then value of coda-Q is changed. In this study, we find that coda-Q increases constantly according to the increasing stress and behavior of coda-Q has a tendency according to direction of the stress respectively (i.e. stress is loaded vertically or horizontally). In conclusion, we are able to estimate relative magnitude and the direction of the stress. It might lead a new method in underground monitoring.

ground deformation associated with the eruption at the Showa crater of Sakurajima, which has been active since 2006. Using the Mogi’s spherical pressure model, a volume change of magma chambers can be estimated from the displacement, tilt, or strain observations near the ground surface. The tilt and strain data observed 36 hours before an eruption in April 9, 2009, are analyzed using the Mogi’s spherical model to reveal the behavior of magma leading to eruption. From these data, there seems to be a time lag in the inflation between the two magma chambers at a depth of 4km and 0.1km, respectively. Moreover, the volume change of the shallow source is about one order less than that of the deep one. A system which consists of shallow and deep magma chambers and several vertical conduits connecting them is numerically modeled to investigate the mechanism of the time lag and why the difference in the magnitude of the volumetric changes in the two chambers appears as described above. The initial values of magma properties in the deep magma chamber are assumed from the volcanic ejecta of Sakurajima volcano. We assumed that magma is supplied with a constant rate to the deep magma chamber. The pressure limit that the magma starts to ascend to the shallow chamber is assigned to the deep one. In a one-dimensional steady flow model of a magma conduit, we consider the vesiculation of volatile-bearing magma, gas escape and overpressure in the bubble due to the viscous resistance, which largely influences the physical properties of magma. We confirmed that our hypothetical model could explain the time lag of the inflation and the difference in the volume change although our simulation results cannot exactly describe the data. We would like to propose that the numerical simulation could be a powerful tool for understanding the behavior of magma before eruption once ground deformations are well observed.

Self-potential (SP) is electrical potential mainly generated by thermoelectric, chemical and streaming potentials in the subsurface. The flow of groundwater is often recognized as a bigger source of SP. Using this feature, there are many attempts to localize and quantify flows of liquid in the soil, including groundwater. In case that underground structure is homogeneous, electrical current density according to ground-water flow becomes uniform. Therefore SP on the surface increases monotonically from upstream to downstream of groundwater flow. As a basic interpretation of SP, the direction of increase in SP corresponds with the direction of groundwater flow. However, the anomalous fluctuations of SP start to appear for subsurface inhomogeneous groundwater flow due to the non-uniform conveyance of changes. Extra charge occurs on the boundary of these parameters. As a result, local minimum or maximum in the profile of SP generate just above the boundaries. Our simulation shows that the anomalies of permeability and coupling-coefficient in the subsurface are predominant parameters to effectively estimate the distribution of surface SP in the existence of inhomogeneous underground flow. The effect of coupling coefficient on the SP is often simulated on the underground flow scale. Some anomalies of SP are explained by the inhomogeneous of coupling coefficient. Simple model simulation, for example the well-pump model shows the effect of hydraulic conductivity on the SP. However, SP anomalies that are generated by inhomogeneity of hydraulic conductivity are not simulated on the underground flow scale. We compare the difference of these anomalies and study the different feature of SP anomaly that is generated by hydraulic conductivity from it by coupling coefficient.