Yoshibumi Kato

Estimation of ultrasonic scattering attenuation in partially frozen brines using magnetic resonance images

Seismic attenuation is not due entirely to intrinsic properties; a component due to scattering effects is included. Although different techniques have been used to experimentally investigate the attenuation of seismic waves, not so many laboratory measurements of attenuation have taken into account the effect of scattering attenuation. Herein, partially frozen brine as a solid-liquid coexistence system is used to investigate attenuation phenomena. We obtained a series of 2D apparent diffusion coefficient (ADC) maps of the ice-brine coexisting system using a diffusion-weighted magnetic resonance imaging (DW-MRI) technique at −5°C , and found a strongly heterogeneous spatial distribution of unfrozen brine. From these maps, we constructed a synthetic seismic data set propagating through 2D media, and generated synthetic data with a second-order finite-difference scheme for the 2D acoustic wave equation. We estimated ultrasonic scattering attenuation in such systems by the centroid frequency shift method and ...

Laboratory experiments on compressional ultrasonic wave attenuation in partially frozen brines

Often, the loss mechanisms responsible for seismic attenuation are unclear and controversial. We used partially frozen brine as a solid-liquid coexistence system to investigate attenuation phenomena. Ultrasonic wave-transmission measurements on an ice-brine coexisting system were conducted to examine the influence of unfrozen brine in the pore microstructure on ultrasonic waves. We observed the variations of a 150–1000 kHz wave transmitted through a liquid system to a solid-liquid coexistence system, changing its temperature from 20°C to – 15°C . We quantitatively estimated attenuation in a frequency range of 350–600 kHz by considering different distances between the source and receiver transducers. We also estimated the total amount of frozen brine at each temperature by using the pulsed nuclear magnetic resonance (NMR) technique and related those results to attenuation results. The waveform analyses indicate that ultrasonic attenuation in an ice-brine coexisting system reaches its peak at −3°C , at whic...

A poroelastic model for ultrasonic wave attenuation in partially frozen brines

Although there are many possible mechanisms for the intrinsic seismic attenuation in composite materials that include fluids, relative motion between solids and fluids during seismic wave propagation is one of the most important attenuation mechanisms. In our previous study, we conducted ultrasonic wave transmission measurements on an ice-brine coexisting system to examine the influence on ultrasonic waves of the unfrozen brine in the pore microstructure of ice. In order to elucidate the physical mechanism responsible for ultrasonic wave attenuation in the frequency range of 350–600 kHz, measured at different temperatures in partially frozen brines, we employed a poroelastic model based on the Biot theory to describe the propagation of ultrasonic waves through partially frozen brines. By assuming that the solid phase is ice and the liquid phase is the unfrozen brine, fluid properties measured by a pulsed nuclear magnetic resonance technique were used to calculate porosities at different temperatures. The computed intrinsic attenuation at 500 kHz cannot completely predict the measured attenuation results from the experimental study in an ice-brine coexisting system, which suggests that other attenuation mechanisms such as the squirt-flow mechanism and wave scattering effect should be taken into account.

Spatial investigation by magnetic resonance imaging and estimation of scattering attenuation in partially frozen brines

Summary Seismic attenuation results are not entirely due to intrinsic properties; a component due to scattering effects is also included. Although different techniques have been used to experimentally investigate the attenuation of seismic waves, few laboratory measurements of attenuation have taken the effect of scattering attenuation into account. Herein partially frozen brine as a solid-liquid coexistence system is used to investigate attenuation phenomena. We obtained a series of two-dimensional apparent diffusion coefficient (ADC) maps of the ice-brine coexisting system using a diffusion-weighted magnetic resonance imaging (DWMRI) technique, and found a strongly heterogeneous spatial distribution of unfrozen brine. From these maps, we constructed a synthetic seismic data set propagating through two-dimensional media, and generated synthetic data with a second-order finite difference scheme for the two-dimensional acoustic wave equation. We estimated ultrasonic scattering attenuation in such systems by the centroid frequency shift method and assuming that the quality factor (Q-value) is independent of frequency.

Laboratory measurements of ultrasonic P-wave attenuation in partially frozen brines by using sweep signals

Summary We demonstrate a method that derives a more accurate measurement of ultrasonic attenuation by using sweep-type signals than by using impulse-type signals. We obtained spectral amplitude of the sweep signal in frequency-time domain using the continuous wavelet transform (CWT) and estimated attenuation in the time-frequency domain using the spectral-ratio method. The advantage of this method is independent on the effect of windowing. We used partially frozen brine as a solid-liquid coexistence system to investigate attenuation phenomena. Ultrasonic wavetransmission measurements on an ice-brine coexisting system were conducted to examine the influence of unfrozen brine in the pore microstructure on ultrasonic waves. We observed the variations of a 100–1000 kHz sweep signal transmitted through a liquid system to a solidliquid coexistence system, changing its temperature from 20°C to –15°C. We quantitatively estimated attenuation in a frequency range of 350–600 kHz by considering different distances between the source and receiver transducers. Finally we demonstrated the possibility of sweep signal to estimate attenuation.

Laboratory Experiments On Ultrasonic Wave Attenuation In Partially Frozen Brines

In this study, we conducted experimental measurements by use of partially frozen brine. We observed the variations of a transmitted wave with frequency content of 350–600 kHz through a liquid system to a solid-liquid coexistence system, changing its temperature from 20 °C to -15 °C. We used the different distances between source and receiver transducers to estimate attenuation quantitatively. At the same time, we estimated the total amount of frozen brine at each temperature by nuclear magnetic resonance (NMR) technique and related them to attenuation results. The waveform analyses indicate that ultrasonic attenuation in ice-brine coexisting system meets its peak at the temperature of -3°C where the ratio of liquid phase to total volume in ice-brine coexisting system is maximum. Finally we can see the high correlation between attenuation of ultrasonic waves and the existence of unfrozen brine. Thus, our laboratory experiments demonstrated that the ultrasonic waves with such frequency range are affected by the existence of unfrozen brine.

The estimation of subsurface carbon stocks for the Kyoto Protocol by ground-penetrating radar

Every advanced country will have to make efforts to reduce greenhouse gases in the atmosphere in compliance with the Kyoto Protocol. It has been decided in Bonn that the countries can estimate the amount of forest absorption generously, because it is difficult to achieve the target for the reduction of carbon dioxide emissions only by means of innovative environmental technology and enhanced energy efficiency in the society. However, uncertainty about the amount of carbon stocks in forests is large. As it is desirable to leave out uncertainty and estimate the amount conservatively in the spirit of the Kyoto Protocol, an accurate estimation would lead to large amounts of the estimated carbon stocks. In the forest, the underground carbon stock is larger than the amount above the ground and its uncertainty is very large. Until now, there has not been an effective method to estimate underground carbon stocks, and the only relatively good one is the method using models from the vegetation above ground. Thus, we suggest an estimating method using ground-penetrating radar (GPR) for improving the estimation method. Using the nondestructive inspection technology like GPR, we can reduce the estimation cost and obtain the quantified data. GPR surveys can classify the soil layers and identify the roots of trees (Rokugawa et al. (2000)).