Koichi Sassa

Seismic attenuation tomography and its application to rock mass evaluation

Abstract We developed two methods of ray-based seismic attenuation tomography. One is amplitude attenuation tomography, which uses amplitude attenuation of the first arrival P-wave. As amplitude is easily influenced by various factors other than viscosity, we removed the effects of velocity structure on wave amplitude, such as transmission loss and diffraction, in the analysis. Decrease of wave frequency during propagation is estimated to obtain the Q-value directly. The other method is pulse broadening tomography, which uses broadening of rise-time or pulse-width of the first arrival P-wave. This method is based on the rise-time principle, which describes pulse broadening with travel time in viscous media. In this method, attenuation information can be obtained by using time data only. The combined use of velocity and attenuation characteristics can be a powerful tool to characterize an in situ rock mass. Therefore we integrated the seismic travel time tomography and the seismic attenuation tomography to a series procedure, and applied it to field data obtained in a mine. The result of travel time tomography agrees with the rock condition estimated by the geological observation at the sites. Faults and fractured zones were detected by the attenuation tomography.

Velocity and amplitude of P-waves transmitted through fractured zones composed of multiple thin low-velocity layers

Abstract Fractured zones and faults in rock masses influence the velocity and amplitude of P-waves propagated through them. In the frequency range of seismic exploration, wavelengths of P-waves are comparable to or longer than the thickness of fractures or faults. Fractures can be modeled as thin low-velocity layers in homogeneous high velocity material. We carried out model experiments on P-wave propagation through a low-velocity zone composed of many thin low-velocity layers, with the direction of P-wave propagation normal to the layers. The experimental models were made of aluminum rods and acrylic resin disks: they correspond, respectively, to rock and materials filling fractures such as clay. Next, we numerically simulated one-dimensional wave propagation through the low-velocity zone by using the communication matrix method. Waveforms obtained by this model calculation show good agreement with those obtained by the model experiments. The waveform and the amplitude of the transmitted P-waves vary with the number and the thickness of low-velocity layers. The time-average equation does not provide a good velocity estimation, because the velocity decreases with the increase in the number of low-velocity layers in spite that the total thickness of layers remaining the same. The distribution of low-velocity layers has little effect on velocity and waveform. Effects of low-velocity zones on waveform, amplitude and velocity of the transmitted P-wave vary with frequency of the incident P-wave. We concluded that the wave transmitted through the low-velocity zone was formed by the superposition of the direct wave and the many waves having multiple reflections at the interfaces of the layers in their path.

Seismic traveltime tomography in anisotropic heterogeneous media

Abstract This paper presents a seismic traveltime tomography method for the analysis of anisotropic heterogeneous media. In this study, anisotropy of slowness is approximated to be sinusoidal with respect to the angle from the anisotropic axis. Anisotropy is represented by the minimum slowness, the maximum slowness and its orientation for every cell. Our method can simultaneously obtain these three parameters. In our first formulation of the anisotropic tomography, the orientation of anisotropy is determined directly from the traveltime data. In the second formulation, the orientation of anisotropy is modified iteratively as well as the maximum and the minimum slowness. The result of the numerical simulation shows that the anisotropic tomography successfully reconstructs the velocity structure having 1–10% velocity anisotropy in case of noise free data. On the other hand, the ordinary isotropic tomography fails to reconstruct the anisotropic structure. Though both the restriction of source-receiver arrangement and the existence of noise affect the accuracy of the reconstructed images, our method can obtain anisotropic information for better understanding of subsurface structures.

Detection of fault structure under a near-surface low velocity layer by seismic tomography: synthetics studies

Abstract We have developed a new method to detect a fault structure under a near-surface low velocity layer (LVL) by seismic tomography. The field study showed that the tomography image reconstructed using borehole-surface configuration had a different result from that of using a crosshole configuration. The image reconstructed by using a borehole-surface configuration showed a decrease in seismic velocities along boreholes, and also the tomogram result using both configurations can not detect the subsurface fault structure. These phenomena are caused by the low velocity layer (LVL) at the top of investigation area. The basic idea hard is based on a downward continuation principle. By knowing the thickness of the LVL and the top of bedrock enables us to place ‘virtual receiver’ and/or ‘virtual source’ below the LVL. In this way, we can reconstruct the image by various tomographic methodologies. As an advantage, this method is easy to be use with the aid of ray tracing methodology. It can also reduce the effect of the near-surface LVL and can maximize the reconstructed image. The final result of our synthetic images by ILST, SIRT, and modified SIRT shows high accuracy and resolution for detection of fault structure under the low velocity layer.

Depth transform of offset VSP data

The present paper proposes a new algorithm for depth transform by use of the equation of equi-travel time Plane of upgoing waves obtained from offset VSP seismograms. It is derived that the equation of equi-travel time plane of waves on VSP seismograms which travel from source to receiving point through reflection point on reflector becomes the equation of the ellipse of which foci are source and receiving points. The procedure of depth transform of offset VSP data is as follows. (1) The average velocities of each layer are determined from the travel times of downgoing waves and their ray-paths from the source to receiving points. (2) The ellipses for each pair of source and receiving point are drawn using travel times of upgoing waves and the estimated average velocity. (3) Each reflector is fixed as the common tangent line to these ellipses. Judging from the model studies for the dipping layers, it was clarified that this algorithm reconstructed the structure with a good accuracy, the short calculation time and the requirement for small size of core memory.

Geophysical Testing for Rock Engineering

Publisher Summary An increasing demand for geophysical testing in recent years has resulted from strict safety specifications for surface and subsurface buildings and facilities, and from developments in earthquake engineering. These applications require that the measurements are made with high accuracy, in order to assist in the definition of complex subsurface geostructures and the distribution and characteristics of thin faults and joints, and also the condition of joint fillings and the rock materials themselves. The refraction seismic method, which has a long history of use, cannot be employed successfully when rock with a high velocity overlies materials of lower velocity, or when there are many differing velocity layers. Moreover, often there is a limit to the accuracy of the velocity values that can be determined using this technique.

Automatic picking of seismic reflection and its application to velocity analysis.

In the present paper, the algorithm for automatic picking of seismic reflection on seismograms is proposed.The procedure of proposed algorithm is as follows.1. The seismograms are transformed to the maximum amplitude seismograms which contain the amplitudes on both all peaks and all troughs.2. The time to start the picking of reflection is assigned on each trace in the maximum amplitude seismograms.3. A lateral continuity search between the traces on seismograms is carried out by use of amplitude ratio, cross-correlation coefficients and semblance across the traces in order to define reflection segments.4. Each segment is classified into grades according to magnitude of amplitude and length of segment.5. If necessary, after some kinds of scaling are applied to all segments for decreasing the segments to be plotted, the segments are displayed to make cross-section.Judging from the results in model studies of the synthetic seismograms and the output from velocity analysis in which this algorithm was applied, it was concluded that the picking of seismic reflection on seismograms was carried out with enough accuracy.

Comments on Corner Fracture in Plastics Caused by Blasting with Two Free Faces

As shown in the previous study on the corner fracturing in rock which was reported in the Journal of the Mining and Metallurgical Institute of Japan, Vol. 83, No. 952, Aug. 1967, the authors clarified already that the corner fracture was not developed essentially in a brittle rock-like material. But it has been reported that the corner fracture is produced actually in metals and in some kinds of plastics. Therefore, in this study, in order to complete the studies on the corner fracture, a plastic material (polymethyl methacrylate) was used as the test material in which the corner fracture was expected to produce, and the stress condition in it in case of blasting with two free faces was analysed by means of the method same as that used in the previous study. The results obtained in this study are as follows; (1) It was clarified both theoretically and experimentally that the corner fracture was developed in the plastic material. (2) There was the difference between the stress distribution in a rock caused by an explosion and that in a plastic material. Therefore, the pattern of the crack produced in the plastics was different from that in the rock. It suggests us that, when the plastics was used as the model material for the studies on rock blasting,. the results obtained might be analysed very carefully.

On the Dynamic Stresses within a Rock Caused by an Explosion

where U(r,ƒÑ)=radial displacement, v(r,r)=rate of change in radial displacement with time, r=distance from the shot point, cL=propagation velocity of the longitudinal wave, A, constants. The experiments were carried out in a tuffacious rocks bearing copper ores and a 45g—cartridge of No. 3 Take dynamite or Tokugiri dynamite was used as an explosive in each measurement. The results obtained are briefly shown below. As to the characteristics of the dynamic stresses, it was found that the radial stress near the shot point (r<about 1.5m) acted as the nonoscillatory compressive stress and this wave shape was almost similar to that of the radial displacement, but at about 2 meters distant from the shot poins, the wave length decreased rapidly and this stress became oscillatory, and the shape of the radial stress approached gradually to the wave shape of the rate of change in the .radial displacement with time with the increase of the distance from the shot point. On the other hand, the tangential stress started generally as the compressive stress, but it changed to the tensile stress at once, and its maximum value acted as the tensile stress . It was recognized that these characteristics agreed with those obtained from the theoretical calculation using Sharpes theory. The values of about 600kg/cm2 and 100kg/cm2 were presumed respectively as the radial and tangential peak stresses at the point 0.5m distant from the shot point, and by comparing theses values with the results of the crater tests, which was carried out at the neighbouring place where the stress wave had measured, it was found that magnitudes of the stresses estimated by these experiments were reasonable.