S33B-1094 1340h
Detection of the Temporal Variations of Structure Sensitive Bodies by the Active Monitoring
Observable phenomena that cause temporal variations of stress field related to generations of earthquakes and volcanic eruptions could be mainly the reflected wave from scattering sources. The heterogeneity in the lithosphere originated from both stress state and heterogeneous distribution of fluid-bearing rocks can be the scattering sources. Temporal variations of the scattering sources due to the structure sensitivity of rocks are essential characteristics of seismogenic regions as well as the active volcanic regions. The active geophysical monitoring would be the essential tool to detect and clarify such an evolving process that governed by the structure sensitivity of rocks in the crust and upper mantle. Among many structure sensitive phenomena, probable changes in the reflected seismic or electromagnetic signals are expected in the temporal variations of impedance and anisotropic dispersion of the transmitted signals in the subduction zone where the scattering sources are evolving associated with the movement of the fluid mainly composed of supercritical water in the crust and upper mantle conditions. Recently discovered slow slip events and deep non-volcanic tremors in the subduction zone could be one of the most challenging targets to clarify their characteristics by using the active monitoring techniques.
S33B-1095 1340h
Travel time estimation from a transfer function in frequency domain: the revised Sompi event analysis
We have developed a method to extract "events" localized in time domain from a transfer function in frequency domain, which is a basic analysis method of ACROSS (Accurately Controlled Routinely Operated Signal System) and referred to as the Sompi event analysis (Hasada et al., 2001, Earth Planets Space). In this method we assume that the complex frequency sequence to be analyzed is a transfer function between the source input and the receiver output sampled at discrete frequencies. Through a kind of autoregressive modeling in frequency domain, we obtain a set of "events" characterized by complex travel time and complex amplitude, where the former expresses travel time and attenuation, and the latter amplitude and phase angle. Though the validity of the Sompi event analysis is confirmed through various numerical tests especially for analysis of dispersive wave, there are some problems in practical application; (1) Identification of wave elements among the different data sets is difficult, (2) Error estimation is not easy. Here we propose a revised procedure by means of weighted least squares fitting, which is based on maximum likelihood estimation. We attempt to improve parameters by weighted least squares fitting, through linearization of model about small variation of parameters. By using this procedure, Maximum likelihood estimation of complex travel times is realized and the problems (1) and (2) are partly solved. On the other hand, there is serious instability in the calculation of least squares. To overcome this problem, some procedure of non-linear optimization may be available.
S33B-1096 1340h
Development of EM-ACROSS and observed transfer function using this system
We have developed and tested a new electromagnetic sounding method, which is called EM-ACROSS (ElectroMagnetic - Accurately Controlled Routinely Operated Signal System). This method uses the same methodology as the seismic ACROSS, i.e. using accurately controlled periodic signals and precise synchronization of the whole observation system. Ogawa and Kumazawa (1996) suggested to use both seismic and electromagnetic waves to visualize the Earth's interior, because the information derived from the each wave is independent. In this study we introduce the EM-ACROSS system and show the results in the diffusion field region. We have developed a prototype system using the GPS clock for synchronization better than 0.1 micro-seconds. The EM signals are transmitted from one of the two crossed dipole sources in Tono Mine (Gifu, Japan). The receiver is located at 700m distance from the source. We successfully tested the transmission of several sets of sine waves, and obtained the transfer function in the frequency range from 0.01 to 500 Hz. Data stacking showed the reduction of the observation noise as expected from statistics. We have been operated this system for more than one year. From the results of the operation, we obtained the averaged transfer functions with good accuracy and also their temporal variations. The signal to noise ratios of the averaged transfer functions are better than 10$^3$. We could evaluate the resistivity of the area from the phases of the transfer functions. The evaluated resistivity is about 100 Ohm-m, which is the typical value at the test site. We could see temporal variation in the phase of transfer function. Although the changes are small as 1% of the averaged value, they are smaller than the confidence limit of averaged value. We could conclude the variation is meaningful. We found these variations correlated with the rainfall.
S33B-1097 1340h
Rotary Transmitter For Active Monitoring By Seismic ACROSS (Accurately Controlled Routinely Operated Signal System)
The optimum designing is discussed on the seismic rotary transmitter, and several different operational modes and signal types are extensively tested to acquire the larger amount of new information through the tensor transfer function between the signal source and receiver for the structure exploration and monitoring of the deeper horizon of the Earth's crust. The shear wave transmission by controlled source has scarcely been made so far, whereas it is supposed to carry invaluable information on dispersion and polarization anisotropy that are more sensitive to the small temporal variation in the physical states of the polycrystalline rocks under stress. One of the successful systems is a single rotary type with a vertical rotation axis (generated force: 2x10$^{6}$ N at 25 Hz), which is installed within a large 'ground coupler' (steel reinforced concrete block (3.5x6m$^{2}$ and 2.3 m in thickness) embedded into the tertiary formation at Tono test site, in Japan. The large contact area of the coupler with ground guarantees the stability of transmitting characteristics by reducing the fatigue and non-linear effect at the contact zone even for the transmission of SH wave with large amplitude. Successive alternation of clockwise and anticlockwise rotations at every hour enables us to obtain a set of 6 components of transfer functions : H$_{ba}$ (b = radial, transverse or vertical, in received wave polarization referred to signal source, a = radial or transverse excitations from the source to any receiver station located at any azimuth), continuously over years. An optimization method of FM signal design is worked to obtain a set of spectral lines with almost uniform level of force amplitude in a prescribed frequency band. Several results of observed transfer functions and its time domain representation are demonstrated.
S33B-1098 1340h
Feasibility of Active Monitoring for Plate Coupling Using ACROSS
Detectability of temporal changes in reflected wave from the boundary of subducting plates in Tokai district with active sources are studied. Based on rock experiments the change in the intensity of reflection wave can be caused by change in coupling between subducting and overriding plates. ACROSS (Accurately-Controlled Rountine-Operated Signal System) consists of sinusoidal vibration sources and receivers is proved to provide a data of excellent signal resolution. The following technical issues should be overcome to monitor the returned signal from boundaries of subducting plates. (1) Long term operation of the source. (2) Detection of temporal change. (3) Accurate estimation of source functions and their temporal change. First two issues have already overcome. We have already succeeded a long-term operation experiment with the ACROSS system in Awaji, Japan. The operation was carried out for 15 months with only minor troubles. Continuous signal during the experiment are successfully obtained. In the experiment we developed a technique to monitor the temporal change of travel time with a resolution of several tens of microseconds. The third issue is one of the most difficult problem for practical monitoring using artificial sources. In the 15-month experiment we correct the source function using the record of seismometers that were deployed around the source We also estimate the efficiency of the reflected wave detection using ACROSS system. We use a data of seismic exploration experiment by blasts that carried out above subducting plate in Tokai district. Clear reflection from the surface of the Philippine Sea plate is observed in the waveform. Assuming that the ACROSS source is installed at the same place of the blast source, the detectability of temporal variation of reflection wave can be estimated. As we have measured the variation of signal amplitude that depends on the distance from an ACROSS source, ground noise at seismic stations (receivers) provide us the signal-to-noise ratio for the signal from ACROSS. The resolution can be estimated only by the signal-to-noise ratio. We surveyed the noise level at the place where reflection from the boundary of subducting Philippine Sea Plate can be detected. The results show that the resolution will be better than 1% in amplitude and 0.1milisecond in travel time for the stacking of one week using three-unit source and ten-elements receiver arrays.
S33B-1099 1340h
Active monitoring of upper crust using ACROSS-seismic array system
Temporal variations of S- and surface-wave travel times were continuously monitored using ACROSS source and seismic array. We made an experiment lasting 5 months at a site near the Nojima fault which ruptured during the 1995 Kobe earthquake (M7.2). Elastic waves generated by ACROSS vibrators are received by two seismic arrays. One is located at about 300m northwest and the other is about 300m southwest of the vibrators. Each array has an aperture size of about 50 m and consists of ten seismometers that are three component velocity sensors with natural frequency of 4.5Hz. In this experiment, we used solar-battery systems to enable the long-term experiment, and we succeeded in continuous data recording without any troubles. To obtain the signal in time domain, in which P, S and some later phases were included, we executed the following procedure in the frequency domain. We extracted the ACROSS signals from the every stacked data. The extracted signal was divided by the force which was generated by the source. In this study, we used the spectrum of the theoretical force calculated from the frequency-modulated rotation. We regarded the result as a transfer function (or band-limited impulse response) between the source and the receivers. Applying appropriate window function and inverse Fourier transformation, we could obtain S wave and big surface wave. To emphasize later part of ACROSS signal, we stacked the data of all N-array sensors for every one hour and transformed its envelope using Hilbert transformation. We may detect some phase around 8, 13, 16 -seconds in the envelope. There were a few candidates for a cause of the phases, random noise or coherent noise, or reflected signals from deeper portion of the crust. We examined these possibilities one by one. The phases were found all through the experiment period. Therefore they must not be due to random noises. Next, we synthesized transfer function between the vibrator and the seismic array to examine the effect of coherent noises theoretically. As a result, we figured out that the later phases should be excited due to coherent noise which should be distributed uniformly over all frequency band. A strong candidate of such noise is uncertainty of the source function which was used in calculation of the transfer function. We estimated relation between the main phases and later phases excited by the uncertainty of the source function quantitatively. In this case, we cannot insist that the later phases were correlated with reflected phase. If they are due to uncertainty of the source function, the amplitude of the later phases, which was three orders of magnitude smaller than the maximum amplitude, suggests that the uncertainty of the source function used in this study should be about 1 percent.
S33B-1100 1340h
Continuous Observation of Travel Time of Seismic Waves Using ACROSS Vibrator and Seismic Array
ACROSS (Accurately Controlled Routinely Operated Signal System) monitors the temporal variation in seismic velocity with high resolution using a precisely controlled sinusoidal signal. Experiments using an ACROSS vibrator and a seismic array were carried out for 1 month in a vault at Mizunami, Gifu Prefecture, Japan. Two ACROSS vibrators were continuously operated with frequency modulation centered at 17.52 and 25.53 Hz, respectively and with modulation amplitude of 2.5 Hz and modulation period of 20 seconds. We deployed a seismic array of 15 seismometers in a cross-shaped vault at an interval of 8 m as crossed array (64_~64m). The distance from the ACROSS vibrator is 2.4 km. Transfer function between the ACROSS vibrator and each seismometer was obtained by applying a deconvolution of the observed data with the theoretical force generated from the source in the frequency domain. We determined propagation direction and apparent velocity of dominant phases using the semblance method. The P and S waves arriving at 0.65 and 1.25 seconds were identified with apparent velocities of 4.0 km/s and 2.2 km/s, respectively. They were interpreted as refracted P and S waves traveled along the velocity boundary between a sedimentally layer and a basement layer. Temporal variation in the travel time of the refracted P and S waves was calculated using cross-spectrum density method. Travel time change was derived from phase along in the frequency domain (Ikuta et. al., 2002). Uncertainties of travel time change of the P and S waves were evaluated within 0.1 ms and 0.05 ms for 1 month observation. The temporal variations showed a similar pattern to the variation of atmospheric temperature. The travel time delayed when the atmospheric temperature decreased. The daily variation had a value of about 0.5 ms at the maximum. Phase variation measured on the source foundation also showed the similar pattern, with smaller value about 0.1 ms at the maximum. The apparent velocity did not show such a daily variation. The result implies that the daily variation, possibly related to the variation of atmospheric temperature, was caused not only around the source but in its propagation. The sensitivity analysis derived from scattering theory suggests that the refracted wave can be affected by the velocity change near the surface. The result indicates that the change near the surface can cause the temporal variation in travel time.
S33B-1101 1340h
4Deep Seismix
4D, or time-lapse, seismic reflection imaging is rapidly becoming a mainstream tool in monitoring oil field production. The same technology offers considerable potential in addressing issues central to active tectonics in deeper crust. In this study, we employ acoustic finite difference wave equation modeling using MATLAB (CREWES) to evaluate the issues that constrain the feasibility of time-lapse imaging of seismic and volcanic systems. Such systems are the most likely to have temporal variations which occur on time scales where such surveys may have operational practicality. Among the processes modeled are changes in inflation (or deflation) of magma sills at midcrustal depths, fluid pressure in the deep seismogenic zone, and offset of potential marker horizons by aseismic creep in slow earthquakes. While the latter would seem to be beyond practical consideration, at least at the present time, differential seismic sections produced for a variety of magma inflation models indicate that monitoring of magma movements at depth is a realistic goal. Possible seismogenic variations in deep faults zones are perhaps more problematic, though our model suggests scenarios wherein useful results may be obtained. In all of these cases, resolution is perhaps less of an issue than S/N. Noise sources include both ambient noise, and systematic property variations in overlying media. The latter may be addressed by standard signal enhancement procedures, both in acquisition and processing. The former may be addressed by "registration' of data against natural deep markers. While one can envision a number of significant practical hurdles to time lapse imaging of deep processes, this study indicates that it is not an unreasonable goal. Moreover, the deep crust may prove to be considerably less `static' that we normally assume.
S33B-1102 1340h
Multidisciplinary Approach To Earthquake Prediction Using Vibroseismic Sounding Of Seismic-Prone Zones
The existence of a near-surface crack formation zone (besides a source zone) was predicted by numerical modeling of the initial stage of rock destruction in the concentration zone of tectonic stresses. This made it possible to explain some contradictions of the multidisciplinary approach to earthquake prediction. It was shown that one should admit the possibility of formation of numerous geophysical anomalies-precursors due to changes in the fluid saturation, electric conductivity, and density of the surface crack formation zone. These anomalies can reflect the development of the surface zone, although they are only indirectly connected with the stresses and crack formation in the source. To find quantitative relations between anomalous geophysical fields at the surface and the field of stresses and deformations in the area of the source, it was proposed to use the "active seismology method". This method allows vibroseismic monitoring of the stressed-deformed state of the Earth's crust mass involved in the crack formation and other rheological changes in the vicinity of the future source. This multidisciplinary prediction technology implies the medium-range determination of the epicenter areas of developing sources by using geodetic and other anomalies and subsequent detailing of the source rheology monitoring by vibroseismic sounding of selected zones. The combination of the data of deep seismic monitoring with rich information about fields-precursors at the surface opens up new possibilities for short-range prediction. Now, an experimental system of active vibroseismic monitoring of the Earth's crust, which includes powerful 100 - ton vibrators, mobile seismic arrays for the recording of vibrosignals, and computer systems for the processing of vibromonitoring data is being developed. The system was tested in experiments, where the influence of the Earth's tidal deformations of the crust (of the order of 10-7) on the velocities of seismic waves was determined. These experiments have shown that the seismic monitoring system with powerful vibrators has high sensitivity and allows us to select relative variations of velocities of seismic waves of about 10-5 - 10-6 in an area of 300-500 km around the source. It makes possible direct monitoring of the state of stresses in an area of 100 000 km2 to detect regions and phases of critical stresses as an earthquake precursor. Some local geophysical anomalies are not precursors of direct action. They mainly characterize the dynamics of the fissured surface zone, but not the rheology of the source. It is likely that the most accurate method for estimating the relation between the surface and source dilatancy zones by using a common field of stresses is vibroseismic monitoring of changes in the anisotropy of the intermediate medium's layer in seismic prone zones. The influence of geological peculiarities of this zone can be investigated by creating vibroseismic test sites in heterogeneous zones (transformed fracture; rift zone; intraplate zone, etc.). The coordination of works by the International Cooperation Program of existing test sites in Parkfield, (California, USA), Baikal, (Siberia, Russia) and Hebei Province (China) can provide the necessary information.
S33B-1103 1340h
Experiments On Vibroseismic Monitoring Of The Earth's Crust In Siberia.
The long-term experiment on vibroseismic monitoring of the Earth' crust was begun in 1997 in Thom'-Kolyvan' fold zone. As excitation sources two vibrators of 100 and 40 ton force were used. The recording was carried out by sensors on source platform at a depth of 3m under platform and in "Novosibirsk" seismological station at a distance of 49km from vibrator. Besides temperature measuring was carried out by a set temperature detectors from subgrade under vibrator up to 3m depth. The experimental work are focussed on study of long-term experiment features with high-power vibrators in conditions of seasonal effects of a ground state, as well as on studies of time variations of seismic wave travel times and on test of developed hardware resources. The fixed vibrator is located in the open and is working for years without installation modifications. The mobile vibrator was located near, and a year later it has been located in underground pavilion. Thus a temperature stabilization of a ground under vibrator has been achieved. The seasonal variations of frequency characteristic of vibrator radiation have been investigated. Only process of a freezing and thawing of a ground was demonstrated to have effect on vibrator radiation. There is both a winter type of performance and summer one and transition periods between them. The seismograms of remote zone test cyclical modifications winter/summer too. Travel time attributes of waves have no dependence on process of freezing/thawing of a ground. Dynamic behavior has a pronounced relation with perfect recurrence of dynamics for the same level of freezing or thawing at different years. The fixed vibrator works in operating condition, when the radiant resonance is in operating frequency range. Taking into account that the resonance is most sensitive to the ground state under a source in experiments with 40-ton vibrator the working range of soundings was removed after resonance spectral region. In this case on measurements in near zone the essential stabilization in time of radiation spectrum is observed. However measurements in far field have shown the same seasonal modifications of seismograms. Dependence on processes of ground freezing and thawing has remained. The placing of 40-ton vibrator in underground pavilion stabilized even greater a signal spectrum, leaving in a medium, but the seismograms at a distance test as before seasonal modifications of wave dynamic characteristics. It has been found by more detail researches that the freezing and thawing of a ground have effect not only on radiation spectrum, but also on the directional diagram of the source. To remove influence of ground seasonal modifications to a spectrum is much easier, than to stabilize an invariability of the directional diagram. The surprising thing is that the seismograms of different years for identical level of ground freezing surprising coincide with each other on wave dynamics. As a result of experiments is shown that for medium monitoring when working with vibrator there is a high accuracy of determination of seismic wave travel times, but the dynamic characteristics of waves test seasonal variations connected to a freezing and thawing of breeds under a source. Their use demands a control over the process of freezing and thawing of a ground.
S33B-1104 1340h
Vibroseismic Monitoring of Active Volcanos
The paper considers theory, methods and experiments aimed at creation of a monitoring system of active volcanos with the use of powerful vibroseismic sources. We propose a concept of creation of a system for studying the geometry of magma chambers, deep faults in the neighborhood of volcanos, dynamics of eruption processes using the methods of vibroseismic sounding, the latter based on powerful controlled sources with the force, acting on the ground. The results of calculation of a seismic field for a medium, which is elliptic inclusion info a homogeneous half-space simulating a magma region, are presented. In this case, a volcanic medium is approximated with the help of a number of, rectangular plates. Special attention is being given to development of method of vibroseismic monitoring of living volcanos, which, in our opinion, will help in measuring the rates of magma elevation in channels and in predicting the time of eruption of a volcano in question in combination with other geophysical, geochemical and geological methods.