NG51A-1193
Active geophysical monitoring of hydrocarbon reservoirs using EM methods
Marine controlled-source electromagnetic (MCSEM) technology has been successfully established as an effective tool for offshore hydrocarbon (HC) exploration. In this paper we consider another application of the MCSEM method for HC reservoir monitoring. We demonstrate that EM methods can be successfully used for the monitoring of producing wells in connection with the enhanced recovery of hydrocarbons. We have developed a new powerful EM modeling technique based on the integral equation method with an inhomogeneous background conductivity (IE IBC). This new method and the corresponding computer software make it possible to model the EM response over a realistic complex model of a sea-bottom HC reservoir. The numerical modeling results demonstrate that the MCSEM method has the ability to map changes in resistivity caused by the production of hydrocarbons over time. In addition, the EM data help to visualize the changes in the location of the oil-water contact within the reservoir. This result opens the possibility for practical application of the EM method in HC reservoir monitoring.
NG51A-1194
Fast imaging of marine controlled-source EM data using time domain electromagnetic migration
The application of electromagnetic (EM) methods in petroleum and mining exploration requires development of appropriate imaging techniques, which provide the means for fast but accurate evaluation of the observed data. Time domain electromagnetic (TDEM) methods are widely used in geophysical exploration. However, practical interpretation of TDEM data is mainly based on a simple one-dimensional (1D) inversion of the data at any given observation point. Due to increased interest in the time-domain technique in offshore petroleum exploration with a controlled-source of the EM field, the development of multidimensional interpretation of TDEM data is required. In this paper, a fast approach to TDEM data interpretation using the method of TDEM migration is introduced. The computation of the migration field is based on downward extrapolation of the observed field in reverse time. An effective method of EM migration based on the operator of an integral transformation in the spatial-temporal domain is examined. A migration geoelectrical image is constructed using the convolution of the background and migration fields. Also, the technique is extended for interpretation of TDEM data observed both in land surveys with layered background media and in marine hydrocarbon exploration by introducing an adjoint land geoelectrical model for a given model and a corresponding adjoint field. The accuracy of this imaging technique is demonstrated successfully using synthetic TDEM data simulating mining exploration and hydrocarbon exploration in land and marine survey environments, respectively. The results ensure that this imaging technique works reasonably well.
NG51A-1195
Estimation of 3D effects of seafloor topography in marine magnetotelluric data recorded in Eastern Nankai
Subsurface resistivity structure in the vicinity of subduction zone is useful information that characterizes not only earthquakes generation mechanism but also the distribution of natural resources such as gas hydrate. Since the generation of both earthquake and natural resources could all be related to the migration of interstitial fluid, it is justified to use an electromagnetic method that is sensitive to the existence of water. In the course of our analysis for marine MT data acquired at Eastern Nankai, we confirmed that the three dimensional geographical features influence the MT responses according to the value of skewness. It is said that there are the three dimensional geographical effects when the skewness value becomes beyond 0.3, while the value in Eastern Nankai was between 0.3 and 0.6 in almost all frequencies at the all sites. Therefore, it is indispensable to accommodate 3D topographic feature in our data analysis. To analyze the crustal and mantle resistivity structures around the Eastern Nankai, off the Tokai region, Japan, marine magnetotelluric (MT) soundings were carried out in 2007. The Eastern Nankai is located at the plate boundary where unconsolidated sediments on the subducting Philippine Sea plate are underplated to the southwest Japan arc above a megathrust seismogenic zone. In analyzing time series data, the MT response (the apparent resistivity and the impedance phase) of TM-mode in Eastern Nankai performs lower quality than that of TE-mode does. First of all, we performed a two dimensional analysis from selected TE- mode data for constructing a resistivity model because we need more complicated procedure in TM-mode analysis. Our two dimensional inversion model from TE-mode demonstrates that the electrical resistivity gap at the plate boundary coincides with the seismic reflector obtained by a seismic reflection survey. The resistivity structure of Eastern Nankai has small scale horst-graben type undulation caused by high and low resistive sedimentary rocks comparing with that of Central Nankai, i.e., one of the other subduction zones of the Philippine Sea Plate. These resistivity features have a possibility to explain the difference in the conditions and materials related to the generation of megathrust earthquakes. Then, we confirmed the resistivity of Eastern Nankai is higher than that of Central Nankai. The results suggest that the plate around Eastern Nankai is older or colder than that of central Nankai, which is consistent with the other tectonic studies. Obviously, the seafloor topography is much more complex in Eastern Nankai than in Central, and could be the cause of the TM-mode that is strongly skewed in that region, while the TE-mode is not so. Between TM- and TE-mode, we found a single 2D structural model may not satisfy both data at the same time. We performed three dimensional foward calculations in many models whose shape look like the feature of the bottom of the sea in Eastern Nankai, and tried to optimize our analysis that leads us to the most appropriate fitting of data. The difference between the MT response observed and calculated from the most suitable model by the three dimensional foward code is the key to make the correction to the effects of seafloor topography on ocean bottom MT data. What to do next is the consideration of these missfit and the hybrid of the three dimensional forward code and two dimensional inversion code. This method makes it possible for MT responses to be more accurate and reliable.
NG51A-1196
Structural exploration using longwave radio-clock time-signal
VLF methods have been used for one dimensional survey that ties records of the single point measurement with subsurface structure. Since VLF electromagnetic wave is not stable due to various effects in the propagation, subsurface structural exploration using VLF methods has limitations in resolution and in the applicability depending on the place of surveys. To overcome some of the limitations, we propose to use standard-time longwaveelectromagnetic transmissions (JJY in Japan), that could be more stable than VLF, for the exploration of underground structure. Once radio time-signal receivers have become popular, we may distribute many receivers in a wide area to record continuous time signal simultaneously for estimating subsurface resistivity distribution. Continuous measurements, moreover, might improve measurement efficiency and S/N ratio. In our study, we applied numerical experiments to confirm the method to work. First, we created a test data set composed of air and heterogeneous half space earth for which JJY signal propagates. Then, we estimate the distortion of time signal on the surface of the half space to evaluate the characteristics of underground response to JJY and to see the availability of JJY standard electromagnetic wave for structural exploration as well as for a VLF method. We used electromagnetic wave of 20 kHz as a VLF wave and 40 and 60 kHz as JJY standard electromagnetic waves and evaluated the resolution of the methods derived from the skin depth and the influence of the geometry for various combination of the orientation of anomalous structure, the propagation direction of radio wave, and the orientation of two- dimentionally aligned receivers. To estimate the influence of the geometry between the orientations of structural anomaly and the propagation direction, we evaluated the characteristic response of the survey as a function of difference angle of the orientations. Our results show the following confirmation: (i) there are little influence on the attenuation of the electromagnetic radiation if observation point is located above the resistivity anomaly, (ii) - higher the frequency becomes, shallower layer the influences come from, and (iii) the smaller difference angle becomes, better the sensitivity of survey becomes. Therefore, we conclude that the structural anomaly runs in the direction of radio wave propagation, the most ideal survey would be conducted as known well for electromagnetic surveys. Our study suggests that JJY signal or any other continuous time signal could be used for the estimation of subsurface resistivity distribution. In the future, we try to extend the method to VLF-MT for subsurface structure and to apply it for field data.
NG51A-1197
Evaluation of seismic reflection characteristics from non-asperities along the subduction zone to actively monitor subduction zone behavior
It is widely accepted that shallow-intermediate depth major earthquakes along subdution zones occur in
asperities surrounding by non-asperities. Non-asperities might comprise liquid-like layer which can generate
strong seismic reflections. Rapid change of physical state in the non-asperities might trigger seismic slip
along asperities because of high strain accumulation there. Strong seismic reflections were observed along
the Japan Trench (Fujie et al., 2002) and the Nankai subduction zones (Iidaka et al, 2003). The observation
of strong reflections along the subduction zone suggests the presence of liquid-like layer which comprises
non-asperities. We showed the possibility to detect change of physical state in the strong seismic reflection
zone in the Tokai subduction zone using ACROSS seismic system (Tsuruga et al., 2006). To perform a better
observation, it is necessary to know seismic characteristics assuming presence of liquid-like phase along the
subduction zone. For this purpose, we performed waveform simulation.
We examined the seismic refraction and wide-angle reflection phases in the subduction zone. In a structural
model, the trench axis is located at x =100km at the center of the model, and an oceanic plate subducts
beneath the forearc basin between 0 and 100 km. The zone between 100 and 200 km is pure oceanic
region. The oceanic crust has 7 km in thick. The thickness of forearc basin is thinning toward the trench axis.
Above the subducting plate, 500-m thick decollement with Vp=1.6-2.2 km/s is placed. Vp at just top of the
oceanic mantle is 8.0 km/s. The seismograms and travel times were calculated by 2D-FDM (Larsen, 2000)
and graph method (Kubota et al., 2005), respectively. Assuming appropriate Vp, Vs, density and Q-values
structural models, we computed shot-gather records using 4-Hz Ricker wavelet explosive sources placed at
the ocean bottom and receivers aligned at 30-m below the sea surface. Grid space in space is 30 m, and
time step is 2 ms. At particular locations, we can recognize the strong reflection from the decollement with
negative polarity due to the negative impedance contrast as follows:
1) At 100 km from the trench axis, clear reflection from the decollement at 6-km below the ocean bottom and
PmP from the subducting oceanic Moho are identified. Reflection from the decollement has large amplitude
between offset distance of 0 and 30@km, but PmP does large amplitude between 30-50 km. Pn traveling in
the oceanic mantle has clear appearance. 2) At 50 km, characteristics of shot-gather records have similar
characteristics of ones at 100km case. 3) At 100 km (trench axis), synthetic waveforms at the offset distance
> 100km are similar to the seismic records at typical oceanic crust. Large Pg and PmP are identified. Pn is
seen for both sides of trench axis. Reflection from PmP around 0-offset is weak, but it has very large
amplitude at 20-40 km by wide-angle reflection.
The thickness of decollement used is a little thick (0.5km) in our numerical test. If 16-Hz wavelet is used as a
source similar to real observation, such layer has about 125 m in thick.. Although we assume a pure liquid
layer, we can expect strong seismic reflection from the target layer even for not-pure liquid layer. Through
the above simulation, we can choose appropriate observation method for active monitoring of the subduction
zone dynamics.
NG51A-1198
Characteristics and Temporal Change of Green Function Acquired by Seismic ACROSS Signal from the Morimachi Transmitting Station
We report the preliminary results obtained by the ACROSS signals observed by Hi-net and T-system within 100 km from the Morimachi transmitting station built in 2006 (Kunitomo et al., This AGU meeting). The seismograms acquired by this work are not those of the ordinary sense but they are the Green function between the source and observation site defined within a specific frequency range (from 3.5 to 7.5 Hz in the present case). Their waveforms show significant azimuthal variation together with the small temporal variation. We note the large coda part of the first S wave arrival towards NW direction, since a clear reflection phase from the upper boundary of the Philippine Sea plate has been recognized by the explosion experiments in 2001 for seismic profiling along this profile (Iidaka et al., 2003). The identification of the noted reflection phase is supported further by ray tracing analyses on the several candidate velocity structures with lateral heterogeneities. The annual variation of travel time is recognized for many phases. For example, in N.MRIH station (distance: 2.9km), the annual variation amounts to about 1msec in the first kick of S wave, and about 5msec in the wave packet at 1 second after the first S wave arrival. This may suggest a possibility of reverberation in the surface layer influenced by seasonal effect as mentioned by Kunitomo and Kumazawa (2004). Close examination of the data analysis has led to so many different types of very small temporal variations in the order of 0.1msec in travel time, which are the interesting subjects for the studies in detail to identify their natures, sources and causes. Such works are demanded to identify to remove the effects originated from the shallower crust near the surface in order to detect the subtle temporal variations in the deep earthquake field. We conclude that the seismic ACROSS works as a potential tool to study the dynamic natures of the Earth crust, in particular for the remote active monitoring of the earthquake prone zones.
NG51A-1199
Simultaneous Near-Field Monitoring During Active Monitoring by seismic ACROSS
ACROSS (Accurately Controlled Routinely Operated Signal System) is a tool for active monitoring of underground structure and state, in which we transmit and receive stationary signal of seismic or electromagnetic wave continuously and monitor the temporal variation of the propagation characteristics. At the present time, the seismic ACROSS transmitters are in continuous operation at the plural sites in Japan and several experiments attempting to monitor underground structures or seismogenic zone by seismic ACROSS have been carried out. Although various temporal changes have been detected by ACROSS observations, most of those changes are considered to occur at the rather shallow part of the ground. We propose a experimental procedure to monitor the neighborhood of a receiver during receiving ACROSS signal. In this procedure, near-field monitoring takes a passive way by use of the ambient noises. The accurate control of the ACROSS signal enables us to deal with the signal and the background noise separately. Because the spectrum of the ACROSS signal is consisted by a group of equally spacing line spectra whose widths are very narrow, we can analyze the remaining parts of the spectra as ambient noise. We developed a simple algorithm for interpolation of the data in which the frequency components of ACROSS signal are absent. The seismic array enables us to distinguish the arrival direction of seismic wave. Array analysis of the ACROSS signal provides us with the information about the propagation path of the arrival phase which involves the temporal change. On the other hand, analysis of the background noise recorded by the seismic array provides the information about the structure near the receiving site. We present the application of the dispersion analysis of the surface-wave phase velocity and seismic interferometry to retrieve the Greenfs function between two seismometers in the array through numerical experiments. In order to apply this procedure to actual observations, some arrangement is necessary, especially in the data stacking system.
NG51A-1200
Seismic ACROSS Transmitter Installed at Morimachi above the Subducting Philippine Sea Plate for the Test Monitoring of the Seismogenic Zone of Tokai Earthquake not yet to Occur
Here we report the first seismic monitoring system in active and constant operation for the wave propagation characteristics in tectonic region just above the subducting plate driving the coming catastrophic earthquakes. Developmental works of such a system (ACROSS; acronym for Accurately Controlled, Routinely Operated, Signal System) have been started in 1994 at Nagoya University and since 1996 also at TGC (Tono Geoscience Center) of JAEA promoted by Hyogoken Nanbu Earthquakes (1995 Jan.17, Mj=7.3). The ACROSS is a technology system including theory of signal and data processing based on the brand new concept of measurement methodology of Green function between a signal source and observation site. The works done for first generation system are reported at IWAM04 and in JAEA report (Kumazawa et al.,2007). The Meteorological Research Institute of JMA has started a project of test monitoring of Tokai area in 2004 in corporation with Shizuoka University to realize the practical use of the seismic ACROSS for earthquake prediction researches. The first target was set to Tokai Earthquake not yet to take place. The seismic ACROSS transmitter was designed so as to be appropriate for the sensitive monitoring of the deep active fault zone on the basis of the previous technology elements accumulated so far. The ground coupler (antenna) is a large steel-reinforced concrete block (over 20m3) installed in the basement rocks in order to preserve the stability. Eccentric moment of the rotary transmitter is 82 kgm at maximum, 10 times larger than that of the first generation. Carrier frequency of FM signal for practical use can be from 3.5 to 15 Hz, and the signal phase is accurately controlled by a motor with vector inverter synchronized with GPS clock with a precision of 10-4 radian or better. By referring to the existing structure model in this area (Iidaka et al., 2003), the site of the transmitting station was chosen at Morimachi so as to be appropriate for detecting the reflected wave from an anticipated fault plane of Tokai Earthquake, the boundary between Eurasian lithosphere and the subducting Philippine Sea Plate. Further several trials of new transmission protocol and also remote control are being made for the transmitter network of the next generation. The whole system appears working well as reported by Yoshida et al. (2008, This meeting).
NG51A-1201
Receiver function analysis applied to refraction survey data
For the estimation of the thickness of oceanic crust or petrophysical investigation of subsurface material, refraction or reflection seismic exploration is one of the methods frequently practiced. These explorations use four-component (x,y,z component of acceleration and pressure) seismometer, but only compressional wave or vertical component of seismometers tends to be used in the analyses. Hence, it is needed to use shear wave or lateral component of seismograms for more precise investigation to estimate the thickness of oceanic crust. Receiver function is a function at a place that can be used to estimate the depth of velocity interfaces by receiving waves from teleseismic signal including shear wave. Receiver function analysis uses both vertical and horizontal components of seismograms and deconvolves the horizontal with the vertical to estimate the spectral difference of P-S converted waves arriving after the direct P wave. Once the phase information of the receiver function is obtained, then one can estimate the depth of the velocity interface. This analysis has advantage in the estimation of the depth of velocity interface including Mohorovicic discontinuity using two components of seismograms when P-to-S converted waves are generated at the interface. Our study presents results of the preliminary study using synthetic seismograms. First, we use three types of geological models that are composed of a single sediment layer, a crust layer, and a sloped Moho, respectively, for underground sources. The receiver function can estimate the depth and shape of Moho interface precisely for the three models. Second, We applied this method to synthetic refraction survey data generated not by earthquakes but by artificial sources on the ground or sea surface. Compressional seismic waves propagate under the velocity interface and radiate converted shear waves as well as at the other deep underground layer interfaces. However, the receiver function analysis applied to the second model cannot clearly estimate the velocity interface behind S-P converted wave or multi-reflected waves in a sediment layer. One of the causes is that the incidence angles of upcoming waves are too large compared to the underground source model due to the slanted interface. As a result, incident converted shear waves have non-negligible energy contaminating the vertical component of seismometers. Therefore, recorded refraction waves need to be transformed from depth-lateral coordinate into radial-tangential coordinate, and then Ps converted waves can be observed clearly. Finally, we applied the receiver function analysis to a more realistic model. This model has not only similar sloping Mohorovicic discontinuity and surface source locations as second model but the surface water layer. Receivers are aligned on the sea bottom (OBS; Ocean Bottom Seismometer survey case) Due to intricately bounced reflections, simulated seismic section becomes more complex than the other previously-mentioned models. In spite of the complexity in the seismic records, we could pick up the refraction waves from Moho interface, after stacking more than 20 receiver functions independently produced from each shot gather. After these processing, the receiver function analysis is justified as a method to estimate the depths of velocity interfaces and would be the applicable method for refraction wave analysis. The further study will be conducted for more realistic model that contain inhomogeneous sediment model, for example, and finally used in the inversion of the depth of velocity interfaces like Moho.
NG51A-1202
Decomposition of P and S waves in a vector seismic wavefield using dispersion relationship
We acquire total elastic wavefield using multi-component geophone. Since the acquisition of both P and S wave characteristics yields better insights into subsurface rock properties than of only P wave, multi- component data acquisition is of importance in seismic exploration. It is, however, often assumed in multi- component processing that the vertical and the horizontal radial components contain only P and P-SV wave arrivals, respectively. This assumption may actually be violated with increasing offset due to later phase arrivals. The recognition of P and S arrivals would be of importance to process multi-component seismic data, but not so many trials have been conducted except in vertical seismic profiling (VSP).wavefield Some methods for the wavefield separation have been proposed. Devaney and Oristaglio (1986) proposed a two-dimensional method for separating VSP data using dispersion relationship. Al-anboori, et al. (2005) proposed an approximate wavefield separation scheme based on a data rotation in the ั-p domain and applied to two-dimensional reflection seismic wavefield. Tokunaga, et al.(2006) applied the method of Devaney and Oristaglio (1986) to surface reflection seismic data and introduced a method to accommodate three dimentional decomposition of tri-component geophone data acquired in a single line. None of the above method, obviously, could perform the decomposition of three dimentional seismic data. The methodology to decompose a vector seismicwavefield needs to be developed for future seismic exploration. We describe two methods to separate a three-dimensional elastic wavefield recorded at three-component receivers into P, SV, and SH waves. One is a method to separate wavefield into each wave in F-K domain, and the other in ั-p domain. Both of the separating methods are based on a plane-wave decomposition of elastic wavefields using the dispersion relationship. The separation scheme is based on a simple rotation or polarization of data recorded in the Cartesian coordinates to ray coordinates using the incidence angle of arriving waves. Such a rotation is not easily implemented in the x-t domain because each trace contains multiple arrivals with time varying incidence angles. For the sake of applying the above-mentioned polarization, we applied the Fourier and ั-p transforms to synthetic data, respectively. It becomes simpler after the transform into F-K or ั-p domain to determine the incidence angle of the arriving waves. In the F-K domain, observed wavefield is a function of horizontal wave numbers, kx and ky, and the frequency f. Likewise, observed wavefield becomes a function of the horizontal slownesses, px and py, and time t in ั-p domain. We determine the incidence angle of each arriving wave using the dispersion relationship, which define the relation between the wave numbers, frequency, and the phase velocity of the wave in the F-K domain, or apparent slownesses and the phase velocity of the waves in the ั-p domain. These methods require only both P and S phase velocities in the vicinity of geophones. In our study, we apply these two methods to three-dimensional synthetic data created by finite difference method. After these processing, we confirmed that both of the methods could decompose elastic wavefield into each of P, SV, and SH waves. Our results demonstrates that these two methods yield significantly higher S/N ratio and help us to analyze the S wave characteristics. We think that both of these methods would be of fundamental and powerful processing tools for multi-component seismic exploration.
NG51A-1203
Monitoring Subsurface Objects Using Resonant Seismic Emission
The numerical modeling results and field data indicate that some contrast subsurface objects (such as tunnels, caves, pipes, filled pits, and fluid-filled fractures) are capable to trap seismic energy and generate durable resonant oscillations. These oscillations are comprised of surface types of circumferential waves which multiply rotate around the object. Resonant emission of such trapped energy occurs primarily in form of shear body waves that can be detected by remotely placed receivers. Resonant emission reveals itself in form of sharp resonant peaks for the late parts of the records, when all strong direct and primary reflected waves are gone. These peaks are observed in the field data for a buried barrel filled with water, in 2D finite- difference modeling results and in exact canonical solution for a fluid-filled sphere. Computed movie for diffraction of a plane wave upon low-velocity elastic sphere confirms generation of resonances by durable surface waves. We show that resonant emission has characteristic quasi-hyperbolic travel-time patterns on shot-gathers. Inversion of these patterns can be performed in frequency domain after muting strong direct and primary scattered waves. Subsurface objects can be detected and imaged at a single resonance frequency without an accurate knowledge about source trigger time. Imaging of subsurface objects requires information about shear velocity distribution in an embedding medium, which can be done interactively during inversion. Resonant emission data processing is done using KinetiK Professional visualization and processing software.
NG51A-1204
Geysers Valley, Kamchatka: Why Landslide of 3-June-2007 Took Place and What Happens After
Analysis of the hydrogeological conditions of the landslide, which took place in the Geysers Valley, Kamchatka on June 3-rd, 2007 shows that possible reason of this was a long term steam upflow occurred along slightly inclined bottom of the Geyzernaya pumice tuffs unit, which finally resulted into deep hydrothermal alteration of the pumice to highly silicified zeolites and montmorillonite, with corresponding loosing of the stability. Landslide triggers may associate with different events including plumbing magma system pressure increase, seasons waterfloods or steam explosions. Landslide took place in a few minutes yielded 10 mln m3 of mud, debris, and blocks of rocks. As a result of this - some geysers located at lower elevations were sealed under 10-30 m thick caprock and a rock dumb trap Geysernaya river into 20 m deep Lake. One year monitoring of the two key geysers eruptions cycling (Velikan and Bolshoy), lake level and thermal discharge into the Lake floor yield to the following results: 1. Velikan maintains stable geysers activity cycling with an average time period 372 min (31 July 2007 6 July 2008), slightly exceeding time period before landslide - 339 min (17 Aug - 5 Oct 2003). Time period increase after landslide off-load and some response to the barometric pressure maximums - may characterize geyser sensitivity to the stress; 2. Bolshoy maintained geysers activity with average time period 64-84 min during Sept 2007 May 2008 at low Lake levels, the rest of the time cold water from Lake inflows into the geysers channel, terminating its activity. Note, Bolshoy time period before landslide was estimated as 108 min (17 Aug - 5 Oct 2003); 3. Thermal discharge into the Lake floor is sensitive to the Lake level, significantly increasing in the winter time.
NG51A-1205
Vibroseismic Research in Siberia
High-power vibration sources were developed in Siberian Branch of the Russian Academy of Sciences during 30 years for use in active seismology and studies of Earth's deep structure. Most of data is obtained using eccentric 40- and 100-ton vibrators. Current research involving these sources covers Baikal rift zone, Altay- Sayan folded area and Okhotsk-Chukotski regions in Russia. Using one week to 15-20 days recording intervals, the vibroseismic observations were repeated for several years in the south of Baikal Lake and in Novosibirsk region in the framework of active monitoring technology. The total area of vibroseismic monitoring in the south of Lake Baikal exceeds 20000 km2. Such spatial scale allows to control stress changes in the zones of large faults (Obruchevski, Primorski, Bolsherechenski), which repeatedly activate during rift genesis. Near Novosibirsk, the system of vibroseismic monitoring is focused on changes of the Earth crust physical characteristics caused by seasonal changes of water levels in Novosibirsk reservoir. It is oriented transversely and along the spread of reservoir at 50-70 km distances from the source location. Variations of elastic waves correlate with process of earthquake preparation in 2002 and were detected in the south of Lake Baikal. Results of long-term experiment near Novosibirsk suggest that the observed 0.3- 0.6% variations in velocities of longitudinal and shear waves are connected with seasonal changes of water levels (up to 5m) in the biggest reservoir. Overall results suggest possibility of vibroseismic monitoring with small number of high-power vibrators and a large number of recording stations.
NG51A-1206
Variations of Seismic Waves Characteristics in a Friable Medium Under a Vibrational Loading.
Instant variations of the velocities and attenuation of seismic waves in a friable medium subjected to dynamic loading have been studied by new experimental techniques using a powerful seismic vibrator. The half-space below the operating vibrator baseplate was scanned by high-frequency elastic waves, and the recorded fluctuations were exposed to a stroboscopic analysis. It was found that the variations of seismic velocities and attenuation are synchronous with the external vibrational load but have phase shift from it. Instant variations of the seismic waves parameters depend on the magnitude and absolute value of deformation, which generally result in decreasing of the elastic-wave velocities. New experimental techniques have a high sensitivity to the dynamic disturbance in the medium and allow one to detect a weak seismic boundaries. The relaxation process after dynamic vibrational loading were investigated and the results of research are presented.
NG51A-1207
Vibroseismic Monitoring of the Baikal Lake Seismic Active Zone and Taman' Mud Volcano Province
A paper presents the results of the vibroseismic monitoring research in seismic active zone of the Baikal Lake and Taman' mud volcano province. Monitoring of the seismic active central part of Baikal rift zone is carried out since 2004 with the use of the vibrator CV-100 located on geophysical observatory "Souhoi Rouchei" near the lake Baikal. It is used vibroseismic interferometry method based on the seismic sounding of the region by powerful seismic vibrators with a long time radiation of narrow-band harmonic signals. The changes in the stressed-deformed state are determined through the variations of the amplitude-phase characteristics of stationary harmonic wave fields, which are excited in the Earth's crust due to a long-time radiation of harmonic signals of constant frequency from the vibrator. In Taman' mud volcano province the vibroseismic monitoring field works were carried out with the use of vibrator CV 10/180 and mobile recording systems on the volcanoes Shugo and Karabetova gora. The methods of vibroseismic tomography and seismic profiling are used for the volcanoes structure investigations and monitoring of geodynamic processes. The results of the field work are presented.