NG44A-01 INVITED
Recent Progress and Future Direction of Active Monitoring with ACROSS
The first ACROSS system for practical use was deployed in Awaji and Tono test site in 1996 by Nagoya University, and JAEA, respectively. Since then the performance of the system has been tested especially for detecting the temporal variation of propagation property of seismic wave. For the first few years an appropriate way of operation for rotational vibrators were looked for. Frequency modulation together with GPS synchronization is found to be fundamentally important technique for the operation of ACROSS vibrators. The first long-term operation test was made at Awaji from January 2000 to March 2001, in which temporal variation of P and S wave are monitored between the source and the receivers that is 1 and 2 km away. In this experiment some changes due to strong ground motion of distant earthquakes were detected. By analyzing the temporal change in S-wave splitting Ikuta and Yamaoka (2004) concluded that the change was due to sudden movement of groundwater in the shallow part of the ground. Another long-term operation test has been continuing at Tono for more than five years. During this operation observation with seismic array was conducted to test the feasibility to detect reflective waves from the boundary of subducting plate in this region. In this experiment Kasahara et al. (2006) identified reflection wave from the boundaries in the deep part of the crust. A new field test is now conducting by JMA, ERI, Shizuoka University and Nagoya University. The target of the test is to detect a temporal variation of reflection wave from the plate boundary. They deployed ACROSS vibrators in two sites and monitor their signal with nation-wide seismic network Hi-net. The principle of ACROSS vibrator, in which signal is generated with a precise synchronization to GPS, is very useful in many sited where we need to know the temporal variation. Based on this concept vibrators with better portability should be developed.
NG44A-02 INVITED
Magneto Rotation Instability and the Problem of Magneto Dynamo
A new approach to the geophysical Earth's problem is considered, where the main role of geophysical time variation of magnetic force of Earth is due to magneto rotation instability rather than the theory of magneto dynamo. Besides the attention is drawn to magneto rotation instability (MRI) as a source of evolution of Earth's magnetic force. First studied theoretically by E.P. Velikhov in ideal condtions, then experimentally and numerically, MRI is never observed at real laboratory conditions. Most part of experiments is dedicated to the instability Raileigh-Taylor that is very close to MRI: viz., non compressible liquid metal is between rotating coaxial cylinders; permanent magnetic field along the axis is produced by currents beside cylinders. A great challenge is that motion of liquid metal is very far from ideal for MRI proof. So Reinolds Number appears to be large, meantime magnetic Reinolds Number is of order of unity or even less. A ratio = , since viscosity, . At study MRI which often called Standard MRI (SMRI) one more condition is to be satisfied: time of motion of perturbation between cylinders is to be less then the time of magnetic force diffusion. Hence one has two inequalities: - Lundquist Number, -Alfven velocity. It follows that and magnetic forces are to be several KGs. Besides, a difficulty appears due to height of cylinders and boundary conditions on the butts of cylinders. It brings about additional motion, for instance due to Eckman force in usual hydrodynamics and counter currents in unit on checking of SMRN. The way out is either search of modification SMRN (like Goodman), or checking of thresholds MRN (like Taylor) with very large aspect relation, to forget butts cylinder conditions. To escape the difficulty tackled with Eckman force Goodman at Princeton University create rotating butts of cylinders to get closer to configuration Taylor but with larger aspect ratio. For Taylor aspect ratio was of the order of 100, for Goodman and cooperators of the order of one ( rotating butts took place). It was shown that unit at Princeton demonstrates the absence of transfer of substance across magnetic force and is a proof that Reileigh-Taylor instability in any configuration does not transfer a substance across magnetic force. At RRC"Kurchatov Institute" is published results when rotation of liquid sodium is created in chamber with motionless walls due to interaction radial current with axis magnetic force. Unit has following parameters, magnetic force ~250 Gs,the butts appeared to be insulator, radial current . An advantage is that rotating force is volume. The velocities near were achieved, however large radial current brings about toroidal magnetic force, that interacting with radial current creates flow of substance like Eckman for in hydrodynamics. However in this case it is simpler since radial potential one can distribute introducing electrodes as rings on the butts of cylinder.
NG44A-03 INVITED
Active Monitoring Using Powerful Vibrators in Seismic Regions of Siberia
Current methods of mid-term and short-term earthquake forecasts can not guarantee acceptable level of reliability for public announcement. Physical analysis of available precursors has serious problems when precursors contradict each other. However, Nature can't be contradictory – it is our ideas on deformation process in the Earth are contradictory because of lack of data and analysis. In this connection, we see growing interest in measurement and analysis of appearances of reflected waves from focal zones of imminent earthquakes. Methods of active monitoring can provide necessary data and analysis. Authors see the main source of vital information in quantitative and qualitative physical characteristics (of structure and properties of focal zones), which are measured in a process of active geophysical monitoring. We suggest that this approach enables forecasts of earthquakes with acceptable reliability. We give a review of seismicity and seismological observations in the Siberia territory, as well as of geophysical monitoring results obtained for the south of Baikal Lake, in the Altay-Sayan area in the Western Siberia. Special attention is given to the methods of active monitoring with powerful vibrators which have been actively developed in Siberian Branch of the Russian Academy of Sciences. There are data recorded for "near" and "far" fields from powerful 60-100 tons vibration sources which are obtained for the active seismic zones and for the zone of a large reservoir. Usage of powerful vibrators enables not just passive observations of stress built up, but management of average stress changes without affecting the background seismicity.
NG44A-04 INVITED
Continuous Crosswell Seismic Measurement: Applications to Monitoring of Stress and CO2
The crosswell acquisition geometry offers direct access to subsurface materials. As such, crosswell data are ideal for monitoring subsurface processes. We have been investigating the use of semi-permanent crosswell seismic acquisition for continuous monitoring, making use of technologies usually deployed for tomographic imaging. Our two applications have been the monitoring of stress induced velocity changes and velocity changes due to CO2 injection. The use of stress induced seismic velocity changes to understand dynamic processes requires knowledge of the in-situ stress sensitivity of a given rock volume. We have made measurement of stress sensitivity using continuous crosswell travel time measurement at 3 sites for up to 2 months, and have found that continuous monitoring using fixed borehole instrument locations (unmoved, but not permanently installed) can achieve very high repeatability. Signal-to-noise ratio is the key parameter to obtaining travel time precision of better than a microsecond. With this precision, we are able to correlate changes in seismic velocity with changes in barometric pressure. Barometric pressure is used as a calibration signal to determine the in-situ stress sensitivity of the rock volume monitored by the crosswell measurement. Initial tests in shallow ground water wells (3 and 30 m depths) were successful in obtaining a stress sensitivity calibration. These tests were preliminary to deployment in the San Andreas Fault Observatory at Depth (SAFOD) boreholes in California. At the SAFOD site a piezoelectric source and a clamped 3-component accelerometer were used at a depth of 1 km in two separate 1 month long experiments. Initial SAFOD results include measurement of stress sensitivity and coseismic velocity changes. The goal of this work is to develop a methodology for monitoring stress increases preceding earthquakes. In separate work, continuous crosswell monitoring with multiple sensors was used to determine the spatiotemporal dimensions of a plume of CO2 injected into a brine aquifer to study carbon sequestration at the Frio site in southeast Texas. In this case the high noise level of an active injection limited the travel time accuracy, but the induced velocity changes due to fluid displacement were easily detected. The development of the plume was monitored for over a week at 15 minute intervals.
NG44A-05
Active EM Monitoring of Sea-Bottom Geoelectrical Structures in the Areas with the Rough Bathymetry
During the recent years, the marine Controlled Source Electromagnetic (MCSEM) method has become widely used for active geophysical surveying of the sea-bottom geological structures in hydrocarbon exploration. A typical MCSEM survey consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The interpretation of the MCSEM data over the complex 3-D geoelectrical structures is a very challenging problem. This problem becomes even more complicated in the areas with the rough sea-bottom bathymetry, because the relief of a sea bottom makes a profound effect on the EM data observed by the receivers located in the close proximity to the bottom. In this paper I introduce a new approach to interpretation of the MCSEM data in the areas with the rough bathymetry. This approach is based on a new formulation of the integral equation (IE) EM modeling method in the models with inhomogeneous background conductivity. The method is based on the separation of the effects due to excess electric current induced in the inhomogeneous background domain, and those due to the anomalous electric current in the location of the anomalous conductivity, respectively. As a result, we arrive at a system of integral equations which uses the same simple Green's functions for the layered model, as in the original IE formulation. However, the new equations take into account the effect of the variable background conductivity distribution. The developed technique allows us to incorporate the known geological structures and bathymetry effects in the method of iterative EM migration/holographic imaging and inversion. This approach provides us with the ability to pre-compute only once the effect of the known geoelectrical structure (e.g., the bathymetry effect) and keep it unchanged during the entire modeling and migration process. Taking into account that precomputing the bathymetry effect constitutes the most time-consuming part of the EM modeling, this approach allows us to increase the effectiveness of the interpretation of the MCSEM data significantly. The method is tested on the typical models of the sea-bottom geoelectrical structures in the areas with hydrocarbon petroleum reservoir and a rough sea bottom bathymetry.
NG44A-06
Seismic Resonant Emission
There are several classes of underground objects which can produce resonant emission after being hit by incident seismic waves. Those objects include tunnels, pipes, buried containers, ground-filled excavations, unexploded ordinances, fluid-filled fractures, mine shafts, and the like. Being high contrast scatterers, these objects are capable of generating strong scattered waves where primary PP, PS, SS waves carry away most of the energy which was brought by incident waves. For both high- and low- velocity objects the primary scattered waves have the same order of magnitude as incident waves. The main difference between these groups of objects is in later arrivals of multiple scattered waves. While high-velocity objects effectively radiate most of the energy soon after impact, the low-velocity objects trap some fraction of incident wave energy in the form of circumferential waves which propagate rotating along the interface between the object and the embedding medium. Circumferential waves include surface Rayleigh-type waves (propagating mostly in the embedding medium), Stoneley waves (propagating mostly in the fluid, if present), and Frantz waves (body waves trapped in the object because of its curvature). Strong impedance contrast ensures small radiation loss for circumferential waves and they slowly decay in amplitude while rotating inside/around the object. Some circumferential waves exist in the high-velocity objects but their amplitudes decay very fast because of strong radiation in outer medium. Most of the secondary (multiply reflected from an object's boundaries or multiply circled around the object) resonant-scattered energy radiates in the embedding medium as shear waves. The possibility of neglecting P- waves in late scattering arrivals simplifies imaging as is demonstrated for the field and modeled data of the example. Resonant emission phenomenon provides an effective tool for active monitoring for a number of applications such as tunnel detection, hydrofrac development, mining operations etc.