NG44B-01 INVITED
Change of seismic process under irradiation of the crust by electromagnetic discharge and the problem of seismic hazard mitigation.
The effect of high-energy electromagnetic pulses emitted by a magnetohydrodynamic generator used as a source for deep electrical sounding of the crust on spatial-temporal structure of seismicity is explored. It has been shown that 2-6 days after effect of electromagnetic pulses there is statistically significant activation of relatively small earthquakes in the regions of study. The total seismic energy of initiated earthquakes is five- six orders of magnitude higher than the energy transmitted by the generator to the radiating dipole. The spatial correlation of earthquake density for each two contiguous years as a function of time was studied. Five-six years periodicity of changes in spatial distribution of seismicity was detected. It was shown that the effect of electromagnetic pulses increases the stability of the spatial distribution of earthquakes over time and simultaneously speeds up cycles of its transformation, which develop on stabilization background. The time change of the fractal dimension of the spatial epicenter's distribution was investigated too. It was found that the fractal dimensions fluctuates about a level 1.48 for the time period before the generator experiments. The beginning of the regular irradiation of the are under study by electromagnetic pulses is followed by gradual falloff in values of fractal dimension inside near field area of the radiating dipole. The decrease of the former is observed during all period of conducting of the electrical sounding. Therefore, the effect of electromagnetic pulses causes the increase of the spatial clustering of the earthquakes. Then fractal dimension gradually increases to a background level. Note that the time interval when the values of the fractal dimension were less that the background level, coincides with the time period in which increasing the stability of the spatial distribution of seismicity over time was observed. The results of the study show that action of high energy electromagnetic discharges radiated by the generators causes substantial changes of spatial temporal property of the seismic process of earthquake source zones and accelerates the release of energy stored in the crust due to the activity of natural tectonic processes, thus serving as a kind of trigger. Based on the energy balance assumption for the crust, we conclude that a man-made increase in part of the seismic energy radiated in the form of flow of relatively small earthquakes leads to an additional release of tectonic stresses, thereby diminishing the likelihood of catastrophic events (or at any rate, reduces the energy of such events). If this assumption will be confirmed by further investigations, the detected effect could be used to develop a technique of decrease of seismic hazard by artificial discharge of tectonic stress by actions of electromagnetic pulses on the crust in the earthquake source zones.
NG44B-02 INVITED
Oil Sands Characteristics and Time-Lapse and P-SV Seismic Steam Monitoring, Athabasca, Canada
A vast amount of oil sands exists in the Athabasca area, Alberta, Canada. These oil sands consist of bitumen (extra-heavy oil) and unconsolidated sand distributed from surface to a depth of 750 meters. Including conventional crude oil, the total number of proved remaining oil reserves in Canada ranks second place in the world after Saudi Arabia. For the production of bitumen from the reservoir 200 to 500 meters in depth, the Steam Assisted Gravity Drainage (SAGD) method (Steam Injection EOR) has been adopted as bitumen is not movable at original temperatures. It is essential to understand the detailed reservoir distribution and steam chamber development extent for optimizing the field development. Oil sands reservoir characterization is conducted using 3D seismic data acquired in February 2002. Conducting acoustic impedance inversion to improve resolution and subsequent multi-attribute analysis integrating seismic data with well data facilitates an understanding of the detailed reservoir distribution. These analyses enable the basement shale to be imaged, and enables identification to a certain degree of thin shale within the reservoir. Top and bottom depths of the reservoir are estimated in the range of 2.0 meters near the existing wells even in such a complex channel sands environment characterized by abrupt lateral sedimentary facies changes. In March 2006, monitoring 3D seismic data was acquired to delineate steam-affected areas. The 2002 baseline data is used as a reference data and the 2006 monitoring data is calibrated to the 2002 seismic data. Apparent differences in the two 3D seismic data sets with the exception of production related response changes are removed during the calibration process. P-wave and S-wave velocities of oil sands core samples are also measured with various pressures and temperatures, and the laboratory measurement results are then combined to construct a rock physics model used to predict velocity changes induced by steam-injection. The differences of the seismic responses between the time-lapse seismic volumes can be quantitatively explained by P-wave velocity decrease of the oil sands layers due to steam-injection. In addition, the data suggests that a larger area would be influenced by pressure than temperature. We calculate several seismic attributes such as RMS values of amplitude difference, maximum cross correlations, and interval velocity differences. These attributes are integrated by using self-organization maps (SOM) and K-means methods. By this analysis, we are able to distinguish areas of steam chamber growth from transitional and non-affected areas. In addition, 3D P-SV converted-wave processing and analysis are applied on the second 3D data set (recorded with three-component digital sensor). Low Vp/Vs values in the P-SV volume show areas of steam chamber development, and high Vp/Vs values indicate transitional zones. Our analysis of both time-lapse 3D seismic and 3D P-SV data along with the rock physics model can be used to monitor qualitatively and quantitatively the rock property changes of the inter-well reservoir sands in the field.
NG44B-03 INVITED
Recent Progress in EM-ACROSS for Monitoring of Deep Crustal Activities in Tokai Region
We have been developing an active monitoring system named the electromagnetic (EM-) ACROSS (Accurately Controlled Routinely Operated Signal System) in order to detect an actively changing properties of deep crustal rocks in Tokai region. The EM-ACROSS has been applied so far only to the change of geological environments down to about 1 km depth with the electric dipoles of 150m-4A and 180m-10A at Tono and Horonobe test sites of JAEA (Japan Atomic Energy Agency), respectively. The essential factor for the observation system is to enable us to acquire the high S/N transfer function data between the source and receiving site of electromagnetic diffusion waves, which are reflected back from the depth of 20 to 30km near the subducting plate boundary beneath this region within a reasonably short period of time. We have studied the required conditions in detail: (a) the signal transmission by the larger electric dipole of the order of 106 Am in the frequency range 0.1-20Hz, (b) technology system that suppresses noise at every aspect of data analyses and further (c) the use of network of the receiving sites spanning 20-30 km to get out of the complexity originated from the lateral heterogeneity of the conductivity in the crust. Then we expect to reach S/N of about 5 by the data stacking of one week. In order to reach the target noted above, we have installed the electric dipole with larger moment (560m-20A) in Shizuoka University campus and continuous transmission experiments has been made for two years with observation sites located about 20km in distance from the source. Recent results of the observation and the technical problems will be discussed for the next development.
NG44B-04
Large-scale electromagnetic modeling for multiple inhomogeneous domains
We develop a new formulation of the integral equation (IE) method for three-dimensional (3D) electromagnetic (EM) field computation in large-scale models with multiple inhomogeneous domains. This problem arises in many practical applications including modeling the EM fields within the complex geoelectrical structures in geophysical exploration. In geophysical applications, it is difficult to describe an earth structure using a horizontally layered background conductivity model, which is required for the efficient implementation of the conventional IE approach. As a result, a large domain of interest with anomalous conductivity distribution needs to be discretized, which complicates the computations. The new method allows us to consider multiple inhomogeneous domains, where the conductivity distribution is different from that of the background, and to use independent discretizations for different domains. This reduces dramatically the computational resources required for large-scale modeling. In addition, by using this method, we can analyze the response of each domain separately without an inappropriate use of the superposition principle for the EM field calculations. The method was carefully tested for modeling the marine controlled-source electromagnetic (MCSEM) fields for complex geoelectrical structures with multiple inhomogeneous domains, such as a seafloor with rough bathymetry, salt domes, and reservoirs. We have also used this technique to investigate the return induction effects from regional geoelectrical structures, e.g., seafloor bathymetry and salt domes, which can distort the EM response from the geophysical exploration target.
NG44B-05
Active Seismic Monitoring of the San Andreas Fault at Parkfield
A unique data set of seismograms for 720 source-receiver paths has been collected as part of a controlled source Vibroseis experiment San Andreas Fault (SAF) at Parkfield. In the experiment, seismic waves repeatedly illuminated the epicentral region of the expected M6 event at Parkfield from June 1987 until November 1996. For this effort, a large shear-wave vibrator was interfaced with the 3-component (3-C) borehole High-Resolution Seismic Network (HRSN), providing precisely timed collection of data for detailed studies of changes in wave propagation associated with stress and strain accumulation in the fault zone (FZ). Data collected by the borehole network were examined for evidence of changes associated with the nucleation process of the anticipated M6 earthquake at Parkfield These investigations reported significant traveltime changes in the S coda for paths crossing the fault zone southeast of the epicenter and above the rupture zone of the 1966 M6 earthquake. Analysis and modeling of these data and comparison with observed changes in creep, water level, microseismicity, slip-at-depth and propagation from characteristic repeating microearthquakes showed temporal variations in a variety of wave propagation attributes that were synchronous with changes in deformation and local seismicity patterns. The main lesson learned from Vibroseis experiment is that changes were clearly observable in the locked part of SAF, which has relatively little natural seismicity could otherwise be used for monitoring of travel-time and attenuation changes. The creeping part of the SAF northwest of Parkfield is not expected to accumulate stress and it is also heavily instrumented. Monitoring of this region revealed no significant changes in seismic signatures. Remarkably in 2004, the expected M6 earthquake at Parkfield occurred and nucleated well into the locked SAF section, well to the southeast of the Vibroseis/HRSN monitoring experiment which was primarily centered on Middle Mountain. This result suggests that active seismic monitoring can be a useful tool for detecting stress changes associated with the nucleation of larger earthquakes even when observations are made over the events nucleation zones with low natural seismicity. Numerical modeling studies and a growing number of observations have argued for the propagation of fault-zone guided waves (FZGW) within a SAF zone that is 100 to 200 m wide at seismogenic depths and with 20 to 40% lower shear-wave velocity than the adjacent unfaulted rock. FZGW are also capable of assessing the degree of fault continuity and other properties of complex FZ geometries such as fault jogs. The SAF in the Cholame valley where 2004 M6 earthquake nucleated, is characterized by such complexity and because FZGW also primarily propagate within the core of fault zones, active continuous seismic monitoring using guided waves is our proposed solution for earthquake studies in Parkfield area.