Seismic Monitoring of Geodynamic Activity in the Baksan Canyon

Verkholantseva t.V. 1, Verkholantsev f.G. 2

1 Mining Institute of the Ural Branch of the Russian Academy of Sciences, Perm, Russia; 2 Geophysical survey of the Russian Academy of Sciences, Obninsk, Russia

tata_ver.89@mail.ru

The Baksan Neutrino Observatory, Institute for Nuclear Test of the Russian academy of sciences is the physical observatory for neutrino researches. The observatory is located in the Baksan canyon, Caucasus. Undersurface construction are located in two galleries which lengths are of 3670 and 4045 m.

The area of Baksan Neutrino Observatory is characterized by rather high intensity of natural geodynamic processes. One of manifestation of them is amount of small seismic events that were formerly revealed in July 2006 during short- period observation. In June 2011 the stationary seismic network was installed in the mine. It includes 7 stations (one triaxial and six vertical) that aims to monitor both local and regional seismicity of Elbrus volcanic area.

Observations show good conditions for microseismic monitoring. It was found very low level of ambient noises close to NLNM (Peterson, 1993) in frequency range up to 5 Hz. Some experiments in the mine were carried out to determine velocity parameters of rocks. Experiments allowed to define the velocity of body waves: Vp = 6400 m/s, Vs=3700 m/s.

A lot of seismic events recorded during the first 6 months confirm a high tectonic activity of the region. All events may be divided into 2 groups. The first one includes regional events at the distance of tens and hundreds km. The second one collects all local events hypocentral distance of which doesn't exceed 7 km and local magnitude goes down to -3. Localization of the last events (about 80% of total amount) allows to distinct areas of different geodynamic activity.

According to analysis of epicentral distances and azimuths of wave propagation two areas were revealed where most seismic sources are located. The first of area is as for as 2 km to the north-west from monitoring system (azimuth 280 ° - 310°) and includes 4% of all events. The second one is situated in 3-4 km to north-east and east from the observation points (azimuth 65 ° - 90 °) and is presented with 96% of events.

The microseismic data inversion for events location and anisotropic parameters estimation

Yaskevich S.V.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk State University, Novosibirsk, Russia

Yaskevichsv@gmail.com

Microseismic monitoring is widely used in modern industry to image hydraulic fracturing operations. It is assumed that microseismic events occur due to fracture development and accurate location of their hypocenters is an important task of microseismic data processing. The geometry of event locations yields, probably, the most reliable information about created fractures. One of the most popular microseimic monitoring systems is a single array of three-component receivers placed in adjacent vertical well. The main advantage of this system is that receivers are located close to events which results in the improved signal-to-noise ratio. The main disadvantage is a lack of azimuthal aperture. Thus accuracy of event location in azimuthal direction depends on the quality of polarization analysis that is usually based on P-wave hodogram procesing. Conventional approach to microseismic data processing starts from isotropic velocity model building based on data from perforation shots. This model is later used for microseismic event location [1]. Such an approach may lead to serious biases in locations when the real medium is anisotropic and even produce unphysical isotropic velocities [2]. Importance of working with anisotropic velocity models is now widely acknowledged in microseimic community.

We will demonstrate how the use of isotropic media parametrization can affect quality of the fracture imaging for horizontally transversely isotropic (HTI) media. WE perform simultaneous event location and anisotropic parameters estimation from a joint kinematic inversion of perfshot and microseismic event data.We want to show that this type of inversion may lead (given good receiver array aperture) to a better imaging of the fracture and reliable anisotropic parameters estimation. Anisotropic HTI parameters also provide important information as they are usually related to rock physics, lithology, internal fractures etc. Kinematic inversion is performed by iterative minimization of a standard least-squares traveltime misfit We use ray tracing to compute traveltime data assuming that we can pick only direct P- and SV- waves for all events. We take true azimuthal directions defined by straight rays in this case. Then we added Gaussian noise with standard deviation of 1 ms to traveltimes and 3 degrees to azimuths. We made 50 independent inversions with different realizations of random noise. When performing the inversion, we impose constraints: event locations to be within 50 m of their true positions, velocities VP0 and VS0 to be within 20% of their true values. As initial guesses we use isotropic models with velocities chosen randomly within outlined bounds. For real data one can choose “true” velocities from acoustic data and remove constraints on the event location. In our work we use the following strategy of kinematic microseismic data inversion: simultaneous event location and anisotropic parameters estimation from a joint set of perforation shot and microseismic event data. Iterative optimization of a least-squares traveltime misfit is used for inversion.

We tested this inversion strategy on several synthetic models with a vertical receiver array in HTI medium. Data consisted of arrival times for direct P-, SV- waves from one perfshot and a set of microseismic events aligned in horizontal direction.

Research was partially funded by the Russian Ministry of Education and Science.

References:

1. Eisner, L., P. Duncan, W. M. Heigl, and W. R. Keller, 2009, Uncertainties in passive seismic monitoring: The Leading Edge, 28 (6), 648–655..

2. Grechka V., and A.A. Duchkov, 2011, Narrow-angle representations of the phase and group velocities and their applications in anisotropic velocity-model building for microseismic monitoring: Geophysics, 76 (6), WC125–WC140.