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A map of the maximum azimuthal gap (AG) within a ten station network. 2 Concentric circles around the site show the survey area 3 Fig. 4. A map of the maximum azimuthal gap (AG) within a ten station network. Concentric circles around the site show the survey area.
Source publication
This study simulates automatic event detection and location performance
of a micro-earthquake network centred around a future power plant site
in Finland, Fennoscandian Shield. Simulation of the event location
capability is based on a relationship derived between event magnitude
and maximum detection distance. Azimuthal coverage and threshold
magni...
Citations
... Kuusamo (Riekki) (KU6) stations located 56 km and 251 km from the epicenter, respectively. The event was also observed by all OBF stations (Valtonen et al., 2013) that were all located <100 km away from the epicenter with an azimuth range of 187°-284°(south to west-northwest). The azimuthal gap of the event was only 49°, and reliable observations were available from as many as 42 stations, the farthest ones being in Åland (AAL) and Kevo (KEV), at 584 and 560 km distance. ...
We present an overview of the seismic networks, products, and services in Finland, northern Europe, and the challenges and opportunities associated with the unique combination of prevailing crystalline bedrock, low natural intraplate seismic background activity, and a high level of anthropogenic seismicity. We introduce national and local seismic networks, explain the databases, analysis tools, and data management concepts, outline the Finnish macroseismic service, and showcase data from the 2017 M 3.3 Liminka earthquake in Ostrobothnia, Finland.
... The usual procedure in geothermal exploration or volcano monitoring is to deploy a number of seismometers (6 or more) covering a prospect area or volcano, and record continuously over a period of time at sampling rates typically higher than 50 Hz. Often the seismic network is designed following a heuristic approach with some basic guidelines: the expected microseismic events should be located within the array forming an azimuthal gap of less than ∼180 • (Valtonen et al., 2013), and the average inter-station spacing should be of the average hypocentral depths. ...
We constructed a network optimization scheme based on well established survey design tools to design and qualify local and regional microseismic monitoring arrays dedicated for geothermal exploration and volcano monitoring. The optimization routine is based on the traditional minimization of the volume error ellipsoid of the linearized earthquake location problem (D-criterion) with the twist of a sequential design procedure. Seismic stations are removed one by one to obtain networks for constraining the locations of multiple hypothetic earthquakes with varying local magnitudes. The sequential approach is simple and allows the analysis of benefit/cost relations. Cost curves are computed for all hypothetic events to reveal the minimum optimal number of stations given specific design experiment objectives.
The scheme is first demonstrated on three test design experiments. Later, we use the routine to augment an existing seismic network for monitoring microseismicity in a geothermal field in NE Iceland (Theistareykir). The resulting 23 station network would become the backbone of a reservoir behavior and exploitation activity study. Hypothetic event locations and magnitude relations are taken from a previous regional seismicity study and coincide with geothermal injection and production areas. Sensitivities are calculated with a known 1D velocity model profile using a finite-difference back-ray tracer, and body wave amplitudes are computed from known local magnitude relations. Finally, expected earthquake location accuracies are calculated via multiple Monte Carlo experiments.
The design routine is later used to qualify an existing seismic network located in SW Iceland (Reykjanes). The seismic array is reduced to strategic positions, and benefit and expected accuracies are quantified to observe whether costs could have been optimized had a previous network design experiment been performed.
Overall, we explore quick and flexible tools for designing and qualifying networks for many applications at various scales.
... The event solutions are published on the Institute's web page within 15 minutes of occurrence and, for significant events, automatic alert message is distributed to the seismologists. As is seen in Figure 5.16, the automatic data processing system provides event detection and location capabilities down to magnitude 1.0 within the network (threshold magnitude calculation method by Valtonen et al., 2013). ...
Jari Kortström, Marja Uski and Kati Oinonen report on the Finnish National Seismic Network for the Summary of the Bulletin of the International Seismological Centre.
... Although a qualitative visual comparison of epicentral latitudes and longitudes of Finnish and Swedish earthquakes proved that the duplicate problem cannot produce a strong bias to results, we used the event time, latitude, longitude and magnitude data to filter out all obvious duplicates. Altogether 66 events were removed as being duplicates, and 15 events for other reasons as explained in Appendix B. Both in Finland and Sweden, catalogued earthquake magnitudes are local magnitudes (M L ) which closely correspond to the original Richter magnitude but have been determined using a modified function better suited for modern seismometers (Valtonen et al. 2013). ...
Being far from plate boundaries but covered with seismograph networks, the Fennoscandian Shield features an ideal test laboratory for studies of intraplate seismicity. For this purpose, this study applies 4190 earthquake events from years 2000–2015 with magnitudes ranging from 0.10 to 5.22 in Finnish and Swedish national catalogues. In addition, 223 heat flow determinations from both countries and their immediate vicinity were used to analyze the potential correlation of earthquake focal depths and the spatially interpolated heat flow field. Separate subset analyses were performed for five areas of notable seismic activity: the southern Gulf of Bothnia coast of Sweden (area 1), the northern Gulf of Bothnia coast of Sweden (area 2), the Swedish Norrbotten and western Finnish Lapland (area 3), the Kuusamo region of Finland (area 4) and the southernmost Sweden (area 5). In total, our subsets incorporated 3619 earthquake events. No obvious relation of heat flow and focal depth exists, implying that variations of heat flow are primarily caused by shallow lying heat producing units instead of deeper sources. This allows for construction of generic geotherms for the range of representative palaeoclimatically corrected (steady-state) surface heat flow values (40–60 mWm−2). The one-dimensional geotherms constructed for a three-layer crust and lithospheric upper mantle are based on mantle heat flow constrained with the aid of mantle xenolith thermobarometry (9–15 mWm−2), upper crustal heat production values (3.3–1.1 μWm−3), and the brittle-ductile transition temperature (350 °C) assigned to the cutoff depth of seismicity (28 ± 4 km). For the middle and lower crust heat production values of 0.6 and 0.2 μWm−3 were assigned, respectively. The models suggest a Moho temperature range of 460 to 500 °C.
... The results of the study (Tiira et al. 2011; Valtonen et al. 2013) suggest that a minimum of ten seismic stations is required (Figure 1), if the network is to have the event location threshold of approximately ML 0.0 and the AG smaller than 180 @BULLET within 25 km distance from the study site. ACKNOWLEDGEMENTS This study is a part of a local seismic network project funded by Fennovoima Oy. ...
This study simulates automatic event detection and location performance of a micro-earthquake network
centered around a site selected for a future power plant in Finland, Fennoscandian Shield. Simulation of
the event location capability is based on a relationship derived between event magnitude and maximum
detection distance. Azimuthal coverage and threshold magnitude are computed for different station configurations and the results are presented as contour maps. An optimal configuration of ten seismograph
stations is proposed for further on-site survey.
We investigate the resolvability of a microseismic event location given a recording array composed of vertical distributed acoustic sensing (DAS) boreholes. We use a modified source-scanning algorithm that takes into account both P and S waves. We transform the brightness maps it produces into probability density functions (PDFs), over which we carry out a resolution and uncertainty analysis. We apply this approach to microseismic events recorded by two vertical DAS boreholes as part of the Frontier Observatory for Research in Geothermal Energy (FORGE) project. We show that for the specific acquisition geometry in FORGE, the horizontal location of the events cannot be determined, but their depth can, similar to results obtained with a single borehole. Using synthetic examples, we show that the recording array’s geometry is the limiting factor in the determination of the horizontal location. We investigate various possible recording geometries composed of idealized DAS-like vertical boreholes with varying locations and depths. We find that, besides the number of recordingd boreholes, their depth is the main factor influencing the location estimation uncertainty. The number and position of the boreholes mainly influence the spatial distribution of the PDF, whereas the boreholes’ depth mainly influences its size. Despite the simplicity of our analysis, it highlights the influence of the monitoring array design for microseismic events’ locating using vertical DAS arrays.
