ML20238B755
| ML20238B755 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 07/31/1987 |
| From: | WESTON GEOPHYSICAL CORP. |
| To: | CLEVELAND ELECTRIC ILLUMINATING CO. |
| Shared Package | |
| ML20238B749 | List: |
| References | |
| NUDOCS 8708210447 | |
| Download: ML20238B755 (25) | |
Text
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THIRD QUARTERLY REPORT CEI SEISMIC MON!TORING NETWORK APRIL 16 through JULY 15,1987 Prepared for CLEVELAND ELECTRIC ILLUMINATING COMPANY JULY 1987 i
i
,,,,.(ar) l5 Weston Geophysical CORPORATION 8708210447 DR e70917 ADOCK 05000440 PDR 1
TA_BLE OF CONTENTS ,
Page /
I LIST OF TABLES i LIST OF FIGURES i
1.0 INTRODUCTION
1 2.0 SEISMIC NETWORK 1 3.0 OBSERVED SEISMICITY FROM APRIL 16 to JULY 15, 1987 3 3.1 Epicentral Area of January 31, 1986 Earthquake 3 3.2 Injection Wells - Main Epicenter Corridor 3 3.3 Other More Distant Earthquakes Recorded 4 4.0 DISCUSSIONS 4
5.0 CONCLUSION
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LIST OF FIGURES FIGURE 1 Telemetered Network Station Configuration FIGURE 2 Reponse Curve for the L-28LB Geophone FIGURE 3 Remote Site Interconnection Diagram FIGURE 4 Data Flow Diagram at Recording Center FIGURE 5 Automated Seismic Telemetering and Recording System FIGURE 6 Seismicity for Period April 16 through July 15, 1987 FIGURE 7 Cumulative Seismicity FIGURE 8 Comparison of the May 1 and 2. 1987 FORD Seismograms LIST OF TABLES TABLE 1 Station Locations of CEI Network TABLE 2 Microcarthquakes in the Injection Wells - Main Shock corridor TABLE 3 Earthquakes Outside the Wells - Main Shock Corridor 1
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1.0 INTRODUCTION
As requested by the U.S. Nuclear Rtgulatory Commission, Cleveland Electric Illuminating (CEI) continues to monitor the seismic activity in a small area of northeastern Ohio that includes the Perry Nuclear Power Plant, two deep injection wells operated by CALHlo, and the epicentral region of the January 31, 1986 earthquakes. This third quarterly report, prepared by Weston Geophysical for CEI, covers the time period from April 16 to July 15, 190L It also includes a brief description of the new digital array installed and tested during this period. This telemetered array will replace the portable MEQ-800 seismographs that CEI has operated since February, 1986, 2.0 SEISMIC NETWORK During this last quartet, CEI has begun operating and testing its new digital array 'AUTOSTAR' { Automated Seismic Telemetering and Recording System], designed by Weston Geophysical. Figure 1 shows the configuration of the five 3-component stations of the digital array, and Table 1 gives the corresponding coordinates. For this period, an average of eight MEQ-800 vertical seismographs were used as back-up.
One MEQ-800 was maintained at each of the five telemetered stations; normally three others were operated near the center of the array to improve the detecticn and location capabilities, as in the previous months.
The principal objectives of AUTOSTAR, specifically established to comply with CEl's commitments to the NRC are the following:
- 1. Detection and location within a small aperture array of all microearthquakes including those with magnitudes as small as -0.5 )
if passible, or at least 0.0.
- 2. Automated digital data acquisition, compatible with the high l frequency content of expected signals at short epicentral distances.
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- 3. Safe and versatilt: archiving of digital files.
- 4. To supplement the digital system, continuous analog recording that allows visual inspection of the complete time histories and facilitates identification of false alarms.
- 5. On-site capability of preliminary process,ing of data through interaction between operator and system.
To meet these objectives. AUTOSTAR incorporates these respective features:
- 1. Five stations surrounding the area of interest, each one with three 4.5 Hz L-28 Mark products sensors, orthogonally oriented inside a borehole probe. Each borehole is lined with PVC through the glacial sediments; the boreholes extend 10 to 15 feet into bedrock and the probes are clamped to the walls of bedrock. The geophones are critically damped with a sensitivity of 1.336 volts / inch /sec; the response curve is shown on Figure 2.
- 2. At each remote site, a Refraction Technology RF 97-02 remote data acquisition unit digitizes the signals at 250 s/sec; the data is then serially transmitted through NEC DSP 14400 modem via a D1 conditioned telephone line. At the recording center, five similar modems receive the data and pass it to the RT 44-03 interface unit, which analyzes the incoming data according to preset triggering criteria selected and controlled by a Wescomp" data processing computer.
- 3. After the identification of a potential seismic event, or trigger, digital files of 60 sec. in length are then directed by the We8 Comp
- to disk and magnetic tape storage. Pretrigger memory assures the digital recording of the event from its beginning.
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4 l 4. The RT 44-03 interface unit, through D/A converters, provides continuous analog outputs for all flfteen channels. At the discretion of the operator, selected channels, e.g. one from each l station, can be displayed on Geotech visual RV-301 helicorders, controlled by AR-320 amplifiers.
- 5. AUTOSTAR offers the option of editing the archived disk files and building up a removable disk filled with wanted events only. It also gives the operator the ability to examine stored files, to filter, normalize, scale up or down and print any file or selected parts of files. Finally, phase arrivals can be picked on the CRT n.onitor and used to locate hypocenters through FASTHYPO. Spectral analysis can also be performed.
Figure 3 shows the instrumental layout of the remote siter; Figure 4 illustrates the data flow at the Ic,;ording center; Figure 5 gives a sketch of the layout at the Recording Center.
3.0 OBSERVED SEISMICITY FROM RPRIL 16 TO JUI.Y 15, 1987 3.1 Epicentral Area of Januar_y 31, 1986 Earthquake _
No seisaic activity was detected in the immediate area of the January 31, 1986 earthquake. The last detected af tershock occurred on February 12, 1987.
J 3.2 Iniection Wells - Main Epicenter Corridor Five microcarthquakes occurred during this quarter; two on May 1 (Mc =
and one on July 8 (MC =
-0.6,1.3), two on May 2. (Mc = -0.6, 0.4),
-0.2). Four events are very close in time and also space, as shown on Figure 6. Two events plot almost superimposed, giving the appearance of one event. The last microcarthquake prior to May 1, 1987 had occurred on February 28, 1987. An updated list of the sixteen microcarthquake {
locations is ptesented in Table 2.
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L other More Distant Ear _thquakes Recorded 3.3 1
Although the CEI micronet is essentially designed to monitor low level seismicity within its small aper ture, it is also capable of collecting valuable data on larger events occurring at greater distences outside the aperture. The digital network triggered on the southern Illinois event of June 10. mbig = 5.1, 670 km away. On June 18, the micronet recorded digitally an carthquake (Mc = 2.7) located about 65 km away in northwestern Pennsylvania. With additional arrival times from other observers in New York and Canada, a preliminary solution was calculated. Recently, on July 13, the micronet triggered on the Mc =
3.7 earthquake that occurred near Ashtabula. Ohio: it also triggered on several of the larger af tershocks. This earthquake was felt mildly in the epicentral area but it was not felt at the Perry Plant site. This is in agreement with CEI's report that the strong motion instrumentation at the Plant did not trigger. Because this earthquake and some of its af tershocks were digitally reccrded by AUTOUTAR. a few MEQ-800's were immediately deployed around the epicentral area in order to collect additional near source data, which could be used in conjection with the digital network data to refine the epicentral locations. Epicenters for j both regional events nave been plotted on Figure 7. They are listed on Table 3 with other regi-onal events, east and west of Perry.
4.0 Discussions l
The occurrence of different types of earthquakes during the last quarter l has given CEI the opportunity to begin a performance assessment c,f its new digital automated network. This is useful since the system is still l in its " shake-down" period, with the back-up of the MEQ-800 portable l array providing a basis for comparison.
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The digital array triggered on the two larger microcarthquakes [Mc = 1.3 and 0.4] that occurred within the aperture. It recorded the two swaller ones [Mc = -0.6] on two visual recorders; it recorded the July 8th event (Mc =
-0.2) on all four visual recorders but the extremely small amplitudes at two stations explain why it did not trigger. Because one of the five stations was not operating, it is too early to establish the ;
triggering threshold of the digital array. It is expected that visual scanning of all analog seismograms will lower the automated detection threshold to magnitude Mc = 0.0 or less.
The first four events occurred closely in time, i.e. within twenty four hours. As shown on Figure 6, they are also very close in space. From Table 2, the location errors suggest that they could well have the same origin. The fifth event is located very close to the others.
When these events are included with those obtained during the previous six months using only the MEQ portable net data, they locate at the northeastern end of what appears to be an elongated cluster. As shown on Figure 7 which presents the cumulative reismicity recorded by CEI instruments since January 31, 1986, an aseismic zone of about 3 to 4 km exists between the two injection wells and this cluster of microearthquakes: because of this gap, any causal correlation with the CALH10 wells remains at this time hypothetical and uncertain. A longer aseismic zone is also apparent between the January 31, 1985 earthquake cluster and the southern end of the elongated cluster. Thus our prior position that these events are not associated with the January 31, 1986 earthquake remains valid. j l
The quality of the data collected by AUTOSTAR is much higher than that obtained with portable MEQ instruments. First, the time resolution is improved by the use of a single clock as opposed to individual clocks requiring daily corrections; the reading error is also minimized since the cursor readout has a .004 see accuracy. Secondly, the phase identification is facilitated by the availability of horizontal components and the versatility of the playback options, e.g. filtering 1186J *5+
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and sceling up. Figure 8 illustrates these statements. It shows the j two events as recorded at Station FORD with MEQ-800s and an S-13 vertical seismometer, on May 1 and 2, 1937 (Julian day 121 and 122), and l the playbacks produced by the new digital system. The arrival times and polarity of the P-phase are clearer on the digital records. The S-phase is equally unreadabic on the MEQ vertical record and the vertical component of the digital net, but it is remarkably sharp on the horizontal components of the digital net. Furthermore, the similarity I
of the digital signals of both microcarthquakes, even though the magnitudes are different (1.3 and 0.4), confirms the results of the hypocentral solutions, i.e. the events have almost identical foci. >
These readings and identification advantages suggest that, from now on, higher hypocentral resolution can be achieved and should contribute to a i
better understanding of the local seiemicity. l t
In view of the superior quality of these new data, a clearer picture of the seismic activity in the vicinity of the calhio injection wells will become available. More precise locations of the hypocenters located with AUTOSTAR data will be compared with the previous results bared on hEQ-Vertical data enly.
Finally, the new network collected numerous digital files from regional earthquakes outside its aperture. These files contain valuable 1 information for future research on observed accelerations in the eastern United States, particularly in the higher frequency range.
5.0 CONCLUSION
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During the last quarter, only five microearthquakes occurred within the area of interest monitored by the new digital network. No conclusion can be reached at this time on the nature or causal mechanism of these i events. j l
The new system AUTOSTAR recently installed has performed satisfactorily during the shake-down period and collected high quality data. It has also detected other regional events outside its aperture. {
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