ML18003A581

Seismic Monitoring Program,First Data Rept, 770930-790331

ML18003A581
Person / Time
Site:	Harris
Issue date:	04/30/1979
From:	CAROLINA POWER & LIGHT CO.
To:
Shared Package
ML18003A580	List: ML18003A579 ML18003A581
References
NUDOCS 7905070480
Download: ML18003A581 (16)

Text

FIRST DATA REPORT SHEARON HARRIS NUCLEAR POWER PLANT SEISMIC MONITORING PROGRAM

SUMMARY

OF DATA SEPTEMBER 30, 1977 - MARCH 31, 1979 CAROLINA POWER 6 LIGHT COMPANY APRIL 30, 1979

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INTRODUCTION On January 6, 1976, Dr. Walter Butler notified Carolina Power & Light Company (CP&L) by letter that the fault discovered at the Shearon Harris Nuclear Power Plant (SHNPP) is not a capable fault as defined in Appendix A to 10CFR Part 100. The NRC also requested that seismic monitoring be performed at the site to confirm the NRC staff conclusion that the proposed reservoirs at the site will not cause fault movement during and after filling.

In response to this, CP&L submitted a proposal on February 13, 1976, to establish a seismic monitoring network which would encompass the SHNPP plant site area. The network was installed and became operational on September 30, 1977, in order to obtain definitive baseline data prior to the filling of the reservoirs.

During this 18-month period of monitoring, no earthquakes have been detected in the plant, site area. However, the data collected has shown that the plant site area and surrounding region are extremely "noisy" from cultural sources during week day working hours (8 a.m. to 6 p.m., Monday through Friday).

We have recorded and identified 552 cultural seismic events 289 quarry explo-sions, 182 SHNPP construction related blasts, and 81 explosions from highway and pipeline construction. Since extensive effort has been required to identify these cultural events and document their arrival patterns and signatures, we will take benefit from the experience we have acquired in identifying cultural events by applying the following criteria:

Events that occur overnight and on weekends with an S-P less than five seconds and all events that occur within the network perimeter will be pursued for specific identification and included in the quarterly reports.

Events that occur outside the network perimeter during week day working hours and which display the character of a typical cultural event will only be listed in a log containing their first arrival time and remarks concerning their probable origin. These events will not be discussed in the quarterly reports.

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By so using the experience we have already gained in categorizing cultural events, our seismic analysts can apply a greater portion of their time to studying events which have a higher probability of being natural phenomena and reduce the time consumed by locating construction sites and personnel with reliable knowledge of blasting times.

1.0 Descri tion of Monitorin System The monitoring network consists of an array of four stations, each identified by a number, 1 through 4, encompassing the SHNPP site area (see Figure 1 and Table 1). Each station contains a Teledyne Geotech S-13 vertical component, short period seismometer. Station No. 4 also has two Model S-13 seismometers installed in the horizontal mode one aligned north-south and the other east-west. Data from Station Nos. 1 through 3 is transmitted via UHF radio to the SHNPP meteorological tower. Data from Station No. 4 is transmitted to the meteorological tower via commercial telephone lines. At the meteorological tower, data from all stations is multiplexed and trans-mitted by telephone lines to a central recording facility located in the Center Plaza Building in Raleigh, North Carolina. The data is recorded on 16mm microfilm by a Teledyne Geotech RF 400 Develocorder on a 24-hour-per-day, 7-day-per-week basis with only brief interruptions for routine periodic maintenance. A single visual channel is also displayed on a Geotech Helicorder drum recorder.

Figure 2 shows the system's measured frequency response using the vertical component seismometer at Station No. 4 at its normal gain settings.

This frequency response is common for all stations when adjusted for magnifi-cation variations imposed by the local background noise conditions at each station. Figure 1 shows the operating magnification at 1 Hz for each station, including the two horizontal components-of Station No. 4. It should also be pointed out that each component of the network is dually recorded at low ampli-fication for obtaining good records of unusually strong signals.

2.0 Unconfirmed Events The Company's seismic analyst views the film reel for each day and logs the time, date, reel number, and suspected source for every detected

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event with an S-P time of less than five seconds. Periodically, he calls all the known source locations and obtains a list of dates and times for their blasting. He then correlates the dates and times of detected events with reported explosive blasts in order to confirm that each recorded event is actually an explosion and not a natural seismic event.

Some cultural sources that are responsible for confirmed explosions that have been recorded are six local rock quarries, three excavation sites at the Shearon Harris Nuclear Power Plant, seven construction locations by the North Carolina Department of Transportation, and several locations along a new gasoline pipeline construction corridor.

Experience has shown that some quarries and construction companies keep very poor records of their blasting activities, often either'istakenly writing down the wrong date or time for an explosion or forgetting to log it altogether. In these cases, the seismic analyst has considerable difficulty confirming an event that is suspected of being an explosion. Nevertheless, out of more than 594 recorded explosions, only 42 events remain unconfirmed.

This number of unconfirmed events is not unexpected in a culturally active area.

All of these 42 events occurred outside the perimeter of the network; 36 are located along a four-lane highway under construction about 30 kilometers from the plant site, and 5 of these events are located along the construction route of a petroleum pipeline. In addition, all of the events exhibit the following characteristics of cultural origin:

All occur only during working hours.

All occur from Monday through Saturday, never on Sundays or- holidays.

All display signal characteristics similar to confirmed cultural events.

Since all events which occurred within the plant site area (network perimeter) have been identified as construction explosions, their arrival times are not listed and there are no epicenters or magnitudes to report.

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3.0 Detection Threshold The detection threshold in terms of ground motion amplitude that can be detected above the natural background noise has been determined at each seis-mometer station employing two methods. The first method involves measuring the normal background noise level and assuming a minimum signal to noise ratio of 2.0 is required for event detection. A minimum signal amplitude of 15 milli-microns zero to peak is required for detection at the quietest station (Station No. 2), and an amplitude of 48 millimicrons zero to peak at the noisiest site (Station No. 3) during nighttime hours. The detection threshold would be sig-nificantly higher during weekday working hours. lf it is valid to use Nuttli's magnitude formula for Lg at very short distances, then the detection threshold at 1 Hz using the minimum detection amplitude is calculated to be Mb = .8 at Station No. 2 and Mb = .3 at Station No. 3.

The second method of threshold determination involves the extrapolation of regional events to local distances. To the extent that it is valid to extrapo-late measured amplitudes of regional events to infer signal levels at local dis-tances using Nuttli's formula for Lg, we estimate a detection threshold for the monitoring network to be on the order of Mb = .5 to Mb ~ -1.0 for signals at 1 Hz.

4.0 Conclusions A seismic monitoring network has been installed which encompasses the Shearon Harris Nuclear Power Plant site. The network has operated continually for 18 months with a detection threshold capable of detecting earthquakes with magnitudes less than Mb ~ 0. Hundreds of cultural events originating from quarries and highway and pipeline construction have been identified. However, no earthquakes have been detected in the plant site area.

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MERRY OAK BONSAI.

ABOARD NEW HILL RAII.ROAD AUXILIARYRESERVOIR VS I FRIENDSHIP IZ 134,000 1

SPILLWAY I'LANT SITE IZ )69,000)

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R ESERVOIR MAKEUP LINE hIAIN RESERVOIR IZ 213,000)

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MAIN DAhI SPILLWAY BUCKHORN DALI A 3 m

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SHNPP Seismic Network Station Locations Elevation Site Ft. M. Latitude ~Lon itude 282 86.0 35 37 '5'IN 78'58'49"W 35.63N 78.98W 245 74.7 35'33'17"N 78'59'23"W 35.55N 78.99W 310 94 ' 35o35 36 78'54 '6 "W 35.59N 78.90W 305 93.0 35 39'57"N 78'54'16"W 35.67N 78.90W

.Table 1

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