3. PG&E'S Planned Seismic Studies

As discussed in sections 3.2. and 3.3. below, PG&E proposes to undertake detailed on-shore the two-dimensional (2-D) and off-shore 3-D seismic surveys of the area surrounding DCPP. PG&E states that there are challenges to performing effective seismic surveys and it is therefore essential to carefully identify the areas to survey and to plan how the survey will be conducted. For example, there are areas where quality results may not be possible due to the presence of tightly folded and steeply dipping basement rocks that would not be well imaged by either 2-D or 3-D seismic survey techniques. In addition, the area around the "Shoreline fault zone" recently identified by the US Geological Survey (USGS) is located in shallow water and is not accessible by large vessels used for marine surveys due to dense marine vegetation and submerged rocks.1 Characterization of the "Shoreline fault zone" and areas that would not be well imaged due to the geology will therefore rely on other geophysical work (e.g., multi-beam echo sounding, magnetic and gravity surveys) and seismicity work (i.e., locating earthquakes) conducted as part of the LTSP Update.

The seismic line locations for the on-shore 2-D survey will have to take into account road access, land ownership, and environmental issues. Both the 2-D and 3-D surveys will be based on geologic models developed during the PG&E LTSP Update along with available on-shore and off-shore industry data. These data will be used to determine how best to image the features of interest and design the seismic data collection (e.g., geometry of seismic sources and receivers, the number and spacing of survey lines, etc.). Areas that have complex geology and dipping strata and or faults may require multiple or larger survey areas than those areas with flat layers and simple geometries.

The seismic survey design cost estimate is $500,000. This includes the cost to purchase necessary industry seismic line information, reprocess the industry data, design the 2-D and 3-D surveys, and evaluate permit requirements and property access agreements.

Seismic imaging is a tool used by geologists and geophysicists to image subsurface geologic formations. Sound waves produced by pneumatic devices called "air guns," or by other means, bounce off underground rock layers and are detected and recorded by ultra-sensitive instruments at the surface. The timing and intensity of these reflections are used to map the location of subsurface structures such as folds and faults. Sophisticated 3-D seismic surveys are based on a grid of closely spaced survey lines that create a high-definition three-dimensional picture of the subsurface geology. Interpretation of these data provides useful information that can help discern new geologic features and constrain uncertainties associated with known fault zones, including geometry (i.e., fault length, width, and dip), location, and fault activity or slip rate. The effectiveness of the 3-D survey is largely dependent on how well the subsurface geology can be imaged.

PG&E proposes to conduct an off-shore 3-D seismic survey. The scope of this task will require acquisition of necessary State of California permits for seismic sources that exceed the 2-kilojoule energy limit, preparing an environmental impact report, renting a ship and crew, and conducting data collection over the defined off-shore survey area as well as data interpretation and integration. The LTSP Update has focused on using high resolution bathymetry, magnetic and gravity data, and existing 2-D seismic reflection lines to determine off-shore fault geometries, locations, and slip rates. The results of the 3-D seismic survey will be integrated with these LTSP Update results.

The cost estimate for the off-shore surveys is $11 million. This includes $1 million for deployment (obtain permits, secure survey ship and crew, relocate the survey ship to the survey area) and $10 million to perform the actual survey, which involves data collection, processing and interpretation (e.g., integrating results with other geological and geophysical data in a geologic information system platform) to characterize the earthquake fault zones. Costs are largely dependent on the size of the survey area that will be defined in the survey design phase. For this estimate, PG&E assumed a survey area of 400 square kilometers which roughly covers the area between Point Buchon and Point San Luis, and out beyond the Hosgri fault zone.

PG&E's ability to proceed with the off-shore studies is conditioned upon obtaining all required permits for the project, including permits from the State Lands Commission and the California Coastal Commission. The permitting process has the potential to delay the anticipated schedule for completion and increase the costs of performing the seismic surveys and studies. As discussed in section IV below, PG&E requests authority to recover its actual costs of conducting the surveys, including any increase in costs associated with the permitting process, including the costs of delay. In addition, it is possible that permits necessary to implement the additional seismic surveys and studies may not be granted at all. In such an event, PG&E would refund to customers any unspent funds collected in rates. The LTSP would continue in effect and fully satisfy NRC regulatory requirements and ensure Diablo Canyon's safe operation.

As with the 3-D seismic imaging techniques, 2-D seismic survey is performed by bouncing sound waves off underground rock layers to reveal geologic structures. Unlike the dense spacing of 3-D surveys, 2-D surveys are recorded along a single line to create a 2-D image or cross section, similar to an X-ray. By combining several 2-D images on parallel lines, a quasi 3-D image can be created. The sound waves or sources for the on-shore 2-D surveys are created by using either small explosive charges in shallow "shot holes" or by large vehicles equipped with heavy plates that vibrate the ground. 2-D surveys are preferred over 3-D surveys on-shore due to the increased difficulty and cost of deploying instruments in rugged terrain as well as land ownership and environmental issues. The scope of the LTSP Update did not include performing new 2-D seismic surveys. It included detailed (surface) mapping of the geomorphic, marine, and fluvial surfaces, and utilizing and interpreting existing reflection lines to provide evidence of fault geometry and slip rate.

PG&E proposes to conduct a series of on-shore 2-D seismic surveys to image relevant geologic features in the area such as the Los Osos fault zone. As with the 3-D seismic survey, the on-shore surveys will require permitting for siting of shot points and seismic instrument deployment and PG&E's ability to proceed with the 2-D survey is conditioned upon receiving all necessary approvals.

The cost estimate for the on-shore 2-D seismic surveys is $2.03 million. This includes $380,000 to deploy (obtain permits, set up survey lines, shot points, and observation locations) and $1,645,000 to perform the actual survey, which includes data collection, processing, and interpretation. These costs are based on establishing up to four survey lines on-shore with reasonable efforts to obtain land rights and necessary permits.

In addition to the seismic surveys, another advanced technique to explore fault zones near DCPP is to install OBS units that will more accurately locate offshore earthquakes (in conjunction with the existing on-shore seismometers). By combining earthquake locations with surface and subsurface geologic features, obtained through other geological and geophysical data collection techniques such as the 3-D seismic surveys, PG&E will be able to more accurately image fault zones. This knowledge is useful to DCPP as calculated earthquake ground motions at the plant site that are used for assessing structures, systems, and components are largely dependent on the distance of an earthquake fault to the plant site facilities.

Currently, the only seismometers at DCPP are on-shore, installed east of the fault zones of interest. PG&E proposes to purchase and install up to four OBS units off-shore, on the west side of the known fault zones, to provide the critical seismological station coverage necessary to improve the quality of present earthquake monitoring locations. The earthquake location uncertainties will be reduced by having seismometers on both sides of the earthquake fault zones.

The total cost for purchasing and installing four OBS units is $2.05 million. This includes $1.51 million to manufacture and deliver the instrument packages and to obtain necessary permits for installation; and $330,000 to install the OBS units off-shore (determining best locations, having divers install at the predetermined locations, and providing maintenance over a three-year period). PG&E will process data for all earthquakes during the three-year period after OBS installation. There is a manual process to obtain raw data after an earthquake, integrate the data with on-shore recordings, and determine the earthquake locations. This task is estimated at $120,000. Modeling and data interpretation is estimated at $90,000 over the three-year period. The modeling involves determining the fault focal mechanisms (i.e., fault motions such as strike slip) and improving off-shore velocity models. Data interpretation includes analyzing how the OBS data fit with current fault interpretations.

In addition to the costs specific to the surveys and OBS installation, PG&E estimates $1.15 million to manage and administer the project. This includes $500,000 to develop a final report, which will present figures that display the results of the surveys, and which will undergo internal and external review (e.g., NRC, PG&E external advisory board). The report will be integrated with the final LTSP Update report that will be the basis for adopting any appropriate changes to the tectonic model and seismic hazard at DCPP. In addition, PG&E estimates $650,000 in additional labor and facilities resources over the three-year period from 2011 to 2013 to: (1) provide technical guidance and review of outside consultant work, (2) ensure the work follows the established scope of the project, (3) utilize DCPP facilities and personnel for support during the 3-D and 2-D seismic surveys and (4) manage costs and schedule. These project management costs are not included in PG&E's 2011 general rate case application (A.09-12-020) as they are specific to implementing the CEC recommendations from the AB 1632 Report.

1 In 2008, ongoing PG&E and the USGS studies indicated the possibility of a minor offshore fault. PG&E's response was to evaluate possible safety implications by studying this potential fault in terms of existing seismic analysis. The data shows that the fault is bounded by existing seismic analysis; PG&E believes that the result of any potential ground motions would be minor and have little, if any, impact on the plant. In 2009, the NRC conducted its own independent evaluation and concluded that potential seismic activity from the Shoreline Fault would be within the plant's existing station designs and does not represent a new challenge to plant operations. The Shoreline Fault will continue to be studied by PG&E, the USGS as well as the NRC as part of PG&E's LTSP activities.

Previous PageTop Of PageNext PageGo To First Page