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CONDUCTING HIGH-RESOLUTION 2D/3D SEISMIC REFLECTION
SURVEYS UNDER NQA-1
William H. Godwin, Fugro Consultants, Inc., Walnut Creek, California
Jamey P. Turner, Fugro Consultants, Inc., Lakewood, Colorado
Daniel R. Ebuna, Fugro Pelagos, Inc., Ventura, California
Stuart P. Nishenko, Pacific Gas & Electric Company, San Francisco, California
Phillip J. Hogan, Fugro Consultants, Inc., Ventura, California
Dan R.H. O’Connell, Fugro Consultants, Inc., Lakewood, Colorado
Abstract
Assessing seismic hazards for critical facilities such as nuclear power plants is challenging and
requires obtaining the highest quality data for analysis and interpretation. For this purpose, Pacific Gas
& Electric Company and their consultant, Fugro Consultants, Inc. acquired high-resolution seismic
reflection surveys along the central coast of California to image potential onshore and offshore faults
and geologic structures near the Pacific Gas & Electric Company Diablo Canyon Power Plant. The
objective of the Central Coastal California Seismic Imaging Project was to acquire seismic reflection
data under a qualified Nuclear Quality Assurance program, recognized by the Nuclear Regulatory
Commission under 10 CFR 50, Appendix B; NQA-1-1994; and 10 CFR 21. Fugro Consultants, Inc.
implemented their NQA-1 Quality Assurance program, which meets the requirements of 10 CFR 50,
Appendix B, and ASME NQA-1, for equipment calibrations, field data acquisition, software validation,
and data processing.
High- and low-energy 2D and 3D high-resolution seismic reflection data were acquired in
onshore areas of Diablo Canyon Power Plant region using Vibroseis, EnviroVibe, and Accelerated
Weight Drop sources and receiver arrays including autonomous nodes and conventional cabled
geophones. Low-energy 2D and 3D offshore data were collected from 2011 through 2012 using P-Cable
methodologies to image fault intersections and buried paleochannels along the central coast. Data
acquisition hardware and processing software and runtime environments were validated and used to
process the onshore and offshore data sets for future interpretation.
Introduction
In November 2008, the U.S. Geological Survey (USGS), working in partnership with the Pacific
Gas & Electric (PG&E) Geosciences Department, discovered a new fault termed the Shoreline fault
zone offshore of the Diablo Canyon Power Plant (DCPP). PG&E evaluated the safety risk the new
feature presented to the plant and submitted its evaluation to the Nuclear Regulatory Commission (NRC)
under the commitment of its current operating licenses. Results of the evaluation confirmed the plant has
adequate safety margin to withstand maximum ground motions postulated to occur from faults in the
region, including the Shoreline Fault (2011).
In 2010 PG&E initiated a project to identify and characterize the geometry, sense of slip, and
activity rate of active faults near DCPP. To accomplish these objectives, onshore and offshore 2D and
3D seismic reflection survey data were collected under Nuclear Quality Assurance (NQA-1) oversight
of all data acquisition and processing, activities. The project is titled “Central Coastal California
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Seismic Imaging Project (CCCSIP).” PG&E owns the project and contracted Fugro Consultants, Inc.
(FCL) to assist in designing the data acquisition, processing, as well as provide FCL Quality Assurance
(QA) management services in support of CCCSIP objectives.
The FCL QA Program directly controlled the following work elements under subcontracts to
FCL: 1) qualification of subcontractors performing nuclear safety-related activities; 2) procurement of
the highest value subcontractors; 3) submittal of QA planning documentation; and 4) surveillance of and
verification of safety-related work. All survey activities followed the procedures and practices outlined
in Project Planning Documents, Data Collection Plans, and Geophysical Work Plans. PG&E performed
a surveillance of FCL’s QA program and approved the program based on the requirements of 10 CFR
50, Appendix B; NQA-1-1994; and 10 CFR 21. The objectives of the respective Onshore and Offshore
programs are discussed below.
Offshore Survey Program
Both the 2D and the 3D P-Cable low-energy offshore survey programs were designed to acquire
ultra high-resolution seismic (UHRS) reflection data in permitted areas in Estero Bay, San Luis Bay, and
offshore Point Sal regions of the Central California Coast (see Figure 1). Seismic data collection began
in 2010 near Point Buchon, resumed in San Luis Bay, Estero Bay, and offshore Point Sal in late 2011
and 2012.
Onshore Survey Program
The onshore seismic survey program was designed to obtain 2D and 3D high-resolution seismic
reflection data in permitted onshore areas of DCPP region. These areas are both on PG&E property and
offsite to the north as far as Morro Bay, east to the Los Osos Valley, and south to the Price
Canyon/Pismo Beach area. Onshore Seismic Reflection Survey (SRS) activities comprised an initial
seismic source testing program and subsequent High-Resolution 2D and 3D Seismic Reflection Surveys.
See Figure 2 for location of 2011 and 2012 surveys.
NQA-1 Background
FCL has developed and maintains a Quality Assurance Program (Program) meeting the criteria
defined by the Nuclear Regulatory Commission (NRC) in 10 CFR 50, Appendix B, “Quality Assurance
Criteria for Nuclear Power Plants and Fuel Reprocessing Plants.” This Program is the top-level policy
document that establishes the manner in which FCL is to assure and achieve the quality of its safety-
related services. NQA-1 is a nuclear power industry standard issued by the American Society of
Mechanical Engineers (ASME) on July 29, 1994, and titled “Quality Assurance Requirements for
Nuclear Facility Applications.”
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Figure 1: Site Location Map of offshore low-energy 3D survey areas (Metzger and Campagnoli, 2001)
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This standard is based upon ANSI N45.2-1977 and its daughter standards. NQA -1-1994 is used
interchangeably for purposes presented in this paper. The foundation for implementation of the FCL
NQA-1 program for seismic reflection survey is a 3-phase process which includes Pre-field planning,
Field performance and Post-field documentation. These three phases can also be used for data
processing, as indicated in Figure 3 below.
Figure 3: Quality Assurance Process Flow Chart
QA Planning
The first and second phases of low-energy 3D seismic surveys were conducted offshore of DCPP
by PG&E during November 2010 to January 2011, and in December 2011, respectively. The third and
last phase of the low-energy 3D seismic surveys was conducted in late summer 2012. PG&E worked in
conjunction with FCL to design the seismic acquisition campaign. For this purpose, FCL prepared
Project Planning Documents, Project Instructions, and Work Instructions to document all phases of the
project design, including the hiring of contractors and subcontractors, acquisition systems and
parameters, active source equipment and shot parameters, survey locations, and acquisition scheduling.
Any project governed by the FCL Quality Assurance Program requires that safety-related data
acquisition and quality control systems have a valid commercial-grade calibration from an A2LA-,
NVLAP-, ACLASS- or LAB-accredited calibration laboratory prior to the use of the equipment for the
project. Prior to acquisition of safety-related data, all equipment selected for data acquisition and quality
control were tested to perform within specifications. Accuracy checks for Differential Global
Positioning Systems (DGPS) and other types of equipment were performed in accordance with
procedures contained in assigned Quality Assurance documents.
Geophysical Data Acquisition Methodology
Offshore Surveys
The UHRS multi-channel seismic reflection P-Cable system, consisting of twelve or fourteen
streamers of hydrophones connected to a single transverse cable and a triple-plate boomer seismic
energy source, was used in December 2011 and in the late summer/fall of 2012. The transverse cable, or
P-Cable, was pulled taut from each end by port and starboard paravanes. The survey vessel used during
the 2011 project was the M/V Bluefin, while the 2012 survey was conducted aboard the M/V Pacific
Star.
Navigational services and equipment were provided by NCS Subsea of Houston, Texas.
Positioning of the survey vessel and towed array was accomplished with a wide-area DGPS utilizing
U.S. Coast Guard corrections. DGPS units on the paravanes, buoys, and seismic energy source
connected to the navigation system through wireless communication. Compasses and depth sensors
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placed on the P-Cable and streamers at strategic locations allowed determination of hydrophone group
positions. These surveys had the advantage of real-time binning, which took positions of common depth
points (CDPs) as they were acquired and plotted them on the survey bin grid as fold (data density). This
allowed the vessel to be steered for optimal coverage.
In order to provide PG&E with a small bin size of 3.125 m by 3.125 m, streamers were spaced
6.25 m apart with a group interval of 6.25 m and shot interval of 3.125 m. This allowed for tight control
of bin spacing by using individual channels or summation of adjacent channels to increase the signal to
noise ratio. The data were acquired at a sampling rate of 0.250 ms and a record length of 750 ms. The
geometry of the array was controlled by ensuring uniform stern-to-paravane and stern-to-source offsets,
as measured using DGPS positions. Source-to-receiver offsets varied by streamer location in the array,
changing weather conditions and vessel speed, but were determined during data processing using
navigational data.
The AP3000 triple-plate boomer seismic energy source consisted of three simultaneously-fired
boomer plates. California State Land Commission (CLSC) regulations required total energy source
levels of less than or equal to 2000 joules (CSLC, 2011). Source energy output levels were continuously
monitored using two Geospace MP-24 hydrophones secured to the forward and aft sections of the triple-
plate boomer sled. Data from the P-Cable system was sent via fast Ethernet to the CNT-2 Controller
onboard the vessel and converted to SEG-D format. The Geometrics software allowed numerous quality
control checks, including calibration of the streamer electronics, leakage and capacitance testing of
hydrophones, display of all seismic channels, gathers of selected channels, and signal/noise
measurements. The seismic observer continuously monitored three representative channel gather
windows, noise levels of all channels, seismic source signature, shot interval timing, streamer depths,
water depth, and system leakage. The GeoEel Solid Digital streamer components, P-Cable, and CNT-2
Controller were supplied by Geometrics of San Jose, California and were subject to equipment
calibration and software validation. The triple-plate boomer seismic source, which required functionality
and source signature testing, was supplied by Subsea Systems, Inc. of Ventura, California.
2011 Onshore Data Acquisition
2011 onshore data acquisition occurred in two phases. First, an illumination study was conducted
to test Vibroseis sweep parameters, Accelerated Weight Drop (AWD) gas pressure and shot stacks,
geophone spacing, noise, offset, and acquisition quality. The objectives of the illumination study were to
obtain empirical data to optimize acquisition parameters for the full scale seismic acquisition study.
Second, the QA geophysical data acquisition campaign consisted of three seismic acquisition
configurations: 1) shallow high resolution 2D surveys with Seistronix EX-6 systems and AWD source
for 4 second data targeting near-surface and potentially steeply-dipping structures, 2) deeper crustal-
scale 2D surveys with Zland nodal systems and four synchronized Hemi-60 Vibroseis source for 11.2
second data targeting large-scale geologic features and deep geometries, and 3) very shallow high-
resolution 3D surveys targeting marine terraces to approximately 0.5 seconds.
AWD data were recorded by a Seistronix recording system using a 9.1m (30ft) group and shot
interval and typically 300-400 channels. A record length of 4 seconds and sample interval of 2 ms were
used to record the AWD data. Multiple impacts (typically 4-5) at each source position were vertically
stacked by the Seistronix recording system. A total of 22 2D seismic reflection lines were acquired
using the AWD source for a total of approximately 103km of AWD data. ProMAX software was used
for preliminary AWD 2D data processing. ProMAX archives contain the AWD data corrected for field
geometry, shot delays, etc. In 2011, over 13,000 AWD shotpoints and receiver stations were recorded.
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Four synchronous Vibroseis Hemi 60 and FairfieldNodal Zland autonomous receiver data were
recorded using a nominal 36.6m (120ft) group and shot interval for 2D profiles and a nominal 91.4m
(300ft) group interval extending away from the road for overland 3D recording when permitted.
Vibroseis Line 115 was recorded with 9.1m (30ft) stations and 18.2m (60ft) shot intervals. Vibroseis
Line 102 used 18.2m (60ft) shot intervals and Vibroseis Line 140 used 18.2m (60ft) shot intervals to
undershoot a large region not accessible by Hemi-60s. Four-to-six 24 sec 5-60 Hz sweeps with 500 ms
cosine tapers were used with an 11.2 sec listen time at each shotpoint. Hemi-60 drive levels were 70%
at most source points and reduced to 30% in areas sensitive to vibration near DCPP and in towns or near
buildings. Slightly less than 4,000 Vibroseis shotpoints and 7,181 Zland node locations were obtained
with a maximum Zland live deployment of 5,050 nodes; full Zland deployment occurred over several
weeks due to permitting limitations. Data extracted from the Zland nodes include active source data
recorded at 50 2D lines plus about 1,300 overland 3D Zland stations.
The Zland array recorded data from 18 earthquakes located by the Northern California Seismic
Network using PG&E and USGS permanent seismograph station data. The Zland node earthquake data
provide about 40,000-50,000 first-arrival times from the 18 earthquakes that can improve constraints on
crustal 3D velocity structure beneath the Irish Hills. Over 3,000 P-wave first-arrival times, and over
1,500 P-wave first-motion polarities were recorded for a magnitude (M) 0.6 earthquake on 18 October
2011. Good ground motion signal-to-noise in the 5 Hz to 30 Hz frequency band occurred not just near
the epicenter but at Zland stations within 0.5 km of DCPP. Adding earthquake data to the active source
data is expected to improve velocity resolution in the middle and lower crust relative to available 3D
crustal velocity estimates (Hardebeck, 2010) and reduce depth uncertainties of depth migrations.
2012 Onshore Data Acquisition
2012 onshore data acquisition activities included an initial start-up seismic source testing
program followed by acquisition of High-Resolution 2D and 3D Seismic Reflection Surveys. Onshore
Start-up Activities began by performing testing and evaluation activities in order to optimize and refine
site-specific aspects of the data collection and vibration monitoring program according to work plans
and test specifications.
Source testing was performed for iSeis Sigma 3 and FairfieldNodal Zland data acquisition
systems. Sigma systems were tested for real-time recording performance of EnviroVibe, AWD, and
hammer-strike plate seismic sources with Seistronix and iSeis Sigma 3 recording systems. Noise, source
walkaway, source parameter testing, and initial source and receiver parameter tests were performed.
GEOVision ground vibration monitoring of EnviroVibe, 45.5kg (100-lb) AWD, and sledgehammer and
strike-plate sources were used to test and document peak ground acceleration, peak ground
displacement, and acceleration response spectra as a function of source operation parameters and
distance from the source at two locations within DCPP. The specific approach and acquisition parameter
selection for the 2012 data acquisition program was designed to gain empirical constraints to optimize
source and receiver acquisition parameters to minimize background noise inputs from DCPP.
Seismic Data Processing Methodology
Offshore Survey Data Processing
In addition to real-time monitoring by the seismic observer, quality control processing of the 3D
UHRS data was carried out at the end of each survey day by an onboard seismic data processor.
Processing was performed using Fugro’s proprietary UNISEIS data processing software. The processing
flow utilized for onboard QC consisted of the following: conversion from SEG-D to UNISEIS format,
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offset determination, trace editing, navigation merge, low-cut filter, statics correction, gain recovery,
CDP sorting, normal move out (NMO) correction, muting, and a brute stack.
A complete processing routine for the 3D data volume was performed by Fugro Seismic
Imaging, Inc. in Houston, Texas again using UNISEIS. During final processing, more sophisticated and
complex techniques were employed, including multiple velocity analyses, swell noise attenuation,
surface-related multiple elimination, signature deconvolution, and a 3D pre-stack Kirchoff time
migration. All 2D seismic data processing was performed by FCL using Seismic Processing Workshop
(SPW) software.
Onshore Survey Data Processing
In addition to real-time monitoring by the seismic observer, quality control processing of the
onshore data was carried out at the end of each survey day by an onboard seismic data processor. Two-
dimensional seismic processing was performed using Green Mountain Geophysics, Inc.
GeoScribe/GMG-Stat and ProMAX data processing software, authorized by the Validation of the
Commercial Software ProMAX and GeoScribe/GMG-Stat Green Mountain Geophysics processing steps
include geometry definition and application with crooked-line geometry, first arrival picking and
refraction analysis, and refraction static calculations of source and receivers. ProMAX processing steps
include data pre-processing, signal processing, data analysis, post-stack processing, and digital output of
final data sets.
Additional processing of 2D and 3D onshore seismic surveys was performed using Fusion
Petroleum Technologies, Inc. Seismic Studio (FULL) and GeoTomo TomoPlus softwares, authorized by
the Validation of the Commercial Software Programs Seismic Studio (FULL) and TomoPlus (PGEQ-PI-
12). The processing flow for Seismic Studio includes reading SEG-Y data and geometry definition, first-
arrival picking, quality, and editing. The processing flow for TomoPlus includes reading SEG-Y data
with geometry and first arrivals, first-arrival checking and review, building an initial velocity model, 3D
first-arrival traveltime tomography, 3D refraction traveltime residual statics solutions, and digital output
of final models and data sets. The TomoPlus 3D velocity model and statics solutions were assigned to
2D profiles as starting points for analyses leading to final 2D time- and depth-migrations.
Conclusions
High-quality 2D and 3D onshore and offshore seismic reflection data were collected and
processed for the first time under an accepted NQA-1 program to support an updated seismic hazard
assessment of the Diablo Canyon Power Plant. These data are of high pedigree as a result of
implementation and documentation of rigorous calibration and validation procedures. As a result, the
data is optimized for geologic interpretation and hazard assessment. The NQA-1 process will allow for
the eventual qualification of other SRS datasets not collected under this program as well.
Acknowledgements
The authors would like to thank PG&E DCPP plant personnel, in particular Richard Van der
Linden and Geoscience QA manager Marcia McLaren. Also, thanks go to offshore crews of the M/V
Bluefin, the M/V Pacific Star and the M/V Michael Uhl, and onshore crews of Nodal Seismic Inc. and
Bird Seismic Services.
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