ML20236C113

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Seismic Reflection Data from Offshore Region Between Point Estero & Point Arguello:Interpretive Data & Discussion
ML20236C113
Person / Time
Site: Diablo Canyon, 05000000
Issue date: 12/11/1975
From:
EARTH SCIENCES ASSOCIATES
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References
FOIA-87-214 NUDOCS 8707290341
Download: ML20236C113 (10)
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geWatory Docket Filt Seismic Reflection Data from the Offshore Region Between Point Estero and Point Arguello:

Interpretive Data and Discussion .

-M.7g Contents i

I. Note on the structure of the East Boundary zone of the offshore Santa Maria Basin and the Hosgri fault, as observed on Western Geophysical Company line W74-12.

Introduction i Discussion Illustrations:

Figure 1 Plot of reflectors and other features on Western Geophysical Company line W74-12 (migrated).

2 Geologic section through the Diablo Canyon Power Plant Site and Adjacent I Area II. Note on Comparison of Fault Locations near the Diablo Canyon Site as Observed on Seismic Reflection Records from Different Surveys Introduction Discussion Table A. Seismic Reflection Lines Surveyed in the Offshore Area Between Estero Bay and Point San Luis Hlustrations:

Plate II(N) Map showing geology and structural relationships v/ithin the coastal and offshore region between Port San Luis and Lopez Point, California, i

annotated to show location of Western Geophysical Company seismic re- l flection lines, and location of the Figure 2 cross section from I, above.

Plate II Map showing geology and structural relationships within the coastal and I

offshore region between Point Conception and Arroyo Grande, California, annotated to show location of Western Geophysical Company scismic reflection lines.

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..  ; 1 is juxtaposed to the east against Acoustic Unit A3 s smic nturns, wM an Mu-preted to represent an upraised block of basement Acoustic Unit A3* * ^ ""

tic Unit A - quivalent rock. The boundary between the basal Acoustic Unit A E #t f 2 2 the offshore section and the inferred A2 -A3 n ar-s n sedon is MerpmW to upm-sent an east-dipping reverse fault. The inference that this boundary represents a fault contact is based on the apparent abrupt large vertical separation across it, and on the I

interpretation that the prominent diffractions present on the unmigrated line W74-12 j record are associated with a fault surface. The reverse fault contact is overlapped I

without break by the upper, Acoustic Unit Ay , part of the offshore basin section. ]

The part of the offshore Santa Maria Basin East Boundary zone that has been 1

affected by post-Pliocene fault movements is referred to as the Hosgri fault or' fault zone. The outer, main break along the post-Pliocene fault zone is indicated on the un-migrated line W74-12 record only by vague diffractions (since it is not well displayed in the acoustic frequency range used to produce the Western seismic lines), but it is clear- l 1

ly identifiable on most sparker seismic reflection lines. Likewise, the details of struc- l l

tural deformation adjacent to this fault are observable on sparker lines (e.g., Figures l 9A,12A,13A, and 14A of Appendix A to Appendix 2.5D to the Diablo FSAR), but not on ]

the Western Geophysical lines. Features appearing on the two versions of line W74-12 that are considered to be artifacts of the data recording and/or processing operations include two water-bottom multiples (on both unmigrated and migrated versions), diffrac-tionr from the Hosgri and the buried fault planes on the unmigrated record, and arcuate ,

patterns of alignments referred tobyWestern Geophysical 'as " velocity diffraction pat-terns" on the migrated record. These latter features are important because they could, i

if confused with true bedding reflections, be the cause of a misleading interpretation of the geologic structure. The velocity diffraction patterns appear as partly overlapping concentric arcuate lines that are centered about point sources located at intervals of about 16,000 feet, ground distance. They are most prominent near the ends of lines, in areas of data dropout, and in zones where slight velocity errors occur in the migra-tion process. These conditions mostly exist where Acoustic Unit A3" "*

Acoustic Unit A2parts of the section are represented in the record. In general, the i velocity diffraction patterns have a much lower degree of coherency than reflections i

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from bedded strata, e re -ca h a-m Amm m oma

The apparent geologic significance of the information contained in the line W74-12 record is that evidence of a pre-upper-Pliocene episode of compressional def-ormation is indicated there. This episode of tectonism probably corresponds to the comprest?onal deformation represented by the deep synclinal folding of the San Luis Range. The evidence from other, higher resolution records showing the upper several j thousand feet of the present Hosgri fault zone indicates that its post-mid-Pliocene history has been characterized by extensional and probably also by Ir.teral slip movements rather than the reverse movements shown at depth on line W74-12.

i

3. Earth Sciences Associates

II Note on Comparison of Fault Locations near the Diablu Canyon Site as Observed on Seismic Reflection Records from Different Surveys Introduction itecords and post-plot track charts from four different seismic reflection surveys of the offshore area near the Diablo Canyon site are available. The track charts and records from three of these surveys (USGS Kelez,1973; BBN,1973-74; Aquatronics, 1974) have been submitted as part of the documentation of geologic interpretations for the Diablo Canyon FSAR. Recordr and track charts for a survey conducted by Western i Geophysical Co. , Inc. In 1974 have also been used for the interpretations presented, but the records themselves, being proprietary, have not been subtitted directly.

Discussion The offshore area near Diablo Canyon has been intensively surveyed by seismic reflection techniques. Within the reach extending about 12 miles from Point San Luis north to the southerly part of Estero Bay, some 55 separate lines have been surveyed and are available for interpretation, as shown on the track chart base to Plate II(N) and in Table A.

These lines were all run in 1973 and 1974. The USGS Kelez and Western records were from parts of regional coastal surveys, while the BBN and Aquatronics surveys were concerned specifically with the area offshore from Diablo Canyon. The Western, BBN, and Aquatronics surveys were all navigationally controlled and post-plotted by Navigation Services, Inc. , using computer-reduced plots of continuous radio-transponder records of the survey ship's position. This system is said to be accurate to within a few feet when good station control is available, as it was in the Diablo area. )

l The original FSAR map of the position of faults in the Diablo offshore area was {

based on interpretation of records from the BBN, Aquatronics, and Kelez surveys. These I l

records were all obtained using sparker type energy sources, which varied in resolution l and penetration from the very high resolution .5 second penetration 3 Kj Aquatronics system to the intermediate resolution 1.5 second penetration 90 Kj Kelez system. The )

latter system achieved nearly 10,000 feet of penetration.

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t u m e a seme aeeee w me

Interpretation of these data resulted in a map showing four main breaks within the Hosgri fault zone. Directly opposite from Diablo Canyon, these breaks ranged from about 2.7 to 4.7 miles distance offshore. The outermost break was seen only in the deeper part of the section, and was mapped as a buried fault.

The records obtained from Western Geophysical were of the computer-processed deep seismic type. These records register to 5.0 seconds penetration, but are of much lower resolution than the sparker records. Because of the nature of the recording and the computer processing, which are designed to maximize the return of information from the deeper part of the section, the Western records show no usable information in the uppermost 500 to 1000 feet of the section. Consequently, faults of small displacement in the near surface part of the section are in many cases difficult or impossible to detect I in such records.

In the case of Western line W74-12, run nearly opposite Diablo Canyon, the chief feature that relates to faulting is a deeply buried steep reverse fault that corresponds to the outermost buried fault shown on Plate U(N). This fault truncates the easterly end of .j the deeper (Lower Pliocene-Miocene Acoustic Unit A ) part of the offshore Santa Maria 2  !

Basin section against Mesozoic basement and upraised Acoustic Uni! A r ek, but is I 2

overlapped by the shallow Upper Pliocene Acoustic Unit A section without break. Smaller breaks that extend to the surface at points closer to shore are only vaguely suggested in the Western records.

Line W74-12 was obtained both in standard format and also with computer migra-tion processing, the latter in order to determine whether significant errors of subsurface geometry and position could be present in the unprocessed record. It was found that the migration caused little change in the apparent position of the buried fault, and that it re-sulted in appreciable degradation of the nearer surface part of the record.

It is considered significant that the major fault break seen on line W74-12 is a deeply buried feature, and that it corresponds to a similar feature mapped at the same location using the three sets of sparker records available previously. These higher resolution records clearly show an unbroken Pliocene section several hundred feet thick extending across this fault, e re woo e gu m m m m /Ammmenomm

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Table A  !

l Seismic Reflection Lines Surveyed i

in the Offshore Area Between Estero Bay and Point San Luis Kelez BBN Aquatronics Western 50 123 SS-1 W74-14 27 SS-2 SS-3 125 48 25-26 46 127 l 23 PB-1 W74-80 l 129 21 l 19 44 17 PB-2 131 15 42 13 PB-3 133 11 VV74-12 38 PB-4 135 9 2

137 4 6 36 8

10 139 12 14 16 18 20 34 141 W74-82 22 32 30 fierrRh federnlreme AeeeentaRee

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145 1

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