ML19276H575

Prefiled Testimony.High Frequency Seismic Methods Inadequate to Locate Fault Traces Existing in Area.Interpolation Between Spaced Seismic Lines Is Speculative

ML19276H575
Person / Time
Site:	Skagit
Issue date:	10/17/1979
From:	Johnson S Oregon State University, CORVALLIS, OR
To:
Shared Package
ML19210E129	List: ML19210E128 ML19276H567 ML19276H572 ML19276H575 ML19276H578 ML19276H580 ML19276H583
References
NUDOCS 7911290437
Download: ML19276H575 (4)

Text

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BEFORE THE ATOMIC SAFETY AND LICENSING BOARD 4 4h;3 g'

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PUGET SOUND POWER & LIGHT ) DOCKET NOS. STN 50-522 COMPANY, et al., ) 50-523

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(Skagit Nuclear Power Project, )

Units 1 and 2) )

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PREFILED TESTIMONY OF STEPHEN H. JOHNSON Oc tobe r 17, 1979 Q. What is your name and occupation?

A. Stephen H. Johnson, Associate Professor of Geophysics, School of Oceanography, Oregon State University, Cor-vallis, Oregon 97331.

Q. What documents have you reviewed in preparation of your testimony?

A. 1. Single Channel records obtained with a 4 kJ sparker and Acoustipulse obtained by Bolt, Beranek and Newman in 1976 for Puget Sound Power and Light Company.

2. High resolution seismic profiles (Uniboom) obtained in 1976 by the USGS (Open file report 76-187).

3. High resolution seismic profiles (Unicoom) obtained in 1978 by the USGS (Open file report (7801061).

Q. What is the substance of your testimony?

.,3-,..==n A. A number of fault trends are proposec to underlie marine areas 7-the mainland near Seilingham, Washincton and the San Juan Islands in ..e

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vicinity of Lummi Island! -

The locations of these lines are plotted on the accomnanyinq map of tne area. The Mobil Oil Company lines U70-(1 through 11) are shown with fault lccations as interpreted by Dobrin (X's) and by Cheney (x's). I cid not examine the Mobil lines.

High' frequency seismic sources (Acoustipulse and Uniboom) are useful for examination of shallow structures. However, records fror these sources are generally of limited value for the study of deep horizons since the hign frequencies recorded (850 - 2900 Hz for the 1978 U.S.G.S lines) are rapidly attenuated and arrivals from greater depths are masked by seismic reverberations in the water layer (water multiples). Thus, attempts with such high frequencey methods to map deep faults observed on the Mobil r.ultichannel lines in the southernStrait of Georgia have not met with muca success. For examole, the ' single fault which Dobrin orcooses to cecss Mobil lines U70-4, U70-5 and U70-7 must cross the 1978 U.S.G.S Line A at time 1943, Line B at time 2210 and Line C at time 1742.  !'y e/. amination of the U.S.G.S. records reveals nc evidence of faulting at Pese times. This observation neither proves nor disproves the interore-

ti',n of Cobrin. The records add no new information except that at these

. : . ions, no vertical disclacemnet of surficial sedirents acoears to be indicated to depths of accroximatelv 75 meters. If tnese records cannot cetect deen faults in the vicinity of where they are seen en .ultichannel rec:rds, then they cannot be used to prove or disprove the existence of sther proposed faults.

I should like to point out a number of lateral uncon c ornities which 2 .: . un en the 1978 U.S.G.S. lines, only one of which has been discussed earlier. Abrupt ternint. tion of nearly horizontal reflectors occur cn

' ine A at times 1852 and 2005 and on Line C at time 1652. Taking into a: count the 6:1 vertical exaggeration of the records, the surfaces have dios of 20', 34 and 16 respectively. These dios are rather shallcw to be intercreted as normal faults in the classical sense, but : suggest t ac they may. recresent faulted and eroded cr stesoly er:ded baserent against which sediments have accumulated. These steenly dioping s.r' aces usually face toward the channels where accumulations of nearly

• ri nntal sediments are observed. The nature of these dipoing surfaces ree:s to be addressec more completely since the unconfomities at tines 1H2 and 2005 on Line A are in line with faults procosed by Cneney to lie resrectively between Lummi and Clark Islands and between Lumni Island ar.c Frances Point.

A second type of lateral change in the sub-bottom characteristics ris;.lts in the abruot disaopearance of reflectors between time 2030 and 2:;5 cn Line A and 2112 and 2140 on Line B. Inis effect is not seen on sijacent soarker lines and may be an effect in the sub-bottom which affects hign frequencies, but its cause is unknown and should be exclained.

Line 12, which lies between W70-7 and W70-9 was searchec for shallow fEts out there is little evidence for them cn the records even though several faults have been prooosed to cross this line. Major offsets

<n;an to exist in the section are not visible on the shalloa ecords and t . efcre lie ce':w the death cf penetration cf the starker cr boomer

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1423 147

Mu er:us s a'.1 faults are seen on BBN lines in the vicinity of S*. cia and Patas !slands. These are probably related to a small platfern tihich acccunts 'or the islands.

Seisric lines in tne vicinity of Vendovi Island also shcw little evidente f faultinc in either the basement or the overlying sedimentary layers. Tnerefcre, if facits exist through this area, they would have to cut strata wnich lie deeper than these records show.

My c:nclusion is that faults with major vertical offsets are not seen on the shallow penetration records. It is my opinion that the high frequency seismic methods are inadequate to locate the trace or traces of a major fault cr faults proposed to exist in the area. Therefore, extracolaticn of these faults into adjoining areas or interpolation betaeen widely soa:ed seis iic lines is a matter of considerable speculation.

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