ML20235B726
| ML20235B726 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Bodega Bay |
| Issue date: | 12/17/1985 |
| From: | Lawson R, Stickel J DAMES & MOORE |
| To: | |
| Shared Package | |
| ML20234A767 | List:
|
| References | |
| FOIA-85-665 NUDOCS 8709240205 | |
| Download: ML20235B726 (39) | |
Text
. - m - _
i
- ZFC2I 07 SEIS4IC SU MEl
,m FECPOSED KUCLEAP PCWER ELAr
~
BCDEGA BAY, CALIFCR' IIA FOR TTIE PACIFIC GAS & ELECTRIC CO.
INTRODUCTION his report presents the results of our seismic refraction-type survey at Bodega Head near Bodega Bay, California. Se purpose of this report is to provide general subsurface infomation Mich vill aid in plant location selection and layout. Foundation recommendations are beyond the scope of this report but shall be discussed at the completion of the second phase of our work.
SCOPE
-s, Se seismic field work consisted of exploring bedrock depth and configuration by running seven continuous seismic lines. Lines A, B, C,an! ?
vere run in a north-south direction and noms 1 to the alignment d the p:ropoce?.
plant sites. Lines D, E, and 0 vere run along the ali6nment of tvo consid :32.
locations for the proposed plant cites. To63ther, the seven eeismic li:.es amounted to 9250 lineal feet of survey. Se locations of these seven lines are shown on the Plot Plan, Plate 1. Be results of the seismic survey ara l
shown on Plates 2 through 6 and 8 through 9, Seismic Sections. Gese plates have a Velocity Section and a Subsurface Section for each d the su:veyed lines.
hey show the velocity data used in the seismic computations and the corre- j
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sponding interpretation of the subsurface. Se procedura and equipment used l 4
s l 1
FIRESTOB5-665 PDR
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in a scismic survey are described in I.ppendix A. The basic theery uced in computing and interpreting seisnic tolocity data is precented in Append!x B.
In addition to the seismic survey, one test borirg was drilled to
' correlate the velocity data. 2hc information cbtained frem the test boring >
1s shown on Plate 7, Los cf F.cring.
GISCC DATA The seismic data and our field reconnaissance indicate that the site is blanketed by sandy soils which are underlain by granitic bedrock. Four velocities are predominant 1000 feet per second--loose surface soils 1600-2200 feet per second--soils (some of these coile are partially cemented
,m
) 4000-5000 feet per,second--vater bearing soils (probably similar soils to above)-
8000-10,000 feet per second--bedrock .,
Because velocities vary largely as the4" eld density of the material, -
the values obtained sppear to be reasonable. The proximity of the' San fairees Fault has caused sc=e local fracturing and altering of the bedrock, which. ,.
accounts for the range of velocities. Seismic results are generally censidered to be 90 per cent accurate. Because of the good correlation of seismic bedrock depth with the test boring, it,is concluded that the depths shown on the Seismic Subsurface Sections are within the~ above mentioned accuracy.
Bedrock consists of quart::-diorite which is. very hard, but locally I f$actured and altered due to seismic activity in the San Andreas Fault Zone. -
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noms u s e.cne ,
te r ur s-evss s s- m.t ue
,+ _ _ - - _ - _ _ _ _ _ . _ _ . _ _ _ _ _ . _ . _ _ _
7 3 ,
.s Bodega Head is believed to be sn uplifted block accociated with old vertical movement of the San Andreas, recent movement has been in a horizontal direction. .
The overburden consists of varying thicknesses of candy soils ,
believed to be old beach sand which, although cemented locally, can be ripped. '
The cemented portions should stand on slopes of.one-half horizontal to one vertical; however, shoring and devatering vill probably be needed for the uncemented, waterbearing soils.
It is anticipated that any excavation into bedrock would require blasting.
CONCLUSIONS The Seismic Subsurface Sections indicate that the bedrock expcsed w.) '
on top of the two small hills dips to depths of 80 to 100 feet in the suddle between the two hills. The bedrock exposed near ground surface on the cliffs to the vest dips nearly t'o sea level on the eastern cliffs. ,This bedroch configuration-resembles a bev1 which has been cut in half.
\
Based on the seismic resultSj it is our Conclusion that there are J no major faults crossing the area investigated. 'fne s=all scarp-like feature near Station 9+o0 on Line C could indicate a fault; however, there is nc evidence of this feature crossing any of the other seismic lines. Minor faults are believed to be present as ajesult of movement along the San Andreas Fault system;. however, it is our opinion that seismic activity e.long these minor faults vould be secondary in importance to seismic activity in t .
the San Andreas Fault Zone itself. We believe that by placing structures l
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,' so that they do net stradils ma.jor faults, the probability' o^ significant stnteturni Osmage from the San Andreas Fault system is remote during the life a* the proponei construction.
She following Plates and Appendices are attached ac.d cc:clete this report:
Plate 1 - Plot Plan Plate 2 - Seismic Sections - Line A Plate 3 - Seismic Sections - Line B ,
Plate 4 - Seismic Sections - Line C Plate 5 - Seismic Sections - Line D Plate 6 - Seismic Sections - Line E Plata 7 - Log of Boring Plate 8 - Seismic Sections - Line F Plate 9 - Seismic Sections - Line G Appendix A .- Method of Performing a Geophysical Suivay Appendix B - h thematical Theory Used in Interpreting Geophysical j Data l l
)
Respectfully tabmitted, DAMES & MOCRB, John F. tickel, .
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34 Q. M Ap d ET STR E ET SAN FRAN CISCO fl. C ALIFO RNI A . DOUGLAS 2 6807 e e PARTNERS: WIL L8 A M W. M C O st E = WILLlaN EN M E BOLL A S S O C I AT E F- O H A R *. E S L . N I C H O L S
- ROBE 8" 3. O A R 't A G H April 20, 1960 Dicific Gas & Electric Co. -
245 Market Street -
San Francisco 5, California
- Attention: Mr. Gordon Coltrin )
l Gentlemen: '
Six copies of the corrected Plate 2 are enclosed which are to be substituted for Plate 2 now bound in our report, " Report h of Seismic Survey, Proposed Nuclear Power Plant, Bodega Bay,
- 4. California, For the Pacific Gas & Electric Co.". Plate 2, entitled,
" Seismic Sections, Line A" was corrected because the intersections L of Line A vith Lines D and E vere incorrectly plotted. Sin'ee the (
bedrock elevations on Lines D and E are considerably more accurately l
) determined than the depth determinations at Stations 4+00.and 9+00 l on Line A, the bedrock elevations from Lines D and E vere used.
L When it was found that the intersections were.misplotted on Line A, 'l this necessitated a change in the bedrock profile.
Seismic data are usually considered to be 90 per cent j accurate. However, when the depth to rock rapidly increases or ,
decreases along a seismic line, as in the case of Stations 4+00 and 9+00 on Line A, the accuracy of the rock profile may be as lov as -1 50 to 60 per cent. Ibr this reason, the mere reliable data from Lines D and E vere used to complete the bedrock profile for Line A.
. Yours very truly, -
1 DAbES & .H00RE. l Robert D. Darragh RDD-GEB BB (6 Copies Submitted)
Attachment ,
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APPENDIX A .
ME:THOD OF PERFORMING A GEOPHYSICAL SURVEY
. GENERAL Geophysics is a tool used to determine t'he subsurface geologic structure of the earth without drilling and extracting cores for visual ex-amination. There are four major methods of perfoming geophysical surveys:
the electrical method, the gravitational method, the magnetic method, and the seismic method.
-., Although there are two vays of making a seismic survey, either by reflection or refraction techniques, the refraction method is particularly adapted to civil engineering purposes for determining the shallow depths to a consolidated layer or bedrock. The seismic-refraction method of performing a geophysical survey is therefore described here. A' description of the physical principles and mathematical calculations employed in the refraction method are described in Appendix B.
The physical principle that materials with different elastic ,
properties will transmit energy at different velocities is used in a seismic refraction geophysical survey. Briefly described, a seismic survey consista l of applying energy to the earth by mecas of dynamite explosions and measuring the time intervals between firing the shots and reception of the r'efraction
~
energy vaves. Dy computing the velocity at which a vave travels through a subsurface material, the trained geophysicist obtains a physical value which f
'] 1 can be used to interpret characteristics of the subsurface.
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g . A-2 FIELD PROCEDURE ON IAND
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Seismic surveys can either be perfomed on land or water. In l
perfcm1Ds a survey on land, the first consideration is to maintain an accurate l I
survey of the ground eurface by obtaining elevations and locations' of the ]
points to be explored. This is done by ordinary surveying methods.
After the' points to be explored have been located in the field, a sho+. hole some three-to ten-feet deep is drilled'at a select.ed location. A seismic cable is then laid out en the ground through the points to be explored.
At each of the exploration points an instrument is attached to the seismic cable to receive the impulses from the dynamite charge. Tbese instruments are called geophones or detectors. In order for the geophones to receive the full impact of the elastic impulses from the charge, it is necessary for them -
to be placed below grass-root depth so they are in contact with solid grcand, The end of the seismic cable is then plugged into a seismograph. Tnis
]
instrument makes a photographic record of the impulses received by the geo-phones.
I After the geophones and cables are connected, the dynamite is l placed in the shot nole and blasting wires are connected. Tne size and depth cf the charge is predetermined by the geophysicist or the man su ervis-ing the field operations. The si::e of the charSes vary from single electric .
blasting caps to perhaps s few po*mds of dynamite. When all of the connections are made and the circuits have been bested, the man chooting the dynamite starts counting, end at a predcte minei u.mber the s,elsregre.ph is c+.,rted and the charge is detonated. h' hen cuf"icient C.sta have been obtain, the seismo-graph is stopped cud the photognpnic record processed.
DELD PROCEDURE ON WA".'ER -!
l,) A seismic survey perfomed on water must have an accurate control 1
of the shot points and the geophone cable, The location of the shot points is i
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accomplished by means of t$iangulation by two transits at predetermined
, locations on shcre. To conduct a survey on a river the 6eophone cable (float-ins type) is held in an off-shore position by one boat located at the upstream
.end of the cabic. The boat operator locates his boat in an upstre'am and down-stree::1 position by means of pairs of targets on shore at appropriate locations.
The boat is located in a cross-stream position by mecn s of a sextant. This location is rechecked by one of the shore transits. The two transits are in constant' radio communication with the towin'g boat. In a bay or area where there in no current or flow direction to the water to stretch the cable to Ats full length in a straight line, it is necessary to use two boats, one at either end of the geophone cable. The location of the cable and boats in this type of survey area are accomplished in the same mnner as in a river (
survey.
_) Two shooting boats are necessary for one shot point at either end of the geophene cable. This is done in order to obtain total equal travel times from each end of the cable to the other end of the cable. These shots are exploded within three to five seconds of each other. ' Enc dynamite shot locations are a minima distance of five feet beneath the water surface, 200 feet from the shooting boat, and 25 feet frem the clesent detecter.
The 12 geophones are'cuspended five feet beneath the surface of the water along the Seophone cable in order to receive the full impact of ,
the elactic wave.
The tvo shooting boats are in telephone co.;;:asier.tien with the seismograph or towing boc.t. The first charse is always set off by the d:r.rn-stream shooting boat or the shooting boat furthest from the seismograph i
boat. The second charge is alway: set off by the upstream shocting 'ocat. l The charges required for Vater vork are clightly lar6er than those for land work.
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m 'After each shoot'er hao primed and located his shot and all circuits have been tested, the seismograph is started and the first charge detonated.
Whensufficientdatahavebeen$btainedtheseismographcperatorsignalsthe shooter and the second charge is detonated. When sufficient data have been ~
obtained the seismograph is stopped and the photographic record processed.
FIELD EQUIPMENT
, The seismic receiving and recording equipment used in these surveys consists of 12 electromagnetic, induction-type geophones, multi-conductor geophone cables, either. land or water type, and a portable 12-trace, battery-powered refraction-reflection type seismograph *. The cables vary in length from 100 feet to 1,350 feet. The spacing of the geophones determines which cable is to be used at any 1ocation. )-
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The seismograph consist's of two main parts, an amplifier and a -
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recording oscillograph. The a=plifier serves to increase the induced electro-l magnetic voltages from the geophones and to improve their quality through automatic-volume-control circuits which balance the wave a=plitude. In 1
addition, the amplifier contains a volt-ohmeter, which is used to test the
]
continuity of the cable-geophone circuit before the charge is detonated.
l The recording oscillograph makes the record of the waves . received by the geophones. 'Ihere are three main parts in the oscillograph: a bank of 12 ]
ballistic galvanometers, a timing system and a photogr,aphic paper drive system.
The galvanometers receive the amplified geophone signals.and direct 12 pin-points of light onto a moving six-inch vide strip of photographic paper.
i Sima.ltaneously, the timing system directs lines of light onto the paper at intervals of 0.01 seconds. The timing system is governed by a temperature-m j *All of the seismic equipment used in this survey, except the land cables, was manufactured by Electro-Technical Iabs, Inc., modified to Ihmes & Moore l
specifications.
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compensated tuning fork and is operated independently from the paper drive '
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,, Other field equipment used in the surveys consists of a daylight ,,,
developing tank, detonators or blasting machines, field telephone and port-
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able FM radio transmitters and receivers.
FIELD OFFICE PROCEDURE If necessary, an office can be set up in the field so the data ,
obtained can be put into a preliminary form which will be of immediate use. {
Preliminary office work consists of picking the time differences between the -
' instant of charge detonation and the receipt of the refracted waves, and the
. plotting ,of these times as ordinates against the distances of the geophones
- from the shot point as abs'cissae. Average straight-line velocity curves can
'?j then be drawn between the points. Utilizing these data, computations can be made as described in Appendix B, to determine the depth to bedrock at the points explored. Preliminary profiles made in the field show bedrock ;
i elevations and ground elevations and are immediately available to interested parties in the field.
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, MATHEMATICAL THEORY USED IN INTERPRETING GEOPiriSICAL DATA *
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1' l GENERAL
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i The energy from a dynamite charge,;progagated in the form of elastic j impulses', obeys the laws'of light wave motion; therefore, it is possible to l 1
analyze the; impulse travel from a dynamite charge by comparison to a traveling' ,,
light wave,' and to derive time-distance relationships. 5 E LIGHT WAVE ANALOGY ,
' Referring to Fig. 2 of Page B-2, let point 1 be the source of light;
^
Vy and V2 thevelocitiesofvaveprogagationinmediumsIandII,ST'thesur-face of contact or interface of the mediums: E , E ,' F , E ,' E , 5 , and M ,
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the several wave paths; and i and r the angles of incidence and refraction, l . .
l respectively. .
l Light, traveling at a velocity, V , in medium I along path E will i 1 1
change direction as it enters medium II and assume a new velocity, V , along 2 .
path E. This change in direction is noted by the angles i and r, and conforms '
to,Snell's Inv. Mathematically, Snell's Law can be written as:
.V 1,V2 (1)
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. sin i' sin r or ,
Y1 sin i , (2)
., sin r V )
2 There is a specihl case where r = 90 degrees when the refracted ray
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W - D A 's A
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. E O s V F
> - A l
.W C I
- M
's D S I N E A S s
a e
zn S N
)
E
/
a' e
t V O 4 S
- s e I s M
/=
e s /s R T p
a-A R ' v T
- U C G 'o/ o c (- C A t A o a t e
1
- R E F I
r .
Y v D m v a t <.
A n m A " . %
y, u. -
C E L n N R f
s ,
U e a o -
2 < M m
a g .
r =
- A
- r R t
s n
.u T1. '
. =
r
- T R s i.. O i e S O
- ,' o. e F r a v o H m
c o ID F
, T 1 t x.
i p c
E
[g.. P y_ /
s c E D
a e u. M eF .
F _
5 .
TI C
S I
7( s t.t s
s A -
B -
s tg,t-O
- ,' g. . .
_ 'c e
- I _ :. d s ,
-C l,^-
ii l
- IL_
g,gg . EO0aN
,e '- y a";;::
4 .; ! : i!
. l l'
B-3 ,
l
. i i
1 travels parallel o and ve'ry close beneath the interface. .In this case, ;
equation (2) becomoo: -
h 5 sin i c Y1
=-
(3) 1 V2 or transposing, equation (3) becomes: .
i N
1 ic = Arcsin V2 i ;
i where e1 is the " critical angle" of incidence. ,
Considering 2V to be greater than V y , it' vill be found that when the wave H impinges the interface at the critical, angle, it vill then con-tinue to travel in the interface at the higher velocity, Y , until reinforce-ment occurs through in-phase contacts with reinforcement waves at AC and AF of the low velocity medium. When this reinforcement occurs, the wave vill be bent into its former medium, I, and continue traveling along paths 5 and M at angics equal to 1 and at velocities equal to V . -
If the total travel time, t, of a single light particle along the path ABCD and the distance X are known, an expression for the distance H can be determined. Thus, let:
~
BC (3) te AB q +g+5 then from the geometry of Fig. 2, it can be seen that:
E=5= H I cos i ,
l and ,
BT = X - 2H tan i;- j further, from equation (4): '
- a V1 1 sh i c . l s
y ;
j
> l' i l
i
. om..se mecn, i 6 t wnpa s .e s eas1**is t s s
s -
j B14 1 1
)
I i
't If these values and trigonometric identities are substituted in equation (5),
(. the result is:
2H (V -
V)! (6) t= p+
2 Vy V2 or H= t- X__ 1 2) (7)
V 2
2 21 l 2(V 2
- V1 ) /2 l
. l
. The refraction seismic method utilizes the time-distance relation- )
ship of equation (7). Transition from the li6ht vave analogy to the refrac-tion seismic method requires only substitution of overburden for medium I, ledge rock for medium II, an explosion of dynamite for point A, Beophonc positions for points D and G, ledge rock surface for interface EP , ground 1
surface for AD3, and ground vibrations or clastic impulses for the wave path representation of light vibrations.
BASIC DEPTH FCWJIA By reference to Fig. 2 on Page B-2, it is seen that vaves can travel j 1
from a shot point to any point on the surface in a straight line throu6h f medium I. Within a certa'in distance from a shot point only waves traveling in medium I can be received. When the distance of a receiver from the shot point i
1 is such that BC is zero or more than zero, refracted waves can also be re- ,
ceived. As this distance is increased, a point is reached where the straight-line waves and the refracted waves arrive at the same time. This distance is called the " critical distance". It is improbable that a geophone would ever j
be placed exactly at the critical distance and hence it is necessary to l
-) determine this distance from the' data obtained in the field. The method of 1
O A mel l' 'k C 91 C 1 ??!*
. se
\
m_____________._
SE5&d x:: D.: W,% % :s:J. :d a : % .c.c,u t .; G ' % w . & a ::: n:,.:; s n a., w ..e.:L.a . . - .
t -
t.
r.
g; , $ . .. . -
f J B-5 , j t
m determining the critical distance is illustrated in-Fig. 1 on Page B-2. The time that it takes for the wavec to reach each geophone is plotted against ,
i the distance from the shot point. Tne distance X1 , shown in Fig. 1, is the critical distance. It is located at the intersection of the curves V 1and X1 V_.
c At the critical distance, t1= , but also at the critical distance y1 .
t 1 = t and X1
= X and substitution can be made in equation (7) giving:
Y Y2 g X
. 1 (8)
V 2 21 3
v) k1 2(V 2 -
V ) /2 and reduction of (8) yielde:
H X Y2~ Y 1 (9) 2 V 2+ V1 In actual field practice the dynamite charge ic buried, and point A ly is below the surface at A'. Taus, it is necessary to consider the depth of l the snot point, h, in the depth formula determination. !hking allowance for the depth of the charge gives the final Basic Depth Formala: j
/
Y
\
H=hd h+X 2~ 1 (10)
V V 2+ 1 .
ADJUSTME:NT FORM In instances where points show excessive deviations from the time-distance curves, it is. necessary to make interpretations by use of an Adjust- l ment Forn:ula. Referring to Fig. 4 of Page B-2, the irregularities in the J 1 edge rock surface are indicated by a lead and lag of time in Fig. 3 Because
, increment of the time lead or lag, Et, can.be picked directly frem the plot, the amount of elevation difference, 75', can be calculated.
) With reference to the enlarged section in Fig. 4, let:
J L
D A fvi t S C MOOf* C
. , . - , s .r. e p *
- W 55 S diL A$ D & 2 E d i $$ 5 1$5.'1 $ $ 5 L V ' 5 'l $.5 5 0 D N k $.Th % . & 5 E a s ! M bV'*O'0:'*E' 'bh'5:L._1'l aj '
m- .
R. s, .....__..o._. . . ,: a _ _ . . . . . . . , . . . . . ._ ,. . . ... t..
- v. C . .* ...
y .
h 36
]
I
. t I
.- = JC .. CC, s_.- -
g7 7-2 y 1
I and- l-
.- .i
. , - JC ___CC' sin i n
(where ,
1 V '-
1 sin i = (4) l lI V 2 .
i
'i then
, .I .
, Vy _ [
, E, CC ,, ge ,
yd V
.I 2 1 i
t' l Or .
t
-dtV . i 1
M' = a (11) i
. .. s V i
i-11- 2 I
6 l V i j 8.
2
. l The vertical. displacement' or elevation difference,M', then is given by: l-i ~
~ M' = M' cos 1 (12)
Where j y2 )1/2 cos i = 1- 1 .
.yd 2 ./.
ti
, ( l
,.The result'is the Adjustment Formula:
II 1
h
-dt V i.
JC' = 1 (13) ,
,t y'
2 - 1/2 ,.
1_ l Y
3
(
s e ,
I t
l D i i
. . 1
'namrso moonc ;
. - ... v....... . ... .
L_____-._____---_..-- _ . _ - . - . - . _ . _ -
M i[23AO.iid5%dh2iiD.$UCiGN21$lisdOEdii d !YA*SN N OdfSbih"EU - ^' @ 'L as. - . '
.t', Q ,' '
o -,
1 1 .
il ' e j
3
.O 4
I I
4, -.- ' ,
B-7 i b.
,4 4 j.. .
')
i i
l
,.- VALTDI1II CP THE FORM'JLAE , '
'l Certain assumptions with respect to the velocity of energy travel-- ,
i
)indifferingmediumsdefine'thevalidityoftherefractionseismicformulae.
t Paramount among these is the assumption that the difference in the velocities R in the different mediums is great enough so that the critical distance can I,t i
~hlbereadilydeterminedfromthetime-distanceplots.
4
-ooo- ;
li l
il P
.'l 6
'6 m >
l J' ,.
9 E
D y '
l' if I'
I!
Ii 0,
l
' I i l l .
) \
l
. 1
~
anmu,a moone
. :.,.e,e..r a4 1
%,3 2 2f.hi 2 h 3E O hii h = 5 D l b '4A:.E.1&5i h 2%;5 ?.G &2hl.yl2 $ G M:5< .2brisiLOr '<* 's V" Q% :',-
e.
- l,
.gN, l
2 4
DOCKET 50-205 AMENDMENT NO, 3 PART IV C 6
A
) .
t 6
e
?%
h= -vo s a 4 ** = **v =*u- 3...%,m...,9,,,,,, . , , , , , , . . . , , , , , . . _ , , , , , , , , , . , , _ .
.__.. _ _ _ _ . . _ _ . . _