ML20235F845
| ML20235F845 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 01/21/1976 |
| From: | Crane P PACIFIC GAS & ELECTRIC CO. |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20235F769 | List: |
| References | |
| FOIA-87-214 OL, NUDOCS 8709290340 | |
| Download: ML20235F845 (16) | |
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Q Regulatory 00 pet Elg PACIFIC GAS AND E LE C T RIC C O M PANY
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Docket. No. h2 7 %nh
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Dear Sir:
,t Enclosed in support of our application for an operat.tus license for Units 1 and 2 at the Diablo Canyon Site are 7 copies of the following:
I "A Discussion of the Application of the Migration Process to Western Geophysical Company Seismic Reflection Line W74-12 in the Vicinity of the Hosgri Fault Zone, in the Area Offshore from the Diablo Canyon Power Plant Site."
l 1
This material responds in part to Item 2.22 of the request for addi-tional information transmitted with the Nuclear Regulatory Commission's letter dated November 14, 1975.
The enclosures contain proprietary geologic data, and it is requested that they be withheld from public disclosure pursuant to 10 CFR 2.790.
This data was developed at considerable expense by original research work performed by Western Geophysical Company.
It is of the type that this company customarily maintains in confidence and withholds from public disclosure.
The release of this proprietary data would grant competitors access to the information without incur-ring any expense.
Kindly acknowledge receipt of the above material on the en-closed copy of this letter and return it to me in the enclosed addressed envelope.
8709290340 870924 Very truly yours, PDR FDIA CONNDR87-214 PDR rw Enclosures l
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CC w/o encs.:
ASLB Parties
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A Discussion of the Application of the Migration Process to Western Geophysical Company Seismic ReGection Line 12 in the Vicinity of the Hosgri Fault Zone, in the Area Off-shore from the Diablo Canyon Power Plant Site Introduction The basic concepts of " Common Depth Point" (CDP) seismic reDection data acqui-sition, " velocity analysis", and the transformation of seismic data from a timewlistance domain to a distance-distance domain will be discussed in this report. These concepts will facilitate a detailed discussion of observed seismic reGection velocities and the ef-fects of velocity variations on the apparent location of geologic structures associated with the Hosgri fault zone adjacent to the Diablo Canyon nuclear reactor site. ReGectors observed on seismic section W74-12 will be used as examples to quantitatively show the errors in reDector positioning resulting from geologically reasonable variations of RMIL_
velocity values extant in this area.
Common Depth Point Processinst The techniques used by Western Geophysical Company to determine offshore sub-surface velocities are an integral part of the Common Depth Point (CDP) staMng proce-dure used to improve the signal-to-noise ratio of seismic reGection data. The CDP pro-cess was first introduced by Mayne in 1962. The technique improved S/N ratios by signal averaging, but did not lower the resolution of the seismic data (a problem intrinsic to all previous signal averaging methods). The CDP method utilizes a geometry of shot points and receiver positions such that a single reflection point may be recorded along several different ray paths (see Figure 1). The seismic traces recording a common depth point are moved along a vertical time axis until identical reflection points are aligned. The traces are then summad (" stacked"). Ideally, the reflectors add constructively, and random perturbations sum to a low average valve. The not signal enhancement is equal to the square root of the number of traces summed (Mayne,1962).
Velocity Analysis L
Accurate velocity data are crucial in normalizing CDP travel timts to provide proper reflector align==t before stacking. Figure 2 shows how a high frequency signal can be degraded by stacking with time misalig==ents as small as 4 milliseconds (Dee,
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1975), CDP " gathers" are constructed to facilitate the accurate determination of ve-I locities. A " gather" is a
.efall the traces from the same common depth point arranged with the shot times (t = 0) forming a horizontal 11ne (Figure 3). CDP traces gathered and arrayed in order of increasing distance from the energy source will show a common resection point as a hyperbola. The hyperbola characteristic of each renec-tion point can be fitted by the equation (Cook,and Taner,1968):
T,2 = T,2 X
+ p Where T
= the travel time to the reRector T, = the normal incidence travel time X
= the distance frcm the trace to the energy source Y
= the RMS velocity to the reflector.
Thus, it can be seen that the curve fitting process will yield a unique Y to each reflecto/
beneath the common depth point.
Hyperbolae formedbynear surface reflectors will have more curvature than thos'e formedby relatively deep reflectors. Hyperbolaewithyronounced curvatures can be more accurately fitted by a mathematical equation than can those with small curvatures. The amount of hyperbolic curvature displayed on a " gather" panelwul also depend on the length of the geo-phone spread; the larger the spread, the larger the amount of the curvature displayed. Hence, the depthto which a velocity analysis is reliable is directly proportional to the spreadlength.
Velocity Spectral Analysis A " Velocity Spectrum" may be constructed from a number of velocity analyses.
Such a spectrum may be used to determine V and the relative reliabuity of V throughout a time-distance domain seismic record section.. Cook and Taner (1969) concisely des-cribe the procedures used to construct a Velocity Spectrum.
" Velocity Spectral analysis computes and displays the coherent power among a set of common depth point traces (CDP gather) according 2
to various hyperbolic curves as given by the equation T,2 2
X
=To+7 AVelocity Spectrum is computed at a given normal incidence time by hyperbolic searches covering a gate of 50 milliseconds made in velocity increments of 100 feet per seconi ' Itis is then repeated down the n
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record at 24 millisecond intervals. The amplitude of the resulting trace, i
which is indicative tW e6herent power in the spectrum, is measured by a multi-channel filter such as is schematically shown in Figure 4 This figure shows three hyperbolic curves and their corresponding power on the spectrum.
Figure 3 shows anactual example taken from the Gulf of Mexuo in i
an area where reflections are plentiful and mostly primaries."
i Velocity Spectra may be degraded by a number of factors. The lack of coherent reGectors or acoustic anisotropies which disrupt the signals to distant geophones will render the Velocity Spectrum unusable. Diffracted seismic returns and intra-bed mul-I tiple reverberations will cause multiple spectral peaks and/or a broadening and reduction of the power within a spectral peak. Dipping beds will cause the Velocity Spectrum to indicate anomalously high velocities.
Time-Distance to Distance-Distance Transformation The position of a reflecting bed in a time-distance domain will be different from I
that of the reflector in a distance-distance domain. De basic reason for this is Glus-trated in Figure 5.
More complete discussions of the transformation from one domain to the other are given by Slotnick (1959) and Gates (1957). The term generally applied to 1
the transformation process is migration.
l Slotnick (1957, p 63-68) outlines a simple geometric procedure for migration.
I The process involves calculating the true dip of the center of the reflector from the for-mula
\\
sine (= kAx Where o( = the dip of the reGector
)
At = the travel time difference from one end of the reGector to I
the other Ax = the horizontal distance spanned by the reDector on the time-distance record section.
The true position of the center of the renector wul then lie along the radial SS' as shown on Figure 5.
The distance SO will depend upon the RMS velocity M determined I
l wa o
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l for that reflector from a Velocity Spectral analysis.
Using Slotnick's apoa-technique, it can be seen that the position of a mi-grated reflector changes as V is varied. The position change will be a function of V i
I and a<. or j
4 H k,o(.) = ( V) f sin M Where AH is the horizontal change in position of a reGector, AV is the change in Y, and t is the two-way travel time to the center of the reGector.
From this equation, it can be seen that increasing V, t, orecwill increase displacement of the renector from its unmigrated position. Thus, in geologic terms, the higher the rock velocities, the deeper the reflector and the greater its dip, the greater the change in the reDector position as a result of the migration process.
Interpretation of Seismic Profile W74-12 Reliability of Velocity Data
'Ite fleid procedures used in shooting Line W74-12 ded a 46 fold multiplicity of common depth points his imparts a high sensitivity to the Velocity Spectra. In general, the field and processing procedures used by Western in deriving their velocity data are such that the data are of the highest quality obtainable with present production technologies.
It is possible to obtain approximate error limits for the velocities used to migrate Line W74-12. '1his may be done by re-interpreting the velocity spectra for this line and by comparing the values derived from the Western data with velocity data obtained from ustrontos Line PB-4. This line is parallel to and 0.5 miles south of Line W74-12.
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These are the two analyses nearest to the Hongri fault zone. Also drawn on the Veloc Spectra are Earth Sciences Associates
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(ESA) re-interpretations of the Western velocity functions.
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The ESA velocity functions indicate lower velocities than those used by Western, tr.rt nowhere do the ESA s4tffate by more than -10% from the Western values.
Spectrum 268 is amB'iguous in the time interval between 0.4 and 0.8 seconds.
The first small peak of th$ broad double peak in this region is interpreted as a multiple reflection. The Western velocity values definitely appear to be about 8% too high in this region. In the time interval from 0.8 to 1.6 seconds, ths V values are well defined.
The precursory peaks in the interval from 1.1 to 1.2 seconds are probably caused by multiple reverberations associated with the reflectors in the 0.8 second range. The broad trailing peaks in the 1.2 to 1.4 second interval are inferred to originate from dif-fracted energy associated with the buried reverse fault west of the Hosgri fault. Below 1.6 seconds, the quality of the velocity spectrum diminlabes as the reflectors in this time interval display less coherency than the reflectors in the overlying strata. Below 2.0 seconds (the region of the Acoustic Unit A-2, A-3 interface), no useful velocity data are observed.
Velocity Spectrum 220 displays are of lower quality than those of Spectrum 268.
Below 0.5 seconds, the peaks are very broad, possihty a result of multiple reflections and spurious diffracted energy associated with the Hosgri fault. The ESA interpretation considers velocities below 0.5 seconds to be oflow reliability and data below 0.7 seconds not to be useful.
Velocity data produced by Aquatronics, Inc. for the processing of Line PB-4 show substantially higher velocities than are obtained from the Western data. The velocity functions used by Aquatronics are shown in Figure 6.
The functions labeled NMO 580 and NMO 291 approximately correspond to Western Velocity Spectral analyses 220 and 268, respectively. In comparing NMO 291 with Western Spectrum 268, the Aquatronics veloc-icy is found to be at most 37% higher than the Western values at the 1.0 second time mark.
Comparison of NMO 580 with Spectrum 220 indicates a maximum discrepancy of 21% which is reached at the 0.5 second mart, the point below which lack of reflector coherency is inferred to render both Western and Aquatronics data unusable.
1 The intrinsic reliability of the Aquatronics velocity data is lower than that of the Western data. The data collection procedures used by Aquatronics are directed toward the acquisition of shallow penetration, high resolution data. As a consequence, they cannot utilize the long geophone spreads necessary to obtain 1arge CDP multiplicity. The Aqua-
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I t onics data are 6 fold stacks. Therefore, the divergence of ray paths used to compute their Velocity Spectra are ughJess than those used by Western. The reliability of the Aquatronics velocity is correspondingly diminished, particularly at deeper pen-etrations.
In summary, by comparing Aquatronics velocity data to Western velocity data, an upper bound of velocity error may be postulated, and by re-interpreting Western data, a lower bound may be established. The Aquatronics data set an upper error 4
boundary of +37%. Re-interpretation of the Western data sets a lower error boundary of -10%. The reliability of the Western velocity values used to migrate Line W74-12 is thus postulated to be +37% and -10%.
I Effects of Velocity Variation on the Migrated Location of a Re0ector The following discussion illustrates the effects of velocity errors on the migra-tion of reflectors within the Hosgri fault zone. Additionally, the discussion shows quan titatively the effects of the velocity errors on two hypothetical faults, one of which is co-incident with the Hosgri fault.
The locations of both the buried reverse fault west of the Hosgri fault and the Hosgri fault itself have been accurately plotted directly from the time-distance record section of Line W74-12. These faults have been located by plotting the crests of diffrac-tion cones formedwhen acoustic energyisinferredtobe diffractedfromthe terminations of 1
beds against the faultplane. The crests of the diffractionhyperbolae mark the position of the bed terminations, which act as point diffraction sources, in their proper lateral po-sition on the time-distance plot (Tucker and Yorjston,1973). Thus, the locations of both the buried reverse fault, and the part of the Hosgri fault below 0.5 seconds, are i
mapped on Line 74-12 by a method that is not subject to the velocity-dependent position y
errors that affect structures which must be mapped on the basis of reflector positions.
The uppermost part of the Hosgri fault has been located in the same manner on high
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1 resolution seismic lines adjacent to and coincident with Line W74-12 ne following values and methods were used to evaluate the migration effects of velocity errors. As previously stated, the velocity errors were considered to be -10%
and +37% of the values used by Western Geophysical Company to migrate Line W74-12.
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The algorithm used to migrate the reflectors is as outlined by Slotnick (1959).
The shift in post osateflector resulting from the migration process increases t.
with the depth of the ren
- r. Three deep renectors were migrated using the nominni, and maximum and mininiGm expected velocities. The results of the analysis are shown in Figure 7. Clearly, as expected, the deepest reDector, Number 3 has the widest range of positions. If the V value used was 37% higher than the nominal value, then the center point of the reGector would change about 3500 feet horizontally and 2200 feet ver-tically from its unmigrated position. Also, notice that on Plate I the eastern termina-tion of ReGector 3 appears to be related to the buried reverse fault. After migration, it can be seen that this cannot be the case, as the actual spatiallocation of Reflector 3 is over a mile west of the fault. This example shows the importance of considering the effects of migration when viewing time-space domain record sections.
If the buried reverse fault west of the Hosgri fault was located on the basis.of tb position of adjacent reDectors, its poaltion would change as shown in Figure 6. The exact numeric value of the change in spatial location of the fault depends strongly on th location of the reDectors used to demark it, and on the assumed direction of inclination of the fault. However, in general, lowering velocities will move a fault of this orienta-tion shoreward, and raising velocities wul move it seaward relative to its unmigrated position The example is used to give a quantita-tive picture of the magnitude of the general pr em of migration errors in the vicinity of the Hosgri fault.
The effects of velocity errors on the migrated positions of structures diminishes as the depth of the structure decreases. This is illustrated by the small change in the position of the Bosgri fault shown in Figure 8. The total change in surface position of the Hosgri ikult ranges only over 100 feet. This small error range is due in part to the nearly vertical orientation of the Hosgri fault. If the structure had a greater dip, in-creases in velocity values would tend to migrate the fault seaward, and velocity de-i creases would relocate the fault shoreward. Even if reDector dips were raised to 30 (the range of the steepest dips that can be recorded by the seismic resection technique),
the displacement of the fault structure would not exooed 500 feet. Thus, provided the structores of interest are shallower than about 1500 feet, position errors associated cuA c,t a n e n e Acea,iata.
t with velocity (migration) errors are relatively small.
It is again stress
. the Hosgri fault in the vicinity of Line W74-12 is post-tioned on the basis of di on cones. But, evenif its position was determined by the positioning of adjacent refiectors, its near-surface location would not vary at the most
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by more than 500 feet and probably not more than 100 feet from its presently plotted surface location.
References Cited Cook, E. E., and M. T. Taner (1969). Velocity Spectra and heir Use in Stratigraphic and Lithologic Differentiation. Geophys Prospecting, vol.17, no. 4, p 433-448.
Dee, M. (1975). Digital CDP Procedures Provide New Capability for High Resolution Seismic Data. Paper presented at the Seventh Annual Offshore Technology Con-ference, Houston, Texas, May 5-8.
Gates, J. P.,1957. Descriptive Geometry and the Offset Seismic Profile. Geophysics vol. 22, no. 3, p 589-609.
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-f Mayne, W. H. (1962). Common Reflection Point Horizontal Data Stacking Techniques.
Geophysics, vol. 27, no. 6, p 927-938.
Slotnick, M. M. (1959). Lessons in Sels:nic Computing. Pub. Society of Exploration Geophysicists, 268 p.
Tucker, P. H., and H. V. Yorjston (1973). Pitfalls in Seismic Interpretation. Soc.
Exploration Geophysicists Monograph Series, no. 2, 50 p.
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l l Figure 8. nis diagram shows the relative change in position caused by velocity variatino of two reflectors within the Hosgri fault zone. Since the reflectors are at a relatively shallow depth, the position of a fault structuzw based upon their location will not change greatly. Notice also that the steep dip of the fault causes positt,on of the fault for each of the velocities used to be indistinguishable close to l
one another.
_____ - - __