Open-file 78-591 Open-file 78-591

UNITED STATESDEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

THE UTILITY OF PETROLEUM SEISMIC EXPLORATIONDATA IN DELINEATING STRUCTURAL FEATURES

WITHIN SALT ANTICLINES

By

S. L. Stockton and A. H. Balch

CONTENTS

PageAbstract------ -- --- ------ -_ ---- ---_ ---_ 1

Introduction-- - -- -- - ---- - -- 2

Regional geologic setting-- ------ -- ______ ________ 2

Stratigraphic framework and associated acoustic properties--- --- ---- - 4Investigative procedures-- _______ _ -_______-_-____ , _____ ___ _ _ 4

Seismic field parameters------- ---------- ---- ____ ___- - __ _ ____ 4Seismic data processing--- ______ ____ __ _ . __ __ 12

Interpretation of seismic data-- ------ --_ - 18

Structural framework------ ---- -- -____ _____ _ _ ____ _ _________ }Q

Computer subsurface modeling--- ------- _____ ____ ______ _____ 22

Recommendations---------- -------- _---_-____-___-__ ______ __ _______ ___ ______ 34Vertical seismic profiling------------ -- ------------ --- - - _______ _____ 34

Surface seismic methods--------------- ----------------- -- -- ----- _____ 34

Borehole geophysics----------- ---------------- __-____-__ ____________ _ ___ 35

Conclusions- - -- ---- -- -- ______ 35

References cited------- ______ __ _ __ _____ _ 36

ILLUSTRATIONS

Page

Figure 1. Map showing the approximate limits of the Paradox Basin and the outline of

the Salt Valley anticline study area, Grand County, Utah----- _--__ 3

2.--Location of seismic profile line C over Salt Valley anticline-- -------- 6

3.--Multifold, common-depth-point seismic section along profile line A------- -- 7

4.--Multifold, common-depth-point seismic section along profile line B- -- 8

5. Multifold, common-depth-point seismic section along profile line C- - 10

6.--Structure contours on top of salt of the Paradox Member of the Hermosa

Formation in the Salt Valley anticline--- -- - -- 11

7.--Flow chart of the Paradox Basin data-processing sequence -- 13

8. Frequency analyses-------- --------- --- - _ - - _____ ___ __ __ 14

9.--Diffraction of seismic energy- --- - -- -- 15

10.--Migration of seismic events due to dipping beds- -- ---- -- 15

11. Synthetic seismogram from an acoustic log---- ---- --- - ------------ -_ 16

12.--Section B with synthetic seismogram from figure 11 -____ _ 17

13. Section A. Lines show configuration of faults and salt-anticline

units on flanks--- _---___ ____- ____ __ __ _ _ ___ 19

14.--Section B. Lines show configuration of faults and salt-anticline

units on flanks--- -- -- -- ------ __ 20

15. Section C. Lines show configuration of faults and salt-anticline

units on flanks- --- --- - - -- -- - ---- 21

ILLUSTRATIONS Continued

Page

Figure 16.--Models of thin beds within salt showing thickness and velocityvariations---------------------------------------------------------------- 23

17.--Synthetic seismic sections of thin beds------------------------------------- 24

18.--Model of interbed in salt to observe the effect of the folding ofthin beds in the seismic section-------- ------------------------------- 25

19.--Depth to seismic foci in buried and folded salt interbeds------------------- 26

20.--Model of a typical subsurface salt ridge derived from a composite of

three seismic profiles, borehole information, and a hypothesis about the nature of the shallow faulting---------------------------------- 27

21.--Normally incident ray paths generated for the subsurface model offigure 20------ --- - --- 28

22.--Salt ridge and asymmetrically folded interbeds------------------------------ 29

23.--Snythetic seismic record section showing salt interbeds----------------- -- 30

24.--Synthetic salt-ridge seismic record section produced from theinterbed reflections only---------------- ------------------------------ 31

25.--Salt-ridge model with less complexly folded interbeds----- --_-------_-,--- 32

26.--Less complex synthetic seismic record section showing interbedreflections only------- -------------------------------------- ______ 33

TABLES

Page

Table 1.--Paleozoic and Mesozoic stratigraphy and acoustic properties of thenorthwest Paradox Basin---------------------------------------- ---- ----- 5

2.--Summary of seismic field parameters----------------------------------- ---- 9

UNITED STATESDEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

Mail Stop 954, Federal Center, Box 25046 Denver, Colorado 80225

THE UTILITY OF PETROLEUM SEISMIC EXPLORATIONDATA IN DELINEATING STRUCTURAL FEATURES

WITHIN SALT ANTICLINES

By

S. L. Stockton and A. H. Balch

' ABSTRACT

The Salt Valley anticline, in the Paradox Basin of southeastern Utah, is under investi­

gation for use as a location for storage of solid nuclear waste. Delineation of thin, nonsalt interbeds within the upper reaches of the salt body is extremely important because the nature and character of any such fluid- or gas-saturated horizons would be critical to the mode of emplacement of wastes into the structure.

Analysis of 50 km of conventional seismic-reflection data, in the vicinity of the anticline, indicates that mapping of thin beds at shallow depths may well be possible using a specially designed adaptation of state-of-the-art seismic oil-exploration procedures.

Computer ray-trace modeling of thin beds in salt reveals that the frequency and spatial resolution required to map the details of interbeds at shallow depths (less than 750 m) may

be on the order of 500 Hz, with surface-spread lengths of less than 350 m. Consideration should be given to the burial of sources and receivers in order to attenuate surface noise and to record the desired high frequencies.

Correlation of the seismic-reflection data with available well data and surface geology reveals the complex, structurally initiated diapir, whose upward flow was maintained by rapid contemporaneous deposition of continental clastic sediments on its flanks. Severe collapse faulting near the crests of these structures has distorted the seismic response. Evidence exists, however, that intrasalt thin beds of anhydrite, dolomite, and black shale

are mappable on seismic record sections either as short, discontinuous reflected events or as amplitude anomalies that result from focusing of the reflected seismic energy by the thin beds; computer modeling of the folded interbeds confirms both of these as possible causes of seismic response from within the salt diapir. Prediction of the seismic signatures of the interbeds can be made from computer-model studies.

Petroleum seismic-reflection data are unsatisfactory for mapping the thin beds because of the lack of sufficient resolution to provide direct evidence of the presence of the thin beds. However, indirect evidence, present in these data as discontinuous seismic events,

suggests that two geophysical techniques designed for this specific problem would allow direct detection of the interbeds in salt. These techniques are vertical seismic profiling and shallow, short-offset, high-frequency, seismic-reflection recording.

INTRODUCTION

The Salt Valley anticline, a major linear salt diapir in the Paradox Basin of south­

eastern Utah (fig. 1), is currently under consideration by DOE (U.S. Department of Energy) as a possible location for the isolation of solid nuclear waste. The Salt Valley anticline

is well qualified for further study as a potential waste emplacement site because it possesses certain favorable geologic characteristics and is readily accessible by land transportation.

Intensely deformed, thin (5-70 m) interbeds of anhydrite, dolomite, and black shale sometimes occur within the halite core. Some of these beds are known to contain water and hydrocarbons under high pressure. The major objective of this investigation was to deter­ mine whether the thin beds could be detected by using seismic-reflection data. Additional objectives were as follows:

1. The interpretation and correlation of existing seismic data, borehole data, and surface geology (fig. 2), and the analysis of the growth history of the Salt Valley anticline.

2. The determination of the minimum acceptable seismic resolution needed to detect thin beds and the determination of the maximum thickness detectable by conventional methods.

3. The isolation and identification of seismic signatures from thin beds in an otherwise homogeneous medium.

4. The proposal or recommendation for further studies, if appropriate.Three multifold, 50-km-long seismic-reflection profiles, which were obtained from the

petroleum industry, were extensively processed and interpreted. A subsurface ray-trace

computer-modeling system was used to formulate hypothetical models of both the internal and external structure of a salt anticline. The resultant synthetic seismic sections were used to test the validity of the interpretation and to design a seismic technique especially tailored to the marker-bed problem.

REGIONAL GEOLOGIC SETTING

The Paradox Basin (fig. 1) covers approximately 25,000 km2 in southern Colorado and Utah. The basin extends northwestward from the Four Corners area.

In the deepest part of the basin, the evaporite units have flowed extensively so that

they now form a northwest-trending belt of salt-cored anticlinal structures (Cater, 1970). The Salt Valley anticline lies at the northwest end of this belt. Solution by ground water near the surface of these linear diapirs has resulted in the collapse of younger strata into long graben structures. In the Salt Valley anticline, the caprock of insoluble residue lies at the surface and extends downward for 200-250 m. Additional details on the geologic setting can be found in Kite and Lohman (1973) and Gard (1976).

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STRATIGRAPHIC FRAMEWORK AND ASSOCIATED ACOUSTIC PROPERTIES

The post-Mississippian lithologies in the salt-anticline region and their acoustic

properties are summarized in table 1. The Paradox salt is the principal diapiric rock in

the Paradox Basin. Original thickness of the salt may have been 1,500-2,500 m (LeFond,

p. 21, 1969), but solution and plastic flow have completely eliminated the salt in some

areas while concentrating as much as 4,300 m of halite in the adjacent anticlinal cores.

INVESTIGATIVE PROCEDURES

The investigation of the Salt Valley anticline region was divided into four phases:

1. Acquisition of seismic-reflection data and analysis of field parameters.

2. Processing of experimental seismic data in order to obtain maximum resolution from

the existing seismic profiles.

3. Creation of synthetic seismograms from integrated acoustic logs and their

correlation with the seismic data and other subsurface well-log information.

4. Computer modeling of hypothetical subsurface geology.

Seismic Field Parameters

Three multifold common-depth-point seismic profiles, one directly over the Salt Valley

anticline, were obtained from the petroleum industry (fig. 2, profile line C). Because

lines A and B (figs. 3, 4) are proprietary data, their locations are not shown on figure 2.

The principal area of interest was the deeper flank and basal Mississippian sediments rather

than the interior salt structure. A summary of the seismic field parameters for the three

profiles is presented in table 2. The seismic sections for lines A, B, and C appear in

figures 3, 4, and 5, respectively. Line C (fig. 5) is over the northwestern end of the Salt

Valley anticline where the northwestward-plunging trend is interrupted by a small saddle and

a structural knob (figs. 2, 6). The low-frequency, band-limited appearance of the seismic

data is a result of a Vibroseis 1 source having a sweep of 45-6 Hz. The graben structures in

the Mississippian acoustic basement at the base of the salt were the primary target. Lack of

resolution of closely spaced events at these shallower depths is evident throughout the

section; the edge of the salt diapir is poorly defined. The length of the geophone spread,

as well as the distance to the near trace (>800 m) on line C, further suggests that an oil-

exploration seismic profile is of limited value in the shallow resolution of intrasalt

structure but will provide an excellent diagnostic tool in analyzing the growth history of

the Salt Valley anticline as related to regional tectonics.

Sections A (fig. 3) and B (fig. 4) were recorded using dynamite as a source. The broad­

band nature of this impulsive source is readily apparent, and mapping of very shallow (150-

300 m), very thin (<50 m) reflector beds is possible. Shallow reflections on the flanks of

the salt are clearly defined. A major disadvantage of the use of dynamite or other high-

energy impulsive seismic sources is the inherent lack of control of the exact frequency and

*Use of brand names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

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Table 2. Summary of seismic field parameters

[Leaders ( ) indicate no data available]

Field parameter

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Geophones

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Spread

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Figure 5.--Multifold, common-depth-point seismic section along profile line C. Areas circled are short-segment, en echelon seismic events.

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Figure 6.--Structure contours on top of salt of the Paradox Member of the Hermosa Formation in the Salt Valley anticline. (Modified from Hite and Lohman, 1973.)

11

phase character of the source determined by ground coupling, charge size, depth, and type

of source.

Seismic Data Processing

Extensive analysis of the seismic data was performed in order to establish the processing parameters and sequence shown in figure 7.

Figure 8^A shows an autocorrelation amplitude spectrum of a selected trace from line C. Several such analyses were used in determining the optimum deconvolution parameters and as a quality-control check on the effectiveness of the deconvolution in attenuating redundant

information on the seismic traces. Severe 13-Hz ringing is evident on the seismic traces. A deconvolution-operator length of 0.140 seconds, which spanned roughly 1.5 cycles of the

autocorrelogram, was selected. This operator was thought to be long enough to attentuate short-period redundancies on the data, but short enough to avoid attenuation of closely spaced primary events with similar spectral character. The results, as observed in the frequency analysis of figure 8B_, show excellent spectral whitening and attenuation of the 13-Hz ringing. There are still some longer period redundancies, but common-depth-point

stacking normally will effectively reduce their amplitude.After deconvolution and bandpassing filtering, a velocity analysis was performed at

close intervals along each profile in order to determine the optimum velocity to use in the computer-stacking routines. For nonparallel, dipping reflector beds, like those in the Salt Valley anticline, stacking velocity is related to, but not equal to, actual rock velocity.

Variations of thickness and velocity in the near surface can often introduce time shifts (static shifts) on the recorded trace. They can destroy the desired reflections when the data are stacked. With these data, as near-surface layer characteristics were not accurately known, a processing technique known as "automatic residual static corrections" was used to statistically derive the static time shifts in the data. After automatic static corrections, a tenfold stack was performed on line C; twelvefold stacks were performed on lines A and B.

The recorded time of a seismic reflection from a steeply dipping layer cannot be directly converted to depth because the time does not represent a vertical travel path. The processing techniques of migration (figs. 9, 10) correct for a nonvertical travel path. In order to enhance coherent, dipping events within a range of geologically possible dips, "a coherency filter" routine was applied to the data after migration.

An acoustic log obtained near the profile lines was used to estimate the velocities of the horizons in the stratigraphic column. A synthetic seismogram (fig. 11) was then created and compared to the processed seismic sections. As illustrated in figure 12, the synthetic section compares rather well with the processed section. This gives us a high level of confidence that the data have been processed correctly and that the various seismic horizons are properly identified.

12

CDP Sort

Elevation

Statics

Frequency Analysis

DCONBroad-bandFilter

Velocity Analysis

NMO

Initial mute

Diffraction

Migration

2-DCoherency Filter

Figure 7.--Flow chart of the Paradox Basin data-processing sequence. CDP, common-depth- point; DCON, deconvolution; NMO, normal move out; 2-D, two-dimensional.

13

CDP NO. 280 WINDOW NO. 1

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flMPLITUDE SPECTRUM

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FREQUENCY - HERTZ

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Figure 8.--Frequency analyses. /\, before deconvolution of indicated interval and IS, after deconvolution of indicated interval.

14

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Figure 9.--Diffraction of seismic energy. A^, depth section with sharp acoustic contrast at D, diffracting seismic energy in all directions and B^, corresponding seismic time profile.

Apparent position of dipping reflector A-B.

True position of dipping reflector as seen after migration.

Figure 10.--Migration of seismic events due to dipping beds.

15

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SYNTHETIC

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20000

150°°

,0000

5000

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TWO-WflY TIME

Figure 11. Synthetic seismogram from an acoustic log. Pm, Molas Formation; Php, Paradox Member_of Hermosa Formation; Ph, Hermosa Formation; PC, Cutler Formation; T , Moenkopi Formation; Je, Entrada Formation.

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INTERPRETATION OF SEISMIC DATA

Figure 12 shows the approximate correlation between the acoustic log synthetic and the

seismic-reflection data. This correlation provided the basis for identification of the various reflectors on the seismic sections. This correlation, in turn, allows the interpre­ tation of the growth history of the structure, including an analysis of the tectonic control of sedimentation and the relationship between the salt diapir and the adjacent structure. Further analysis of the data yields possible characterization of the intrasalt structure.

Interpreted sections are presented in figures 13-15.

Structural FrameworkThe average seismic velocities in sediments adjacent to the Salt Valley anticline are

close to that of pure salt. Owing to velocity differentials, little distortion of the

Mississippian base-of-salt can be expected on the seismic time sections. The presence of a deep horst-graben system under the salt (fig. 15) lends credence to the concept of structural influence on, if not initiation of, the diapirism. However, the dramatic thickening of flank sediments toward the synclinal axes on either side of the salt ridge indicates positive

sedimentary control of the upward movement of the halite. The data from profile line B

(fig. 14) show roughly 1.6 s of seismic two-way traveltime from the top of the salt, which is near the surface, to the base of the salt. This time difference indicates that more than 3,500 m of salt is at the anticlinal axis. This thickness implies approximately" the same amount of differential thickness in the adjacent sediments. The thinning toward the anti­ cline of the upper member of the Hermosa Formation, as indicated on lines A and B (figs. 13, 14), suggests that diapirism had already begun by Middle to Late Pennsylvanian time. Initial salt diapirism was structurally influenced; but continued rapid sedimentation, associated with the removal of salt by flow from the flanks of the salt swell, maintained the upward momentum of the salt mass.

Some continuous seismic events appear within the salt on the processed sections. Several short isotime lineups of seismic events are apparent in a general en echelon pattern

in the salt. There are no continuous indications of the presence of a reflector bed in the shallow salt; however, if the folding is of the suspected degree of complexity, these reflector beds could be connected by vertical segments (giving no reflections) or they could be transposition folds. A similar intrasalt reflection character is evidence on lines A and B, although noise obscures these reflections. Another possible origin of these coherent seismic events, which will be discussed in more detail in the modeling section of this report, is that the segments represent buried foci due to concave-upward folding of the high acoustic contrast interbeds. In any case, these phenomena of the seismic sections are, at best, indirect. The exact shape and location of the interbeds is only implied. Because the absence of interbeds, rather than their presence, is of paramount importance in searching for a nuclear waste emplacement site, these phenomena alone may provide sufficient evidence to suggest the presence of an interbed. High-resolution seismic profiles could provide a more direct indication of the nature of the interbeds. Closer spacing of seismic surface

18

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ntic

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ts on

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anks

. Pm,

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Fig

ure

1

4.-

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ctio

n

B (in

terp

rete

d).

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ine

s sh

ow configura

tion

of

fau

lts

and

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nticlin

e

units

on

flanks.

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atio

n;

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ber

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tle

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stations would allow better coverage of complex folding in the subsurface, and higher frequencies should permit finer resolution. Computer subsurface seismic modeling was used as a consistency check on the geologic interpretation and as a determinant in designing a new seismic survey to locate the marker beds.

COMPUTER SUBSURFACE MODELING

Figure 16 shows our thin-beds model and figure 17 shows the resulting synthetic model.

The top two wedges of low- and high-velocity material-vary, respectively, from 0 to 150 m in thickness, and the wavelet closely approximates the impulsive source signature of sections A and B. It can be seen that constructive reinforcement of the wavelet from the salt-interbed boundary with that from the interbed-salt boundary obscures the true nature of the reflectors.

The distortion of the interbeds in the salt may provide a promising indirect indication of their presence. Figure 18 shows a hypothetical model of a severely folded thin bed

imbedded in a saltlike medium. Buried focusing of the reflected energy, due to the presence of thin beds at depth, results in the seismic signature shown in figure 19. This indication of folded interbeds has the advantage of being relatively independent of the thickness of the interbed. This is an indirect, rather than a direct, method of detecting the presence of interbeds in the salt; and it depends on the geometry and degree of closure of the folds. If the shape of the interbed distortion conforms to the assumptions of Hite and Lohman (1973), this technique for mapping the interbeds is feasible.

Figure 20 shows a typical salt-ridge subsurface model derived from a composite of the three seismic profiles, the borehole information, and assumptions on the nature of the shallow faulting. Figure 21 shows the normally incident rays generated for the subsurface model of figure 20. The refraction of the rays at each interface has the effect of scattering the reflected energy from a small portion of the subsurface over a large portion of the time section. The many oblique ray paths indicate that many seismic events shown vertically beneath a source point will represent reflections from a considerable distance off the vertical.

The hypothetical cross section of the Paradox Valley anticline (Hite and Lohman, 1973) was modeled using three asymmetrically folded thin beds within the salt (fig. 22). Second- order folding, laterally varying velocity and density, were superimposed on these thin beds. Structural complexity of the interbeds increased toward the anticlinal core in the manner described by Kupfer (1968). The resultant synthetic seismic section is shown in figure 23. Figure 24 shows a synthetic section produced from the reflections from the interbeds only. Energy dispersion and diffraction are evident throughout the section. Many of the folds were tighter than the 100-m trace interval used in the model, making identification of the buried foci impossible. Diffraction migration of these data would aid the detection of the interbeds, but would require a high degree of precision in the determination of the migration velocities. The diffraction migration of the synthetic data would collapse the diffractions from the tight interbed folding and yield an intrasalt en echelon pattern similar to that observed on the real seismic data. (Compare circled areas on figs. 4, 24.)

22

6,096 m/sec

2,590 m/sec

2,590 m/sec 6,096 m/sec

500 _J METERS

Figure 16.-Models of thin beds within salt showing thickness and velocity variations The velocity of the salt is 4,480 m/s.

23

500METERS

17.--Synthetic seismic sections of thin beds.

24

J KILOMETER

Figure 18. Model of interbed in salt to observe the effect of the folding of thin beds in the seismic section.

25

KIL

OM

ET

ER

Fig

ure

19.-

-Depth

to

se

ism

ic

foci

in

buried

and

fold

ed salt

inte

rbeds.

2

KIL

OM

ET

ER

S

Fig

ure

20.-

-Model

of

a ty

pic

al

subsu

rface

salt ridge

derived

from

a

com

posi

te

of

thre

e

seis

mic

pro

file

s,

bore

hole

in

form

ation,

and

hyp

oth

esi

s a

bo

ut

the

na

ture

o

f th

e

shallo

w fa

ultin

g.

KIL

OM

ET

ER

S

Figure

21 .--Normally incident ra

y pa

ths

gene

rate

d for

the

subs

urfa

ce m

odel

of

fi

gure

20,

KIL

OM

ET

ER

S

Figu

re 2

2.--

Salt

rid

ge a

nd a

symm

etri

call

y folded interbeds.

(Modified

from

Hite

and

Lohman,

1973

.)

1.3

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.

t.S

.

1.6

1.7

1.8

1.9

2.0- p.

t

la. i

,'fJQ

n

J K

ILO

ME

TE

RS

Figure 23

.--S

ynth

etic

seismic

record se

ctio

n sh

owin

g salt in

terb

eds.

3.0

5.0

KIL

OM

ET

ER

S

Fig

ure

24.-

-Synth

etic sa

lt-r

idg

e

seis

mic

re

cord

se

ctio

n

prod

uced

fr

om

the

inte

rbed

reflections

on

ly.

Are

as

circle

d

have

sim

ila

ritie

s

to

ob

serv

ed

se

ism

ic

eve

nts

on

re

al

da

ta (f

ig.

5).

KIL

OM

ET

ER

S

Figu

re 25

.--S

alt-

n'dg

e mo

del

with

less co

mple

xly

folded interbeds.

3.0

KIL

OM

ET

ER

S

Figu

re 26. Les

s complex

synt

heti

c seismic

reco

rd section

show

ing

interbed reflections

only

.

The second salt-ridge model has a subsurface flank identical to that of the model shown

in figure 20 but has less severe folding of the interbeds near the anticlinal axis (fig. 25).

This type of folding would be expected if the principal stress axis were vertical rather than horizontal. The synthetic section, again showing only reflected energy from the inter­

beds, is presented in figure 26. Although the seismic response shows a proportional reduction in complexity, diffracted energy still dominates the seismic section. If, however, the shallower interbeds are less complex, mapping may be possible using conventional or nearly conventional seismic-reflection techniques.

RECOMMENDATIONS

Mapping of shallow, structurally complex reflector beds in an area such as the Salt

Valley anticline requires a custom-designed, integrated geophysical program that will incorporate state-of-the-art techniques, such as vertical seismic profiling; shallow

reflection surveys, possibly using a source with a known signature; and a detailed borehole survey. Potential field methods, such as gravity and magnetic anomaly mapping, would have limited usefulness owing to the suspected broad area! extent of the interbeds and the complexity of the rocks near the surface. Recent advances in seismic data processing can yield highly quantitative analyses of the subsurface and provide greatly expanded resolution of subtle acoustic differences in the geologic profile.

Vertical Seismic Profiling

Until recently, very little has been published on the use of vertical seismic arrays in

the detailing of shallow acoustic boundaries. Studies by Gal'perin (1974), Wuenschel (1976), and others have only recently begun to exploit the advantages of VSP (vertical seismic profiling) as an exploration tool. Some of the advantages of VSP are as follows:

1. Improved signal-to-noise ratio. Placing either the source or the receiver arrays or both in a borehole below the inhomogeneous near-surface reduces wave scattering often encountered when using surface or near-surface arrays.

2. Broader bandwidth is obtainable owing to better rock coupling of both the source and the receiver. Expansion of the frequency bandwidth results in higher resolution, allowing the separation of closely spaced or thin reflectors on a seismic time section.

3. Directional sensitivity of three-component geophones in vertical seismic profiling would make possible three-dimensional analysis of the seismic response. Direction of the reflector from the receiver, dip information, and the determination of reflector characteristics from amplitudes and modes of wave propagation could be obtained.

Surface Seismic Methods

Because the principal depth of interest for nuclear waste emplacement is shallow (less

than 1,200 m), the surface configuration of the seismic spread should have a short offset and a small interval between geophones. When the angle of incidence of the seismic wave with the reflectors is nearly normal, the effects of refraction and mode conversion of the seismic

34

energy are minimized. If the reflected seismic wave is approximately planar when it

reaches the detectors, separation of the reflected energy from noise traveling horizontally

(along the surface of the ground) is simplified. A spread length on the order of 350 m or

less would insure that the angle of incidence would be quite high for a flat reflector. The

short offset would farther insure that the Rayleigh and Love waves generated by the source

would not interfere with shallow, reflected energy. The depths of interest on a shallow

seismic profile would be represented in less than 0.600 s of two-way time on the seismic

section.

Small-interval short-spacing of geophones would have two advantages:

1. A closer reflection-point spacing on the subsurface profile would be equal to one-

half on the geophone spacing. This would allow detection of smaller anomalies.

2. More geophones would allow higher common-depth-point multiplicity and provide better

noise cancellation. Exact field-recording configurations should be established after

sufficient noise analysis to determine the optimum source and geophone configuration for best

noise cancellation.

Seismic-reflection data should be capable of providing very high resolution of the

subsurface. For an interbed 15 m thick and an interval velocity of 3,000 m/s, a seismic

pulse that is 0.01 s wide would be desirable. For such reflectors, frequencies of at least

100 Hz would be desired. Burial of the geophones would optimize ground coupling, and high-

frequency seismic data would be detected.

Borehole Geophysics

All of the surface seismic profiling and vertical seismic profiling could be effectively

tied to the geology by logging wells drilled to the depth of interest.

CONCLUSIONS

From this study, we obtained the following results and conclusions:

1. Analysis of approximately 50 km of conventional seismic-reflection data using surface

arrays and both impulsive and controlled sources indicates that the potential exists for

mapping shallow, high-acoustic contrast, isolated thin beds in a homogeneous salt.

2. Computer ray-trace modeling has aided in the identification of frequency and

spatial-resolution limitations that are present in most petroleum seismic data. For more

detailed mapping of very thin (5-70 m), intensely folded interbeds at depths of less than

750 m, frequencies on the order of 500 Hz and surface-spread lengths of less than 350 m are

recommended. Furthermore, considerations should be given to burial of both the source and

receiver arrays in order to attenuate surface-related noise.

3. Correlation of the seismic-reflection data with available well data and surface

geology in the area of the salt-cored anticlines in the Paradox Basin of southeastern Utah

confirms a complex, structurally initiated, basement-fault-control led salt diapir. Its upward

flow was influenced by contemporaneous deposition of Permian continental clastic sediments in

the adjacent synclines, which were formed by removal of the salt. Complex inhomogeneities

35

near the surface, caused by solution of these diapirs near the crests, are responsible for

distortion of the seismic response.

4. Evidence exists that thin interbeds of anhydrite, dolomite, and black shale are

mappable, either as anomalous amplitudes due to focusing at depth or as short, discontinuous

segments. Computer modeling of folded thin beds in salt confirms both of these as

possible causes for the intrasalt seismic response observed on the seismic-reflection

profiles.

5. The following refinements of existing seismic-reflection methods and their

integration with other geophysical techniques should allow more direct identification of the

interbeds in salt: vertical seismic profiling and a shallow, short-offset, high-multiplicity,

high-frequency, seismic-reflection survey.

REFERENCES CITED

Cater, F. W., 1970, Geology of the salt anticline region in southwestern Colorado, with a

section on Stratigraphy, by F. W. Cater and L. C. Craig: U.S. Geological Survey

Professional Paper 637, 80 p. [1971].

GaVperin, E. I., trans. by A. J. Hermont, ed. by J. E. White, 1974, Vertical seismic

profiling: Society of Exploratory Geophysicists Special Publication 12, 270 p.

[Originally published as "Vertikal'noe seismicheskoe profilirovanie," Moskva, Nedra,

1971.]

Gard, L. M., 1976, Geology of the north end of the Salt Valley anticline, Grand County, Utah:

U.S. Geological Survey Open-File Report 76-303, 35 p.

Hite, R. J., and Lohman, S. W., 1973, Geologic appraisal of Paradox Basin salt deposits for

waste emplacement: U.S. Geological Survey Open-File Report, 75 p.

Kupfer, D. H., 1968, Relationship of internal to external structure of salt domes, in

Diapirism and diapirs--A symposium: American Association of Petroleum Geologists Memoir

8, p. 79-89.

LeFond, S. J., 1969, Handbook of world salt resources: New York, Plenum Press (monographs in

geoscience), 384 p.

Wuenschel, P. C., 1976, The vertical array in reflection seismology some experimental studies:

Geophysics, v. 41, no. 2, p. 219-232.

36