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Seismic 17
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40
Angle of IncidenceO
FIG. 4. P-wave reflectivity at two shale/sand interfaces.
Monzl 1, shale/sand; model 2, anisotropic shale/isotropic
direction for the highestV,, (model 3). Ho wever, with low
VIA mo del l), the P-wave velocity decreases o a m inimum
of 8.8 kft/s at 35 degrees before rising to the 12.5 kft/s
velocity. The effect on th e SV-p hase velocity is even more
pronounced or the low VI3 case, ncreasing 0 percent rom
0 to 4 2 deg rees rom vertical. This behavior controls the
critical angle position.
The secondexample Figure 4) shows he
P-P
reflectivity
of two shale/sandmodelsdemo nstrating he effect of taking
into accou nt anisotropy in the s hale. The is otropic shale
model model 1) shows eflectivity increasingwith offset. n
this case, the shale P-wave velocity is lower than the sand
velocity-a situation common to the Texas Gulf Coast
region. How ever, for an aniso tropicshalewith a horizontal
P-wave velocity 20 percent higher than the vertical
P-
velocity, the horizontal velocity is highe r n the sha le than
the sand and the P-P reflectivity decreaseswith offset
(model 2), reversing the trend expected under isotropic
assumptions.
In conclusion, the tw o exa mples of P-wa ve reflectivity
show that anisotropy must be taken into account n ampli-
tude-offset tudies nvolving shalesand that the presenceof
small amounts of g as in a shale (low V,J could produce
dramaticchangesn reflectivity at incident anglesof explora-
tion interest.
References
Brown?R. J. S., and Korringa, J., 1975,On the depen@ce of the
elasticpropertiesof a po rous ock on the compressllxhty f the
oore fluid: geophysics4 0. 608.
Daley, P. F., aid ‘Hron, g ., 197 7, Reflection and transmission
coefficients or transversely sotropic media: Bull. Seism. Sot.
Am., 67, 661.
- 197 9, Reflectionand transmission oefficients or seismic
waves n ellipsoidally nisotropicmedia:Geophysics, 4, 27.
Gassmann, ., 1964, ntroduction o seismic raveltimemethods n
anisotropicmedia:Pure and Appl. Geophys.58. 63.
Jones,L. E. A., and Wang, H. F., 1981,Ultrasonicvelocities n
Cretaceou s hales rom the W illiston Basin:Geophysics, 6, 288.
Wide-Angle Reflections: A Tool to
Penetrate Horizons with High Acoustic
Impedance.Contrasts
s17.7
Jannis Makris and Jens Thiessen, Univ. of Hamburg , West
Germany
In
the autum n of 1983 a se ismic wide-angle reflection
survey was carried out in a complexarea in the Gulf of Suez
using ocean bottom seismog raphsOBS). The stratigraphy
and velocities are well kno wn only at bo reholes hat bad
reachedbasementhighs.Due to the highoil productivityand
econom ic significanceof this area, extensive conventional
reflection seismic surveys had been performed during the
last decade. They had failed, how ever, to penetrate the
Miocene evaporiteswhich are characterized y high acous-
tic impedanceand are underlain by low velocity layers. In
order o overcom e his difficulty we proposed nd performed
a wide-angle reflection seismicexperiment. The main idea
was to exploit the intense increase of reflected seismic
energy at the wide-an gle ange of incidencenear the critical
point of reflection. This type o f su bsurfacemappingcannot
provide information with resolution com parable o normal
steep-angleseismic techniques. It is, how ever, the only
physical method that can be deployed under the above
mentioned conditions. During the experiment, 120 OBS
positionswere observedand the data were evaluated with
ray tracing techniques, using traveltimes and am plitude
computations.This technique enabled us to delineate the
structures at the crystalline basem ent and perm itted the
compilationof a regional baseme ntmap.
The survey area lies in the middle of the Gulf o f S uez
directly off the Sinai coast. It is characte rizedby strong
tectonizedblocks of thick Pliocene and Miocene sediments
covering a thinner seriesof Eocene to carboniferous ocks.
Inside the Miocene layers, the am ount of
anhydrite and salt
beds increases, so that observed P-wave velocities reach
values of up to 6 km/s. These high velocity formations are
followed by marls, shales,and sandstoneswith low veloci-
ties ranging between 2.5 and 3.4 km /s. In this extreme
situationof several eflectingsalt and anhydritebedsoverly-
ing low velocity ayers the reflectedamountof steep ncident
seismicwave s s so high, that in reflection seismicsections
arrivals from deeper boundariesare weak and maskedby
noise and multiples.
During the last decade he oil industry has spent a great
amountof time and effort to overcome his difficulty without
success. n the following we show that seismic energy
reflectedunder wide-angle ncidence that is, totally reflect-
ed beyond he critical angle of incidence)can penetratesuch
complex structures providing nformation about the deeper
parts of the basins. The basic idea behind the concept
presented n this paper s that even small amountsof energy
penetrating through the high impedance layers may be
observedunder critical angle of incidence see Figure 1).
In a joint venture between Deminex, Essen, and the
Institute of G eophy sics, University of H amb urg, a wid e-
angle reflection experiment using the OBS was designed,
which according o our estimates and by considering he
above mentioned physical facts should provide seismic
information from the deep s ituated crystalline basem ent n
the Gulf of Suez. The program was su pported by the
Ministry of Science and Technology of the Federal Republic
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Seismic
17
673
refracted
longitudinal
----- reflected
. refracted
transverse
-._, _._
reflected
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60°
,
critical
angle
D
FIG. 1. Theoretical energy distribution or longitudinalplane
waves incident on a high acoustic impedance layer, after
Richards (1960). (VI = 3.9 km/s, p1 = 2.4 and V2 = 6.4 km/s,
pz = 2.65).
J
FIG.
2. Locationof seismic ines and ocean bottom seismo-
graphs (OBS).
of Germany. The instrumentationand survey techniquewas
mainly the same as describedby the authors n their paper
presented t the 53rd Annual S EC Meeting in 1983.
The survey
A net of profiles was chosenso that the research argets
were denselycoveredby seismic nformation seeFigure 2).
The lines were laid approximately 3 km apart, running
parallel and perpendicular o the main strike of the struc-
tures. On eac h ine up to 15 OBS were deployed n constant
intervals of 2 km. The shootingwas performed with 4 air
guns of 8 1each, fired at intervals of 100 to 150 m along the
lines. The field work w as completedwithin three we ekswith
a total of 275 km seismic lines observed and 120 OBS
sections ecord ed.For positioningwe used a high precision
Syledis navigation system.
The seismic ections
Wide-angle reflected seismic arrivals need no t be proc-
essed by CDP techniques since the exploited amount of
seismic energy has maxim um values under critical inci-
dence. Furthermore, due to the fact that these events have
very long travelpathsa common-depth-point annot be de-
fined. Instead eachOBS recordsa sectionwhich s compara-
ble to a single shotdisplay of a very long spreador streamer.
The processing f the data was thereforestraightforward nd
fast, including the following optional tasks: predictive de-
convolution, filtering, mixing, traveltime reduction, and
display.
Figure 3 displaysa typical section,recordedby OBS
1
on
profile BB (see Figure 2). The traces were deconvolved,
band-pass iltered, and normalized o their m ean amplitude.
The traveltimes are reduced with a velocity of 4.5 km/s.
Therefore, seismicevents raveling with an averagevelocity
of 4.5 km/s are aligned parallel to the distance-axis.On the
seismic section three main groupsof arrivals, besidessur-
face and water wavesof low velocities,can be distinguished
in Figure 3: Pl are first arrivals, showingapparentvelocities
of about4.5 km/s from distances f 5 km on. P2 appearswith
approxim ately the sam e velocity, but in larger epicentral
distances and h igher traveltimes than the P l signals. P3
arrivals differ from Pl and P2 with m uch highervelocitiesof
about 6.5 km /s, but with comparableamplitudes.
It was shown with ray-tracing computations, hat the Pl
and P 2 arrivals representcritical refra cted waves traveling
inside h e Miocene evapo rites nd their reflected efractions .
The P3 arrivals were proved to originate at the top of the
crystalline basem entas wide-angle reflectionsof P-waves.
For this purp ose traveltimes and synthetic sections were
computed.
FIG. 3. Example for a deconvolved, filtered and time re-
duced OBS seismic section (OBS 1, profile BB).
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674
Seismic 17
FIG. 4. Model with raypaths and calculated raveltimes or
the section of OBS 1 on profile BB.
Evaluation of the sections
The
sections
were evaluated with a fast ray tracing inter-
active program , especially designed or computing travel-
times of wide-angle reflections and diving wa ves for two-
dimensional ateral and vertical inhomogeneous tructures.
By modifying the mo del parameters he calculated travel-
times are fitted iteratively to th e observedones and graphi-
cally displayed. The models are based on the known over-
burden structuresand velocities derived at bo reholes.The
seismicmodelingwas therefore estricted n constructing he
basementgeometry rom seismic ventsgenerated elow the
Miocen e evapo rites. An exam ple is given in Figure 4, in
which the model or the section n Figure 3 is shown ogether
with the raypathsand computed lined) traveltime curves.
One can see hat a basementelement of approximately2 km
length can easily be re solvedby wide-anglereflections.The
complete basement geometry for each line could thus be
constructed and a basement map derived by combining
profilesparallel and perpendicular o the stru ctures.
References
Makris, ., andThiessen,
.,
1983.Offshoreseismic nvestigations f
sedimentary asinswith a newly developed ceanbottomseismo-
graph:Presented t the 53rd Annual SEC Meeting, Las Vegas.
Richards,T. C., 1960.Wide angle eflections nd heir applicationo
finding imestonestructures n the foothills of western Can ada:
Geophysics, 5, 385407.
Color Display of Offset Dependent
97.6
Reflectivity in Seism ic Data
GregoryE. Onstott, Sohio Petroleum Co.; Mile M.
Backus, Clark R. Wilson, and J. il. Phillips, Univ. of
Texas, Austin
The ch ange n s eismic eflection coefficientwith angle of
incidenceof the w ave on the reflectinghorizon can provide
cluesas to the elasticpropertiesof the rockson either sideof
the reflecting nterface. A me thod s
proposed
for encoding
the chang e in reflection coefficient with source-rece iver
offset in a single color display whe re it m ay be readily
interpreted. The technique consistsof generating “partial
stack” sections within three restricted offset ranges and
superimposinghem in the three primary colors red, g reen,
and blue on a color video display terminal. The display is
implemented for some synthetic seismogramsgenerated
from an elastic earth model and for somedeep marine data
taken on the East Coastof the United States.The method s
found to be successfuln its primary purposeof displaying
anom alies n offsetdepende nt eflectivity n a form amenable
to interpretation.The method s also found to be useful as a
quality control on velocity analysis and for distinguishing
multiples rom primary events,
A useful measureof rock properties n the subsurface s
the behaviorof seismic eflectivity as a function of the angle
of incidenceof the seismic wave on the reflectinghorizon.
The variation in reflectivity with incidence angle is con-
trolled by the contrasts n elastic param etersbetween the
rockson either side of the reflecting nterface.This reflectiv-
ity information is normally co llected at great expen se in
exploration seismicsurveys only to
be
thrown away in the
stacking of normal-moveout corrected common-midpoint
gathers.Recent work (Rosa, 1976) showed he difficulty in
obtaining unique m athema ticalsolutions o r th e elastic pa-
rametersof the rocks rom precriticalreflectionamplitudes.
Nevertheless, t is proposed hat the change n reflectivity
with offset can be profitably exploited to spot seismic
anomalies o which other data and geologic ntuition can be
applied o n arrow the range of p ossible nterpretations.
Several recent papers on the subject, by Backus et al
(1982), Ostrander (1982), and G assaway et al. (1983), dis-
cussed he u se of offset depend ent eflectivity in p etroleum
exploration.The latter two paperswere concernedwith the
analysisof seismic bright spot” anomalies,usingamplitude
variationswith offset n common-depth-point athers.Since
it is not practical o examine all of the CDP gath ers n a large
datase t, his approac h s effectively im ited to the analysisof
reflectivity anomalies which a re visible on the stacke d
section. Any stratigraphic hange n rock propertieswhich
does not res ult in a significantchange n the stacked race
amplitude will probably be m issed and thus will not be
examined n the nonzero offset domain. It is sugg estedhat
many economichydrocarb on raps fall into this category.
A data processingand display method is proposed o r
encoding the reflection amplitude variation w ith offset in
color on a single sectionwhere t can be readily observedby
the interpreter.
The
basic method is to form three partial
stack races nsteadof one from each NMO-corrected CDP
by summing he traceswithin three restrictedoffsetranges.
The stacked traces are formed into three comm on offset
range sections and visually superimposedn the primary
colors ed, green, and blue on a color video display erminal.
The resulting image resemblesa stacke d section but indi-
cates by color and b rightness he distribution of reflection
amplitudesover the three offsetrangesat every point in the
section, Such a display allows the interpreter to perceive
chang es n reflectivity over o ffset without having to lo ok at
CDP gathers or multiple partial stack sections,so that the
variations n reflectivity can be rela ted to the s tructureand
depositionalpatterns.
Color may be consideredas a three-dimensionalvector
space, he basisvectorsbeing the primary colors ed, green,
and blue (for transmitted igh t) or, alternatively, hue, satura-
tion, and intensity. A color image is a vector field with a