Wide Angle Reflections

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Seismic 17

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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



Seismic

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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).



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

Wide Angle Reflections

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