12. CORRELATION OF SEISMIC REFLECTORS

James E. Andrews>Department of Oceanography, University of Hawaii, Honolulu, Hawaii

INTRODUCTION

At all sites drilled on Leg 21 vertical reflection profileswere shot using the Glomar Challenger's 30 cu in air gun andsonobouy receivers. At Site 203 two sonobuoys malfunc-tioned and no record was obtained, but data from all othersites (204-210), combined with the nearly continuouscoring carried out at each site, have provided a reasonablydetailed picture of the nature of reflective horizons. Inseveral instances (especially Sites 206 and 210) the correla-tions have shown earlier interpretations of the geologybased solely on reflection profiling to be quite incorrect.

The individual sonobuoy records are presented here withannotations from shipboard work. It was found duringdrilling that "typical" sediment velocities (e.g., 1500 m/secfor the upper 2-300 m of sediment) provided quite accept-able estimates of depth for guiding the sampling programs.The sonic velocities measured on cores in the shipboard labnow permit depth determinations to be refined. Limits onthe depth determinations arise principally from meas-urements on the original records, wavelength considera-tions, and interference patterns. Measurements are easilymade to 0.01 sec using a Gerber expandable scale, and maybe occasionally interpolated to 0.001 sec. This gives generalaccuracies of ± 7.5 meters. A sediment velocity error of 100m/sec (a typical range of variation in a "uniform" section)adds to this by only ±0.5 meters. If such an error existedthroughout a 300-meter section, the error would be ± 25meters; however, it is felt that this does not occur. Thisrange of error is also that encountered due to in situ versuslab temperature and pressure effects on velocity and nocorrections made for these factors. Wavelengths of theseismic pulse at these sediment velocities average 10 meters(150 Hz center frequency at 1500 m/sec), but for thelowest frequencies recorded (10-50 Hz) and velocitiesdeeper in the section (2000-3000 m/sec) may be as great as300 meters, and resolution is limited accordingly. Theeffect of the multiple signals of the bubble pulse from theairgun is such that overlap occurs at or before onewavelength separation of reflecting horizons. As spacingchanges, destructive phase interference may mask reflectorsseparated by intervals of (2«+l)λ/4 (nλ/2 if there is avelocity decrease at the lower reflector). Constructiveinterference occurs for the inverse of these spacings andimpedance changes and can result in spurious and nonuni-form strengthening of reflectors or incorrect apparentspacing of reflectors. Some of thesy problems can be seenin the model studies of Woods (1956). Thus the records aremost correctly regarded as an interference pattern ratherthan a simple representation of geologic structure. It isinteresting, however, that practice shows the range ± 1 0meters to be normally obtained when correlations may beestablished. The major problem then is that patterns do not

always represent similar geology and that not all reflectorsobserved represent real features of the sediment column.

Sonic velocities were measured onboard the GlomarChallenger with a Hamilton frame unit, and density/porosity values were derived from the GRAPE records.Thermal conductivity was measured when heat flow meas-urements were taken in the hole (see Physical Propertysections for each site for data). Reflections occur whereacoustic impedance (sonic velocity × density) changes inthe section. In the sediments cored, velocity and densitydiscontinuities generally coincide as a result of overburdencompaction, and at greater depths, cementation. BecauseGRAPE measurements are continuous, they often showdensity maxima in intervals over which velocity data havebeen averaged, and these points may appear as reflectors.Such boundaries may easily be time transgressive and morea function of sedimentation rates than biostratigraphy. Inrecording the sonobuoy records, a variety of filter settingswas used to investigate the effect of frequency (orwavelength) on the reflected signal. This influence derivesfrom the reflector spacing (constructive or destructivephase interference) and variable attenuation of differentwavelengths. The latter is generally of less importance forthe frequency variations used. Tables 1 to 7 presentsummaries of reflector depths, sedimentological, and litho-logic properties against depth below ocean floor. A timescale (round-trip travel time) adjusted on the basis ofsediment velocities is also shown. The velocities are averagesfor intervals of relatively uniform values, and individualmeasurements may vary by ± 100 m/sec or more.

SITES

Site 204 is on a section of relatively undisturbed crustnear the western margin of the Pacific plate. The acousticbasement on underway profiles is a broad band up to 0.1sec thick which slopes to the west (Figure 1). On thesonobuoy profile (Figure 2) the acoustic basement is seenas a series of reflectors overlain by a generally transparentsection containing one moderately strong reflector.Velocities in the clay sections were measured at an averageof 1483 m/sec throughout the section. Reflector 1 at 47.5meters marks the change from silty clay to glass shard ash.A second transition from the ash to a clay at 70 meters isnear the calculated depth for Reflector 2 at 0.09 sec. At0.14 sec (103 m) acoustic basement was found to be atuffaceous sandstone and conglomerate, with velocitiesincreasing abruptly to 2643 m/sec. The drilling wasterminated at 150 meters due to low drilling rate and theabsence of fossils. The deepest of the reflectors forming theacoustic basement horizon are at or below 315 meters (at2643 m/sec). There is a correlation at 129 meters of one ofthese reflectors with the top of the vitric tuff, although nopronounced change in acoustic velocity is observed at the

459

J. E. ANDREWS

TABLE 1Site 204 Correlations

TABLE 2Site 205 Correlations

25 _

50 _

75 -

00 -

125 -

50 —

Reflectors

1

2

3

4

Phys. Prop.

1483 m/sec

1.28-1.44 g/cm3

2.0-2.5 g/cm3

Lithology

Unit l

Unit 2

Unit 3

Biostrat.

Quaternary

M. Miocene

E. Miocene

Oligocene

" ~

Cretaceous

?

Phys. Prop. Lithology

315 m deepestgood reflector

(@ 2643 m/sec)

break. This suggests that the numerous reflectors below thisdepth may be real, rather than reverberation, and makesestimates of depth to the top of the second layer verydifficult.

Site 205 was selected near the eastern edge of theflattest portion of the South Fiji Basin. Near the site severalsmall basement intrusions occur at the change in slopebetween the basin and the sediment blanket coming fromthe Lau Ridge. Occasionally these are expressed as low hills.The underway record (Figure 3) shows a very thin trans-parent section above a section of acoustically well stratifiedmaterial. A broader transparent zone beneath this liesimmediately above a weak acoustic basement (0.39-0.49sec). The sonobuoy profile (Figure 4) shows a series ofreflectors above an acoustic basement at 0.49 sec (Reflec-tor 5). This reflector is very clear on the record at 10 to 40Hz, but when higher frequencies (40-80 or 160 Hz) wereused, a series of returns appears above this level almostobscuring the breaks. Table 2 presents correlations downthe hole.

Reflector 1 at 62 meters coincides with a velocityincrease of 300 m/sec and is clear at low and highfrequencies. This lies entirely within lithologic Unit 2 at avelocity discontinuity. Reflector 2 at 154 meters is onlyclear when frequencies above 80 Hz are recorded and isrepresented by peaks in the velocity, density, and thermalconductivity profiles down the hole, although no generallithologic change occurs at that level. A third reflector at

3

4

5

6

1.2-1.3 g/ c m 3

1.4-1.6 g/cm3

1544 m/sec

1834 m/sec

ppeak

? no measure

1834 m/sec

1918 m/sec

1684 m/sec

4400 m/sec

Bottom

Unit 1

Unit 2

Unit 3

Unit 4

of Hole

?

L. Miocene

M. Miocene

L. Oligocene

M. Oligocene

209 meters (0.24 sec) is also clearly seen on 40 to 160 Hzportions of the record. No major lithologic changes occurhere, and a gap in the physical property measurementsprevents correlation with small excursions.

The change at 275 meters from the ash of Unit 2 to thechalks of Unit 3 is near a faint reflector at 0.33 sec—about285 meters. An interesting situation is created by theslightly higher velocity (1918 m/sec) in the interval 250 to255 meters followed by the substantially lower velocity of1684 m/sec from 255 to 335 meters. These velocitychanges do not appear as reflectors on the sonobuoy trace,although they suggest a signal should be prominent at 0.30sec where only vague indications exist on the profiles at 40to 160 Hz. In such instances it may be that cancellation ofsignals from adjacent reflectors is possible—the depth fromthe 1918/1684 m/sec surface to the top of Unit 3 is about1.5 wavelengths (although this varies by a factor of 4 in therange recorded). The high over low velocity structure wouldproduce a phase change opposite to a low/high surface sothat («λ/2) should be the appropriate spacing for destruc-tive interference. The top of Unit 4 (basalt and limestone) ismarked by a velocity increase to 4400 m/sec and correlates

460

SEISMIC REFLECTORS

TABLE 3Site 206 Correlations

TABLE 4Site 207 Correlations

Reflectors Phys. Prop. Lithology Bi

1

2

3

4

5

6

7

1530 m/sec

1562 m /sec

1696 m/sec

1870m/sec

2131 m/sec

1870 m/sec

2375 m/sec

Unit 2

Unit 3

Unit 4

Bottom of Hole

Pleistocene

Pliocene

L. Miocene

M. Miocene

L. Oligocene

M. Oligocene

L Eocene

E. Paleocene

Meters 0 .Reflectors Phys. Prop. Lithology Biostrat.

exactly with Reflector 5 using the overlying velocities. Thisreflector is faintly visible at 10 to 40 Hz, and is clearlypresent at all other frequencies. The strong reflectivehorizon at 0.49 sec (perhaps 550 m depth) suggests oceanicsecond layer and supports the contention that Hole 205terminated in an intrusive sill.

Site 206 in the New Caledonia Basin presents majordiscrepancies between stratigraphic boundaries and reflec-tors, as well as between geophysical interpretation andgeologic structure. The site survey and approach tracksshow (Figure 5) an acoustically stratified basin fill of ratherwell defined dimensions with an intrusive structure from adeep transparent horizon. This was initially interpreted asbasin turbiditës on earlier pelagic material. The on-siteprofile is shown in Figure 6. Drilling and quasi continuouscoring showed the entire section penetrated (nearly 0.9sec-711 m) to be rather uniform pelagic sediments with nosign of terrigenous contribution "via turbiditës. The regionalunconformity (see Leg Summary and Conclusions) occursat this site at 620 meters, but is not marked as a reflectinghorizon. Table 3 presents correlations for this site.

Reflectors 2, 5, and 6 coincide closely with velocitydiscontinuities (1562 to 1696 m/sec; 1870 to 2131 m/sec;

1584 m/si

1661 m/s<

3348 m/a

Unit 2

Chert

Unit 3

Chert

L&M Paleocene

- 0.3

- 0.34

and 2131 to 1870 m/sec, respectively). Reflector 5 alsomarks marks the Late Oligocene/Middle Oligocene bound-ary. None of the other biostratigraphic boundaries andnone of the lithostratigraphic boundaries are marked byreflectors. Two velocity discontinuities—one of 200 m/sec(1696/1870 m/sec) and one of 500 m/sec (1870/2375m/sec)—are not seen in the record, probably again due tophase interference and masking. A displaced section ofnearly 40 meters of Paleocene and Cretaceous material alsodoes not produce a signal on the profiles.

Reflector 4 appears to be the top of the "transparent"layer of the site surveys which suggests a possible LateOligocene event as a disturbing influence on the sedimentsbelow, although this is quite a tenuous conclusion.

The possible basement at 0.9 sec was not reached bydrilling, but on the 10 to 40 Hz record there is a suggestionof reflectors as deep as 1.2 sec so that the depth tobasement is conjectural. The displaced older sediments nearthe base of the hole suggest that the possibly thicker fillmay be material slumped from the adjacent highs—perhapsduring Eocene deepening of the basin.

Reflectors 4 to 6 show up -strongly at all frequenciesrecorded, while Reflectors 1 to 3 are best delineated atfrequencies of 160 Hz or above. The lack of coincidence ofreflectors with structure as at other sites (e.g., Site 210)suggests that local or regional tracing of reflectors as timelines in structural interpretation can be misleading.

The presence of displaced material within the sectiondefined as acoustically transparent which forms the bulk ofthe adjacent hill suggests that the lack of reflectivity maybe a function of internal disruption of the sediment during

461

J. E. ANDREWS

TABLE 5Site 208 Correlations

Phys. Prop. Lithology

1

2

3

4

5

6

7

-g -~->

9

1500 m/sec

1586m/sec

1898 m/sec

2345 m/sec

Bottom

Unit l

Unit 2

of Hole

Pleistocene

Pliocene

L. Miocene

E. M. Miocene

E. Miocene

L. Oligocene

M. Eocene

M. Paleocene

E. Paleocene

Maastrichtian

0 Se

0.54

0.575

a period of tectonism within the basin. This possibility wassuggested earlier on the basis of site survey data and will beexplored more fully elsewhere. It does appear, however,that the New Caledonia Basin may well have undergonecompression during early or mid-Tertiary with resultantfolding of some basin structures and perhaps somestructures on the eastern slope of the Lord Howe Rise.

Site 207 on the southern Lord Howe Rise is near the topof a basement high (Figure 7). Reflectors converge ontothis structure (with some deeper ones being truncated) andcan be clearly correlated with boundaries within thesection. The sonobuoy profile (Figure 8) and the correla-tion chart (Table 4) show the local structure. Principalboundaries marked by reflectors are: The Eocene/Oligoceneregional unconformity (Reflector 1) at 135 meter(0.17 sec). In this section there is a displaced section ofEarly Late Eocene (118-120 m) above a 14-meter section ofEarly Middle Miocene (128-142 m) which rests on theunconformity. Immediately below the unconformity achert stringer was encountered (143 m). There is a verysmall change in average velocity (and acoustic impedance)at the depth calculated for the reflector (134.6 m), and anyof the above boundaries fall within extreme confidencelimits of the depth calculations. Since this is one of onlytwo sites (out of five) at which a reflector marks theunconformity, it is unfortunate that the picture cannot be

TABLE 6Site 209 Correlations

Reflectors Phys. Prop. Lithology Biostrat.

Bottom of Hole

L. Pliocene

M. Pliocene

L. M. Miocene ‰_ 0.08

L. Oligoce

further clarified, but it would appear most likely that thechert was responsible. This boundary also marks thetransition between lithologic Units 1 and 2.

The Unit 2/Unit 3 boundary (310 m) is marked byReflector 6 (0.37 sec). This is the top of the silty claystone,but not a point of marked change in sonic velocity,although a density maximum does occur at 305 meters. TheUnit 3/Unit 4 boundary-the top of rhyolite- is alsomarked (Reflector 7). This is a pronounced velocity jumpfrom 1917 m/sec to 4921 m/sec at 360 meters. The Unit4/Unit 5 boundary which is lithologically a transition froma brecciated appearance in Unit 4 to a flow appearance inUnit 5 is marked by Reflector 8 (0.45 sec).

Reflectors 3 and 4 mark velocity increases (1587 to1661 m/sec and 1661 to 1917 m/sec, respectively) withinUnit 2. A chert interval also occurs at the depth ofReflector 4. Reflectors 2 and 5 also are within Unit 2 andare at or near zones where the induration op the coresincreased sufficiently to be remarked by sedimentologistsonboard while opening the cores. Acoustic basement on theunderway profiles is the top of the rhyolite-Unit 4. Thereflectors, in this instance, can be usefully traced to local

462

SEISMIC REFLECTORS

TABLE 7Site 210 Correlations

o _

200 —

300 —

400 —

5 0 0 -

600 —

700 -

Reflectors

1

2

3

4

5

Phys. Prop.

1538 m/sec

1615 m/sec

1974 m/sec

2559 m/sec

2035 m/sec

Lithology

Unit 1

Unit 2

Unit 3

Unit 4

Unit 5

Biostrat.

L. Pleistocene

E. Pleistocene

L. Pliocene

E Pliocene

L. Miocene l

M. Miocene

F. MioceneM. Oligocene

L. Eocene ~ "

M. Eocene

E. Eocene

geologic structures to provide bench marks for interpreta-tions of structural history.

Site 208 is on the northern end of the Lord Howe Rise(Figure 9) and is the only other site (with 207) at which theregional unconformity is clearly marked by a reflector(Reflector 5, 0.575 sec). This reflector appears to truncatesome layers to the east and west of the site and supportsthe hypothesis of an erosional origin for the unconformity.Reflectors are present on the sonobuoy profile (Figure 10)at least 0.2 sec below underway acoustic basement and atleast 60 meters below the bottom of the hole so that thenature of true basement cannot be determined. Only twolithologic units are present, and their boundary at 490meters is marked by Reflector 5 which is also the regionalunconformity. Correlations are presented in Table 5.

Reflectors 1,2, and 6 coincide with increases in sonicvelocity in the cores—from 1500 m/sec to 1586 m/sec to1898 m/sec to 2345 m/sec. Reflector 2 also correspondswith the Late Middle Miocene/Middle Middle Mioceneboundary, and Reflector 6 with the Middle/Early Paleoceneboundary. Reflector 7 marks the Cretaceous/Tertiaryboundary. Other reflectors—3 and 4—are not obviouslyrelated to any stratigraphic or physical transitions, andReflectors 8 and 9 were below the depth of the hole.

Reflectors at Site 209 on the Queensland Plateau are notas well known as those at other sites due to the poor corerecovery in the coarse sediments encountered and the

variations in velocity with sediment type. This places agreat deal of uncertainty on velocity averages and bound-aries. The underway seismic profiles (Figure 11) are markedby a definite basement reflector which rises at the site to0.38 sec (Reflector 7) and a strong intermediate reflector at0.25 sec (Reflector 5). The 0.25-sec reflector is the"unconformity" or "erosional" surface described by Ewinget al. (1970) which crops out on the edge of the plateaunear the 2000-meter contour. The on-site sonobuoy profileis shown in Figure 12, and Table 6 presents the derivedcorrelations. Velocity boundaries have been placed roughlyat midpoints between depths of measured velocities. Onthis basis Reflectors 1 and 2 are not related to observedchanges, while Reflectors 3 and 4 mark the velocityincreases of 1588 to 1819 m/sec and 1819 to 4552 m/sec.Since only three measurements are available in the intervalof high velocities below Reflector 4 (5799,4823, and 3035m/sec), an average is difficult to accept, but does produce areflector depth for Reflector 5 which coincides with theLate Eocene/Late Middle Eocene boundary at 270 meters.This is the depth of a velocity decrease to 1947 m/sec andmarks the lowest level of chert stringers in Unit 2. Thus,geophysical interpretations must be redrawn to recognizetrie nature of this reflector. Mass movement of sedimentsabove the chert has exposed the reflector, but ispost-depositional rather than another interval of nondeposi-tion. The regional unconformity is present at this site near145 m-the top of Unit 2 - but the interval 90 to 180meters is unsampled. Reflector 3 at 100 m may mark thebreak but this is conjectural. In using the velocitiesavailable, Reflector 6 falls at 328 meters, 20 meters belowthe top of Unit 3. There is a small increase in acousticimpedance at this depth.

Site 210 provided one of the most .interesting contrastsof geophysical interpretation with geological structure. Atthis site only one of five reflectors coincides with observedphysical or stratigraphic breaks. The site was continuouslycored to 90 meters depth, and thereafter every other 9meters was cored. Even combining the maximum seismicuncertainty with a possible 9-meter error in an uncoredhorizon boundary does not give sufficient scope toapproach a fit for the unrelated reflectors, and it seemsobvious that this is exactly what they are.

Underway records to the site and from the site survey(Figure 13) show acoustically stratified and transparenthorizons horizontally overlying a rough basement reflector.These were interpreted as abyssal plain turbidites andpelagic oozes overlying basaltic basement.

Figure 14 shows the on-site profile, and Table 7 presentsthe correlations proposed. Reflectors 1, 2, and 3 occurwithin the 480-meter thick turbidite sequence of Unit 1.Throughout this unit layers of nanno ooze several centi-meters (average 8.5 cm) thick occur. These are generallymore highly lithified than the turbidite intervals, and it maybe that phase correlation of reflections from several ofthese are responsible for the recorded signals while inter-ference obscures reflections from the physical propertybreaks. Reflector 3 is associated with an increase infrequency of occurrence of these beds.

Reflector 4 clearly marks the top of Unit 3—lithified tannanno chalks. There is also a marked sonic velocity increase

463

ON

. :f ' f : ! > . • ; • . . . ^ - ^ ' i r . . i . ^ : ^ ' l ^ • i l I : i j s - i ».. :i • i π . iFigure 1. Profile to Tonga Trench from 204 (GC).

SEISMIC REFLECTORS

Figure 2. 204 Sonobuoy.

465

" I S d T : r V "• S " ' • " • " . • > - • • • ' • / ! ' . '• • » " " f . ' . . ". . •* T - f . - > • • s - v j f

• •• ' •. v * r •• !

,- • e

' - :

-a

9 t -':-.'ò % . i• ,m •* -. •a rt

<

SJ

•-*

St]

Figure 3. Pto/ife to 205 (GC) South Fiji Basin.

SEISMIC REFLECTORS

I-! I

Figure 4. 205 Sonobuoy.

467

, 4 i

WWi w • :s»: : :. ?£'

T ! !• # i ' Λ , ' v ia /i^•';•

.;••' | ; •'vv f r •r^':• ' •• • • • • •' • I M. " Ji ?

!? ,"f vV: : vi.?-..• . . i . •.•• : •• f i ' v • • : i t t

• rf•' " ^'– *' l .: * : : « t « •-

551

i;4 ,H I

-hi-*"' • 1 I K « %>3lS

j i \

Figure 5. i>o/z/e to 206 (GC) New Caledonia Basin.

SEISMIC REFLECTORS

w jr,

fryΦ

I

1

^ 1 3 !

•i

•:• i ü l. - &*0. - f i

• •4i;

i

I

I

M!iL• !ft

1 • . .

.

fiI]

Figure 6. 206 Sonobuoy.

469

J. E. ANDREWS

Figure 7. Approach to 207-Site Survey-Southern Lord Howe Rise.

470

SEISMIC REFLECTORS

S i

Figure 8. 207Sonobuoy.

471

to

H r•"~7" " ™~ v • - ~ ~ r • 4 - • n : — — ^ - ^ - r - — >i t ; . f . •• • - ' • ! ' • • ö

: ' : i ' •̂ •' . : . : ' ' ' . 9i - l • ' • I • • • ' i • • • ' ' ; • ** * ! • • • ' • " • ; •; "'••

F f ^ • ; : •:•: • 1 : ;••! • • - j ; ::: . H .••:• •..• I • - ' '

' i rj ; -̂ ÷- r-

.1 | :. .. .; . ;;_;,:_ .: ______: j__! fl • i •••f^ ::;• • < • ; . : / : • , i ; • ! i / i | p | •

;

•. •; I • •• •" - ; : ' •

_4: ...i..|i I ... _.., ,.:..___ :_‰. ^_^_jj . :1

i i i 1 . 7 • ' ' • :; I i : • : » : : • ' • • f :•! " : : ''•! ::

a * "• M sss 5is i5?; | 4 ssi .a|? •j;f ; * 35 »-t

Figure 9. Approach to 208 (GC) Northern Lord Howe Rise.

SEISMIC REFLECTORS

I " . i

i

Figure 10. 208 Sonobuoy.

473

J. E. ANDREWS

••-~—rf

j|;:;

Tl a * * 1 , 1 9 S .V,-•

Figure 11. Approach to 209 (GC) Queensland Plateau.

474

SEISMIC REFLECTORS

Figure 12. 209 Sonobuoy.

475

I o j- - ^?

• .

L»' β "*

r ......

Figure 13. Profile from 209 to 210 (GC) Coral Sea Abyssal Plain.

SEISMIC REFLECTORS

iMΦ yfo!i_L•

.v,

'-«*

Figure 14. 270 Sonobuoy.

All

0 0 M

I I • i ••• • I

Figure 15. Kona Keoki profile over structure near 210. Coral Sea Abyssal Plain.

SEISMIC REFLECTORS

into this unit (1974 to 2559 m/sec). This reflector appearsin underway records as acoustic basement except at verylow speeds when Reflector 5 is present. These tworeflectors are the rough surface thought to be basalt, butare obviously within the sediment section. The roughness ofthe horizons hints at earlier tectonic influences on anexisting basin floor. The clays of Unit 2 are not apparent inthe seismic records. The regional unconformity is present at540 meters and is not marked by a reflector. Velocitychanges at 523 meters (top of Unit 3, Reflector 4) and at565 meters (no reflector) bracket the unconformity. Nodistinct acoustic basement is present on the sonobuoyprofile.

The distinction of stratified and transparent acoustichorizons is not seen in the cores. Rather, the presence orabsence of reflectors appears to be a function of phaserelations in the reflected signals, and no major "trans-parent" or pelagic intervals exist in the section above therough surfaces.

On an uplifted section to the east of the site, the seismicprofile shows the lower rough transparent section upliftedwith the basin fill ponded against it (Figure 15). This area is

a very good candidate for future drilling to establish the ageand early history of the Coral Sea Basin.

CONCLUSIONS

The nearly continuous coring carried out on Leg 21 hasdemonstrated a wide variation in the degree of correlationpossible between acoustic reflectors and stratigraphic and/or physical changes in the sediments. Major changes inacoustic impedance, at times marking major unconformitiesin the sedimentary sequence, may not appear on seismicprofiles due to interval spacing and interference. In othercases it is possible to establish regional interpretation on thebasis of reflectors which do represent erosional surfaces andlithostratigraphic boundaries. Except where unconformitiesare present, biostratigraphic horizons are not usefullydefined. Time transgressive lithostratigraphies are the mostusefully defined horizons. These problems must beaccepted in all geologic interpretations of geophysical data.

REFERENCES

Woods, J. P., 1956. The composition of reflections:Geophysics, vol. XXI, p. 261-277.

479