Chapter 7 Seismie Interpretation r r r (' n " ("') " ("') ("') CHAPTER 7 SEISMIC INTERPRETATION A large volume of seismic data was collected during the course of this project. This was to ensure that maximum benefit was gained from having the equipment and Mr. David Mitchell in Tasmania. As a result, there is considerably more data than I hope to do justice to and I am only presenting a fraction of it here as fully interpreted sections. I have chosen eight complete cross-river profiles (lines 3, 5, 7, 16, 17, 21, 22 and 23) because they are parallel, span a large portion of the study area, and the stratigraphy is largely unobscured by acoustic turbidity. I have also interpreted a further 11 short profiles - these are all affected by acoustic turbidity and are located off Taroona (lines 24 to 34). The short profiles are less informative than the long profiles. Along with the seismic interpretations, for the eight full length cross-river profiles I have displayed a modelled profile ofTMI (except for line 7; there is no magnetic data for most of this line). These magnetic profiles were extracted from the final gridded dataset using the seismic trackpaths as a template, so they are truly coincident. In general terms, the magnetic sources are buried more deeply than the seismic system is able to image, but I hope that it is apparent (despite the necessary difference in display scales) that there are features - particularly faults - that appear to be expressed on both the magnetic models and seismic profiles. For the shorter lines (numbers 24 to 34), I have displayed profiles of TMI without forward models. Some ofthese have sections of 'no data' - this occurs where the seismic profile is outside the magnetic grid. Each profile is presented as a series of two sheets. The first has the uninterpreted, located seismic profile and a profile of TMI. The second has the interpreted seismic profile and the modelled magnetic profile for the long lines, and the interpreted and uninterpreted seismic profiles for the short lines. The magnetic modeling has all been done using dolerite sources only, with relatively low magnetic susceptibilities. Susceptibilities are shown on the profiles. The observed magnetic response is shown in blue, and the calculated response in red. The same points discussed in chapter 4 apply to these magnetic profiles. A Geophysical Investigation of the Derwent Estuary 114
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SEISMIC INTERPRETATION CHAPTER 7 - University … 7 Seismic Interpretation ,~ The colour scheme used for the seismic interpretation is as follows. The solid black lines are traces
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Chapter 7 Seismie Interpretation
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CHAPTER 7 SEISMIC INTERPRETATION�
A large volume of seismic data was collected during the course of this project. This was to
ensure that maximum benefit was gained from having the equipment and Mr. David
Mitchell in Tasmania. As a result, there is considerably more data than I hope to do justice
to and I am only presenting a fraction of it here as fully interpreted sections. I have chosen
eight complete cross-river profiles (lines 3, 5, 7, 16, 17, 21, 22 and 23) because they are
parallel, span a large portion of the study area, and the stratigraphy is largely unobscured
by acoustic turbidity. I have also interpreted a further 11 short profiles - these are all
affected by acoustic turbidity and are located off Taroona (lines 24 to 34). The short
profiles are less informative than the long profiles.
Along with the seismic interpretations, for the eight full length cross-river profiles I have
displayed a modelled profile ofTMI (except for line 7; there is no magnetic data for most
of this line). These magnetic profiles were extracted from the final gridded dataset using
the seismic trackpaths as a template, so they are truly coincident. In general terms, the
magnetic sources are buried more deeply than the seismic system is able to image, but I
hope that it is apparent (despite the necessary difference in display scales) that there are
features - particularly faults - that appear to be expressed on both the magnetic models
and seismic profiles.
For the shorter lines (numbers 24 to 34), I have displayed profiles of TMI without forward
models. Some ofthese have sections of 'no data' - this occurs where the seismic profile is
outside the magnetic grid.
Each profile is presented as a series of two sheets. The first has the uninterpreted, located
seismic profile and a profile of TMI. The second has the interpreted seismic profile and the
modelled magnetic profile for the long lines, and the interpreted and uninterpreted seismic
profiles for the short lines. The magnetic modeling has all been done using dolerite sources
only, with relatively low magnetic susceptibilities. Susceptibilities are shown on the
profiles. The observed magnetic response is shown in blue, and the calculated response in
red. The same points discussed in chapter 4 apply to these magnetic profiles.
A Geophysical Investigation of the Derwent Estuary 114
t5230000.. ! '>.a --:5'$)\.} t t 530000 535000 540000
c::J ~:JlCn... b+di(y 2500 o 2500 Meters Seine TuO:peIh. !!!!! -- Sti'!IllcTr.o<pelht N ""'pili«! Goologyo OJaI.rnay On-shore Geology Polygons o T.Il.-y from 1:250,000 Digital Coverage. D TII••1ll
Grid: Australian Map Grid, Zone 55CJ: f'lrml.n Datum: Australian Geodetic Datum, 1966 9lltOvsRo<:k$
c=J r''''.-yB.._ Ac=J JIor..IC;Do/toIU
Figure 7.1. Trackpaths map for 'long' seismic prOfiles chosenfor interpretation.
A Geophysical Investigation of the Derwent Estuary 115
Youngest sediment (marine sand?). Only present on line 7, ie. close
to Storm Bay
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The colour scheme is similar for the short profiles (24 to 34), with the addition of
Brown Possible landslip deposit, characterised by numerous diffractions.
Green Dipping strata of probable Oligocene age. Also exposed on shore at
Taroona.
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A Geophysical Investigation ofthe Derwent Estuary 117
0
Chapter 7 Seismic Interpretation
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Also, although the age sequence of colours is the same (ie. bright yellow youngest and so
on) on all profiles, the absolute ages of the various packages of sediment may not be the
same on all profiles. This is one of the difficulties in interpreting the seismic data in the
absence of age control and with discontinuous reflections.
Apart from these points, I think the interpretations are largely self-explanatory. There is a
(short) discussion of some of the major features of each profile on the interpreted sections.
Some general conclusions are given in chapter 9.
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A Geophysical Investigation of the Derwent Estuary 118
11 526969mE 5240253mN 11 527456mE 5240471mN I 1 528426mE 5240704mN I I 529331mE 5240893mN I I 530430mE 5241182mN I 1 531338mE 5241438mN I
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526000 528000 530000 534000 Profile of TMI (nT)
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5240000 + + +
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5240000 62650
62550
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5236000 + + 5236000 ~
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5234000
5232000
62150
62050
Derwent Estuary Seismic Reflection Survey
526000 528000 530000 532000 534000
Arrows denote survey direction. Red dots mark position fixes on the seismic record.
61950 0 1000 30002000
Along profile distance (from 526917mE 5240234mN)
4000 5000 Line 3
Figure 7.3.1 119
526969mE 5240253mN� I 528426mE 5240704mN I I 529331mE 5240893mN I I 530430mE 5241182mN I I 531338mE 5241438mN I 'I� 'T'
Frequency I Adjustment
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Acoustic�
Basement�
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Line 3, This line in a good example of how the .=m~ occurrence of acoustic turbidity is associated with
Cl tie ,-obs«vation< ~ ,-_.do" lools :tie" 'l'I.- tjelp .=m~ sediment volume. Although the most recent sediments ~M!J~~ extend across the entire profile, the acoustic turbidity is
526000 528000 530000 532000 534000 I '[ only developed where the sediment has significant volume. There are several generations of channels and 200.0� .......~\ block faulted stratigraphy, particularly to the west. Interestingly, the interpreted fault positions are generally ~---- ~~ I� " in accordance with those modelled on the magnetic 5240oo0~ + I + +, ~ + /5240000 /" .I
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profile. This implies that some of the faults that extend 0.0� '\ nearly to the seatloor have depth extents of hundreds of\ metres, and we.re active during the break-up of the
, I dolerite (possibly in the early part of the Tertiary). This \ f could imply that these faults are long lived, or that early
52380001 + 1/y. _r, /~ \ +~ r5238000 TMI (nl) Tertiary stratigraphy is exposed close to the seatloor in parts of the Estuary. The major channel positions are co-200.0
incident with the major structures (indicated by the In absence of dolerite and offset of dolerite blocks). This 52360001 + '\ + 7 + ) + 'J.5236000�
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implies that these structures have also been long lived, or
\1� -'---- the sediments associated with the faults are preferentially -r---
-� X 527200 528000 528800 529600 530400 531200 eroded during channel incision.
Arrows denote survey direction.� -600 ~~"'530489,6<'V.5NII99.' Figure 7.3.2 120Red dots mark position fixes on the seismic record. 'o<HeIp,p<=F1
I 527279mE 5238967mN I I 528102mE 5239072mN I I 529330mE 5239359mN I I 530480mE 5239625mN I I 531354mE 5239799mNJ I 532476mE 5239933mN I� ""'-'...-, 1,-,; ,~.... 't-' '" ryG-~t'---:,...~ .:..,., , .~:; _:.I;.;:,"'."!\:~;. L" I --,' ... ~,' -',''''--j -',J:' ',\', .£.;.....~f,,"'h - ...
526000 528000 530000 532000 534000 Along profile distance (from 526959mE 5238895mN)� "I
Arrows denote survey direction. Figure 7 4.1 Red dots mark position fixes on the seismic record. •
121 J I
I 527279mE 5238967mN I I 528102mE 5239072mN I I 529330mE 5239359mN I I 530480mE 5239625mN I I 531354mE 5239799mN I� I 532476mE 5239933mN I ")' ~.; . . .
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[J El< Qb«<""""'" !!:- .,.wo""" .I~. "'" ~" tl'" ~ Line 23. There is an extension of acoustic basement off °DI~II.i(BI'I!1 +i~q<iiG:'·H'\.[Q.I-- shore on this section - the lack of response in the
526000 528000 530000 532000 534000 magnetic data over this basement indicates that it is Parmeener Supergroup rather than dolerite. I have interpreted it as Permian. given that there is Permian
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nearby (onshore). There are fault blocks that appear to be rotated to the east of the main western fault (bounding 5240000 +� 5240000 the acoustic basement). There is also a large growth fault to the west, with at least 20m of displacement (using a conservative velocity estimate). The magnetic profile indicates a source below the channel containing the
5238000 +� 5238000 0.0 acoustic turbidity·· this appears to be out of place with \� ~ respect to the r~mainder of the dolerite and could in fact .,~...~
Red dots mark position fixes on the seismic record.� Figure 7.4.2 122
I 527175mE 5237384mN I I 527836mE 5237799mN I I 528632mE 5238205mN I I 529540mE 5238456mN I I 530463mE 5238634mN I I 531323mE 5238767mN I I 532045mE 5238954mN I
Arrows denote survey direction. Red dots mark position fixes on the seismic record.
61950 0 1000 2000 3000 4000
Along profile distance (from 527062mE 5237298mN)
5000 6000 Line 22
Figure 7.5.1 123
I 527175mE 5237384mN I I 527836mE 5237799mN I I 528632mE 5238205mN I I 529540mE 5238456mN I I 530463mE 5238634mN I I 531323mE 5238767mN I I 532045mE 5238954mN I
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I5l ElIo Ql>«<vation<_ !iodd ~"""""" loo!< _ _ ~ :i'I> -I~ Line 22. This profile is quite similar to the previous one
( 526000 528000 530000 532000 534000
DI~[1iI1 ~[~I+I~lo4~ ;\:-1'\.10..1 (# 23), showing the Parmeener Supergroup rocks extending off the eastern shore. In this case, they appear to be underlain by dolerite at a depth of between 300 and.
~ 400m. Again there are growth faults with significant
5240000 + ..,.-- +--,, 5240000 80.0
offset to the west. Note the absence of inferred magnetic sources beneath the channel containing the acoustic
~ ~ turbidity. The calculated magnetic profile does not fit the observed response well in this location and some of the
5238000 + ~--~
;-' + ~ 5238000 0.0
-----f
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observed response is probably due to sources off section (i.e. the bodies on the previous profile terminating to the north of this profile). This could indicate a cross-river structure to the.north of this profile.
For~. press Ft P; "392,12 .. :r=: '129.05 -~: S3ii93.ti "(; S23e811.~r-Red dots mark position fixes on the seismic record. Figure 7.5.2 124
I 527466mE 5236189mN I I 528156mE 5236443mN I I 528883mE 5236816mN I I 529633mE 5237025mN I I 530046mE 5237209mN I I 531391mE 5237812mN I "CJ...-...l-_' .'-- _.!
Red dots mark position fixes on the seismic record. Figure 7.6.1 125
I 527466mE 5236189mN I I 528156mE 5236443mN I I 528883mE 5236816mN I I 529633mE 5237025mN I I 530046mE 5237209mN I I 531391mE 5237812mN I
El ~ Qbsm...... !1Odd ~~ IooIs ,..... ~ tie\> Oo-.~e
...,MJ-"l Line S. On this line the second-to-most-recent channel
DI~11iiiI1 el\'l +I-'\,IOJ;IG-+I"\.I~I (pale yellow) is very broad. It has increased in size down river to this point, and generally migrated further from
526000 528000 530000 534000 the eastern shore. This profile is interesting in that it
5240000 + +
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5240000
0.0
160.0
--
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clearly shows unconformable relationships between at least five generations of channels. There is no great extension of acoustic basement off the shore on this profile. Again, there appears to be growth faulting to the west. The magnetic anomalies here are low amplitude, indicating deep sources. This is borne out by the modeling. There is no deep 'recent' channel against the
5238000 + ~
y,....--' + .............-¥ 5238000 TMI (nT) western shore on this profile - compare this to the next profile..
Arrows denote survey direction. Along profile distance (from 527616mE 5234799mN)
Red dots mark position fixes on the seismic record, Figure 7.7.1 127
I
I 527587mE 5234805mN I I 528647mE 5235062mN I I 529656mE 5235543mN I I 530651 mE 5236015mN I I 531840mE 5236594mN 1� I 533260mE 5236994mN I I~.
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E;J tie Qb....voOon< tlodel ~~ 1""" y_ l!!ndow ~ _Irao;;l Line 16. This line appears to show a very large extension DI~I"ISI"'1 +I~I~~+1'\.1«.1, of basement into the river - but due to the depth of the
526000 528000 530000 534000 reflectors it is difficult to be certain of this interpretation, Note the decrease in size of the 'recent' channel (containing the acoustic turbidity). The second youngest generation of channel fill is very extensive on this
80.0� profile, at both ends. The structural relationships on this 5240000~ +� 5240000 profile are not clear cut, particularly towards the west.
Interestingly, the upper surface of the acoustic turbidity is well beneath the seafloor on this profile - perhaps 5 to !Om below, This profile actually marks the furthest
5238000-1 + 5238000 0.0� southern extension of acoustic turbidity encountered in� the entire survey, and the deepest occurance with respect� to the riverbed,�
TMI(nn
5236000� 5236000+ -a0.0
x 528000 528800 529600 530400 531200 532000 532800
-600 ~ · J~ .',.� \' I Line 16 _ •� -~ -' yI526000 528000 530000 532000 534000� Az =67.0deg
Z(m)Arrows denote survey direction.
-lIOO LI----::c=-::::-:c=----=-----=------=:-----__----=----=--=:-::=--,-------------------...JRed dots mark position fixes on the seismic record. FaH~,pressFI -----: - P:36S1.53 rF:111.61-'-~~'i.2.2/1V:52361-43.(~ Figure 7.7.2 128
I 528005mE 5233483mN I I 528826mE 5233659mN I I 529761mE 5234161mN I I 530960mE 5234762mN I I 531976mE 5235192m!"J I 532955mE 5235649mN I '~'.~' .. '-"''', ':J.!: ",.-",' .." ... .,:.
.. j ~ i 'I..'\.
----~ ~]".......... .,.; .. : ,,''1,. '(. I:~~ -o· I
Line 17. On this line there again appears to be an extension of Permian rocks off the eastern shore. Note the complete absence of acoustic turbidity - the most recent channel sediments are still present, and the channel is in approximately the same position (i.e. the middle of the estuary), but there is no acoustic turbidity. At least three generations of channels are present, and the older generations are more extensive compared to further upriver. There are two prominent blocks separating the three main channels (west, central and east), and I interpret these as non-eroded cores of older Tertiary? sediments. The eastern block appears to be folded but this is probably a combination of erosion ad original repose angles (i.e. drape angles).
Derwent Estuary Seismic Reflection Survey
Line 17
Figure 7.8.2 130
I 528073mE 5231900mN I I 529147mE 5232586mN I I 530135mE 5233160mN I I 531053mE 5233617mN I I 532146mE 5233886mN I I 533106mE 5234124mN I
Red dots mark position fixes on the seismic record. I
61950
0 1000 2000 3000
Along profile distance (from 528350mE 5232081 mN)
4000 5000 Line 21
Figure 7.9.1 131
I I 528073mE 5231900mN I I 529147mE 5232586mN I I 530135mE 5233160mN I I 531053mE 5233617mN I I 532146mE 5233886mN I I 533106mE 5234124mN
I
Seatloor
I I. J:
rt ~ d"" I West I
~25.l
p1 E'" Q!>s«v~ _~ ~""'-"lloo loci< ."..., _ ~ -I~ Line 21. Unlike the previous line, this line only has two DI~II;II 81 ~ 1+14v1Or ~1+1:;'lo..I1 filled channels (not three). This suggests that one has
526000 528000 530000 534000 been diverted in another direction, or two have coalesced to form one (note that the channel to the west is very broad). The most recent cover of sediment on this profile
~ is thin, again suggestive of changes in channel 320.0