Table 1 Acquisition parameters for the seismic reflection survey
End-on and shoot-through Spread type
140–150 No. of channels
100 m Minimum offset
25 m Geophone spacing
Single 28 Hz Geophone type
Nominal shot spacing 100 m
0.5–1 kg dynamite Charge type
3 m Nominal charge depth
Nominal fold 20
SERCEL 348 Recording instrument
2 ms Sample rate
Field low cut Out
250 Hz Field high cut
16 s Record length
Profile length 17 km
Total number of shots 144
There are several dips present in the data suggest- ing that dip moveout DMO corrections should
be made prior to stacking. Attempts to use DMO were made, but the irregular shooting geometry
resulted in that DMO was not successful in im- proving the stack. The final stacked section is
shown in Fig. 5.
Migration of seismic data is necessary to place the reflecting events in their true spatial position.
Wavefield migration of discontinuous reflections such as those in the final stack Fig. 5 is difficult.
Therefore, an automatic line drawing routine was
Table 2 Processing parameters for CDP line
Read SEG2 data 1
Spherical divergence correction 2
3 Spike and noise edit
Air blast attenuation: v = 340 ms 4
5 Trace equalisation: 1000–2500 ms
6 Elevation statics
Refraction statics 7
Surface consistent deconvolution 8
Design gate: 1000–3000 ms Type: spiking
Operator length: 200 ms White noise: 0.1
9 Bandpass filter: zero phase
25–50–120–180 Hz: 0–1000 ms 20–40–100–150 Hz: 500–2000 ms
15–30–80–120 Hz: 1500–3000 ms 10–20–65–90 Hz: 2000–5000 ms
10 Trace equalisation: 1000–2500 ms
AGC: 300 ms window 11
12 FK filter: remove
− 2500–−4500 ms
2500–4500 ms CDP sort
13 14
Velocity analyses 15
Residual statics Trim statics: maximum 1 ms
16 17
Mute 18
Stack 19
AGC: 300 ms window Dynamic SN Filtering
20 21
FX Decon: 20–100 Hz 22
Trace equalisation: 1000–3000 ms at CDP 1
0–2000 ms at CDP 1150
noisy as a result of the ironworks plant there. In addition, wind noise was present during the first
days of acquisition along the eastern half of the profile. These factors resulted in generally better
data quality along the western half of the profile.
5. Data processing
Data were recorded to 16 s with signal penetra- tion to about 3 – 5 s on the majority of the shots.
Therefore, only the upper 5 s of data were pro- cessed and only the upper 3 s of data have
adequate signal penetration along the entire profile. The data were processed at Uppsala Uni-
versity using a commercial processing package Table 2. All data were projected onto a straight
Common Datapoint line CDP for stacking and interpretation Fig. 2b. Raw and processed shot
records show predominantly east-dipping struc- tures in the west Fig. 3 and west-dipping struc-
tures in the east Fig. 4. Tests of various stacking velocities show that more coherent images were
obtained at low stacking velocities in the west and at higher stacking velocities in the east. This is
primarily as a result of the steeper dips in the east.
Fig. 3. Raw a; and processed b shot records from the western part of the profile. Processing up to step 12 in Table
2 is shown here.
structural geometry for the reflecting elements with moderate to steep west-dipping reflectors in
the east and more gently east-dipping reflectors in the west. The maximum dip of imaged reflections
is approximately 60 – 70°. Superimposed on this geometry are higher amplitude, sub-horizontal to
moderately dipping reflections in the western part of the section in the depth interval 3 – 7 km
marked A in Figs. 6 and 7.
TIB rocks probably do not contain significantly strong enough lithological contrasts in an unde-
formed state to generate reflections. However, when sheared, lithological variations related to
formation of mylonites may have developed and these variations can include large enough con-
trasts in velocity andor density to generate reflec- tions Hurich et al., 1985; Ratcliffe et al., 1986;
McDonough and Fountain, 1988. Sheared doler- ites within the TIB rocks may also have generated
some of the reflectivity in the western part of the present profile. This interpretation was also sug-
gested by Dahl-Jensen et al. 1991 for the deep seismic profile to the north.
The higher amplitude reflections in the west at depths between 3 and 7 km A in Figs. 6 and 7
appear to transect the easterly-dipping lower am- plitude reflections. However, these reflections may
emanate from out-of-the-plane of the profile. Sev- eral show a diffractive nature on the time section
Fig. 5, indicating they may originate from the northern or southern ends of near-vertical dolerite
sheets situated south or north of the profile, respectively.
6
.
2
. Comparison with surface geology The bivergent structure of the lower amplitude
dipping reflections is in good agreement with sur- face structural information Fig. 8, with the ex-
ception that the hinge line where the dip changes from east to west is displaced c. 3 km to the west
on the seismic profile, relative to the surface geol- ogy. However, the number of observed surface
structures are limited in the region of the hinge axis and west-dipping structures have been ob-
served south of the profile at CDP 800. Surface dips are generally in the order of 15 – 50° west of
the hinge line and 60 – 80° east of it. More steeply
Fig. 4. Raw a; and processed b shot records from the eastern part of the profile. Processing up to step 12 in Table 2
is shown here.
used to pick the strongest reflections in the final stack Fig. 6. This line drawing of the stacked
section was then migrated at a velocity of 6000 ms and the resulting section Fig. 7 forms the
basis for the interpretation of the data.
6. Results
