Data acquisition Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue1-2.2000:

has concentrated on determining the large-scale crustal structure. The eastern limit of the wide angle reflectionrefraction EUGENO-S profile 3 EUGENO-S Working Group, 1988 is located immediately west of the Sveconorwegian front, north of the study area. Upper crustal velocities along the eastern end of the profile range from 6.1 to 6.5 kms. The profile did not continue far enough to the east to allow the lower crustal velocity structure and depth to Moho to be deter- mined in the area east of the PZ fault. Just west of this fault, the crustal thickness was estimated to c. 40 km and disruptions in the Moho were inferred farther to the west. The EUGENO-S Working Group 1988 interpreted both the frontal defor- mation zone and the MZ to be west-dipping features at depth. These structures were also in- ferred to penetrate the entire crust down to the Moho and to possibly disrupt the Moho boundary. In conjunction with the large-scale refraction profiles of the EUGENO-S project, airgun profi- les were shot on Lake Va¨nern to investigate the MZ. The airgun shots were recorded by numerous refraction stations south of the lake and on two refraction stations east and west of the lake Green et al., 1988b. The airgun shots were also recorded on the Va¨rmlandsna¨s peninsula by a 7 km long multi-channel reflection system Juhlin et al., 1989. The refraction station data Green et al., 1988b provided more detailed information on the veloc- ity structure, dips of interfaces and depth to Moho than the EUGENO-S profile 3. Green et al. 1988b interpreted the Moho to dip about 5° to the east below Lake Va¨nern, reaching a depth of about 43 km below the eastern part of the lake. Several crustal wide-angle reflections were also observed. These were interpreted to originate from c. 100 m thick layers with either a positive or negative velocity contrast of about 0.4 kms and dipping about 10° to the west. The reflection seismic images from the Va¨rmlandsna¨s survey Juhlin et al., 1989 are consistent with the inter- pretation of Green et al. 1988b, where reflections from the upper crust are generally sub-horizontal or dip to the west whereas lower crustal reflectors dip to the east. About 70 km north of the study area, a 100 km long deep seismic profile was acquired in three stages in the mid to late eighties Dahl-Jensen et al., 1991. Reflectivity is weak along most of the profile. However, between the MZ and the PZ fault, strong sub-horizontal to gently east-dipping reflectivity is observed in the upper 2 s TWT 6 km. Farther east, the reflectivity weakens, but has a west-dipping pattern. Stephens et al. 1996 used this change in the dip pattern along the profile as support for the existence of the fan-like structure mentioned earlier. Earthquake studies using arrival times from local events in the Lake Va¨nern area Slunga, 1985 also indicate that the Moho dips to the east. On the basis of these earthquake studies, the Moho depth is estimated to be 44 km below the SFDZ.

4. Data acquisition

The westernmost limit of the seismic profile reported here is located close to the eastern shore of Lake Va¨nern Fig. 2b, immediately east of the PZ fault. In this area, the tectonic foliation dips about 15 – 40° to the east. The profile extends to the south-east where the surface mapped deforma- tional fabric becomes progressively more steeply dipping, and is near-vertical about 1 km east of Bjo¨rneborg Fig. 2b. The easternmost 3 km of the profile crosses an area where the surface mapped foliation dips about 60 – 80° to the west. Data were acquired over an 8 day period in August 1996 Table 1. The eastern half of the profile station 340 – 681 runs along the main road which connects Kristinehamn with De- gerfors via Bjo¨rneborg. No shots could be fired in Bjo¨rneborg station 453 – 551 and this section of the profile was undershot. The undershooting im- plies that information in the upper 500 ms, corre- sponding to c. 1500 m depth, was not obtained below Bjo¨rneborg. The western half station 1 – 340 runs mainly along small roads and across fields. Noise from the main road degraded the data quality along the eastern half of the profile. Geo- phone stations located in Bjo¨rneborg were also 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