Figure 2. (a) Seismic profile P3 shown in red with a total
length of approximately 40 km was acquired in summer 2021. In blue the
common midpoint (CMP) line used to process the data. (b) Geological map
of the study area; dashed lines are mapped faults close to the profile.
Seismicity of the area is also shown. Grey circles have no focal depth
solutions.
Along the entire profile length, a dual-element seismic spread
consisting of wireless seismic recorders connected to 10 Hz vertical
geophones and a micro-electro-mechanical MEMs-based landstreamer was
used to acquire the data with the aim of imaging both near-surface and
deeper structures. This setup was also used during the acquisition of
profiles P1 and P2 (Malehmir et al., 2022). The wireless recorders
(total of 421) were used in an asymmetric split-spread roll-along
acquisition geometry to cover the entire profile length, with an active
spread of 7.4 km. To provide a regular fold and offset coverage, and
plan receiver deployment, different scenarios were evaluated for in what
order the receivers should be picked up. Twenty landstreamer mounted
MEMS-based recorders (2 m sensor spacing) were towed behind one of the
two seismic vibrators used as a seismic source for the survey. The
streamer data provided active quality control and GPS time stamping of
shot records. The two seismic vibrators (Innova 9t Mini-vibs - UV2) were
equipped with an additional weight of 3000 kg to improve ground coupling
and operated in a clock-phase synchronized mode to avoid distortion and
anti-phase vibration. At every source point, the vibrators (each
approximately 7 m long) were positioned with the actual shot position
assigned in between the two vibrators, collocated with wireless
receivers. Sweeps at both vibrators were simultaneously initiated with
initiation of recording of the streamer data and we treat the excited
seismic energy as a single source point. To improve the S/N ratio, four
sweeps per shot location were generated with the sweep parameters.
With 1961 wireless recorder positions, 17546 landstreamer recorder
positions and a receiver spacing of 20 m and 2 m respectively,
approximately 40 km of high-resolution seismic data were acquired. The
nominal shot spacing achieved was 20 m with a total of 1887 positions.
Due to a technical problem, landstreamer recorders were not used during
one day of the acquisition, hence they contain only 1736 shot positions.
A GPS antenna was used for time-tagging and sampling of the landstreamer
data. The time stamps were later used to harvest the corresponding data
from the wireless recorders that were autonomously recording during the
acquisition. Details of the survey parameters and the acquisition
strategy used can be found in Table 1. All wireless receiver positions
were accurately surveyed using a cm accuracy DGPS system.