Figure 5. (a) Unmigrated stacked section of the MEMs-based
landstreamer data. Arrows point at the interpreted bedrock reflection.
(b) First-break traveltime tomography results obtained from the wireless
recorder data. Note the correspondence between the low-velocity
structures and bedrock reflection geometries. (c-d) Highlighted part of
the landstreamer stacked section (a) shown for better display and
quality assurance purposes.
Results from the wireless data are focused on deeper reflection imaging
since they cover larger offsets and are better designed for this
purpose. An unmigrated stacked section of the profile is shown in Figure
6. The most reflective part (D1) is in the central area of the profile,
between CMP 1200 and 2000, where a strong package of reflectivity (also
clearly observable on some receiver gathers, e.g., Figure 3) from 0.7 to
1.2 s with a domed shape pattern is visible. This main reflection on its
eastern bottom intersects a west-dipping package of reflections, but
with albeit lower continuity (F2). They project to the surface at the
location of the Pocheon fault, where also a strong back-scattering of
surface-waves is observed. This interpretation needs to be viewed with
caution as the projection downwards and to the surface may not
necessarily be valid. An important feature of the data is the strong
reflectivity observed in the central part of the profile (D1). While the
nature of this strong reflective zone is unclear, the presence of mafic
dykes and natural geothermal fields, suggesting fluid presence that can
enhance the reflectivity, in the area (Lee et al., 2010) make them a
plausible explanation for the origin of the reflectivity. If the
interpretation of the geometry is correct and if there is any fault
underlying this domed-shaped reflective zone, then there might be a
relationship between the geometry of the domed-shaped reflective zone
and an underlying fault.
The apparent bending of the reflective zone (D1) towards the underlying
planar-type reflectivity may imply a sense of movement or shearing with
a “fault-bend fold” pattern (Figure 7), suggesting that much of the
movement along the planar surface (fault) is then reverse (Suppe and
Medwedeff, 1990). In this interpretation scenario, the reflective
package D1 should be from materials that are isolated and planar in
nature like sills and dykes, which is highly likely as they are present
on the geological map of the area. There might also be fluids in this
area as geothermal fields are known in the central region of the profile
(Lee et al., 2010).
On the most eastern part of the profile, an east-dipping reflection (F1)
is visible from 0.2 to 0.6 s; however, it is not strong and can only be
observed in a portion of the shot/receiver gathers (Figure 6). The
reflection projects to the location of the Wangsukcheon fault (Figure
2). Therefore, it is likely generated by the Wangsukcheon fault plane,
implying a zone of brittle structures.
On the western part of the profile, various sub-horizontal reflections
with a low lateral continuity are visible down to 2.5 s (or
approximately 7.5 km depth). Between CMP 700 and 800, no coherent
reflection is observed, and strong noise is present. This region
coincides with the Chugaryeong fault; hence the lack of reflectivity may
be explained by the sub-vertical nature of this fault (F3). The strong
surface-wave back scattering energy observed at this location further
supports this interpretation. Vertical barriers like faults are natural
features that can back-scatter surface-waves (Blonk and Herman, 1994; Yu
et al, 2014).