Near-surface structure of the Carpathian Foredeep marginal zone in the Roztocze Hills area
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Abstract
Shallow seismic survey was made along 1280 m profile in the marginal zone of the Carpathian Foredeep. Measurements performed with standalone wireless stations and especially designed accelerated weight drop system resulted in high fold (up to 60), long offset seismic data. The acquisition has been designed to gather both high-resolution reflection and wide-angle refraction data at long offsets. Seismic processing has been realised separately in two paths with focus on the shallow and deep structures. Data processing for the shallow part combines the travel time tomography and the wide angle reflection imaging. This difficult analysis shows that a careful manual front mute combined with correct statics leads to detailed recognition of structures between 30 and 200 m. For those depths, we recognised several SW dipping tectonic displacements and a main fault zone that probably is the main fault limiting the Roztocze Hills area, and at the same time constitutes the border of the Carpathian Forebulge. The deep interpretation clearly shows a NE dipping evaporate layer at a depth of about 500–700 m. We also show limitations of our survey that leads to unclear recognition of the first 30 m, concluding with the need of joint interpretation with other geophysical methods.
Keywords
Carpathian Foredeep Seismic imaging Traveltime tomography Near-surface Joint seismic interpretationIntroduction
The reflection seismic methods are well known and successfully used in industrial applications over the decades. They are also more often used in smaller scales to recognize near-surface structures. The potential of high-resolution seismic reflection methods has been proven by number of groups studding unconsolidated or postglacial sediments (Buker et al. 1998; Bachrach and Nur 1998; Steeples and Miller 1998; Francese et al. 2007). Standard reflection methods can resolve structures starting from several meters down to several hundreds of meters (Pullan and Hunter 1990; Steeples and Miller 1990; Feroci et al. 2000; Steeples 2000; Dusar et al. 2001; Sugiyama et al. 2003; Bruno et al. 2010; Green et al. 2010).
To recognize the near-surface geological structures an optimal solution is to utilise wide-angle observation and combine them with high resolution reflection data. A successful applications of this approach (Bruno et al. 2010, 2013) shows, that seismic velocities recognized from observed refractions significantly enhance the reflection image of shallow structures.
Here, we report the result of joint interpretation of wide-angle seismic along 2D profile designed to recognize the structure of the marginal zone of the Carpathian Forefront. Recorded large offsets were helpful not only to estimate velocities, but also to image a deep bedrock reflection, that would not be possible using standard deployment. Moreover, designed survey with wireless stations and especially designed accelerated weight drop source was cost-effective and can be performed in a very short time giving a productive tool for the geological studies. Combination of reflection seismic imaging and travel time tomography, that is the tool to recover a high resolution images, has been used before in tectonic scale (Malinowski et al. 2013), the industrial scale (Majdański et al. 2016; Vesnaver et al. 1999), but also in near-surface studies (Bruno et al. 2010, 2013).
Geological setting
Schematic map showing: a general location of the study region in Poland, b tectonic structures in the area; c location of seismic profile in the marginal zone of the Carpathian Foredeep; d detailed geological map in the vicinity of the profile (after Kurkowski 1998)
This region is dominated on the surface by sedimentary rocks, mainly Cretaceous marls or sandstones and Neogene (Miocene) sandstones, limestones and marls. These rocks were formed in shallow sea with high hydrodynamic energy. Miocene rocks are preserved only in the form of erosional patches along south-western marginal zone (Wysocka and Jasionowski 2006). Carpathian Foredeep in the north eastern part consists mostly of Miocene flat bedded lightly lithified shales, of the same age as the rocks described above, covered by thin layer of Quaternary sediments. In the whole Miocene sequence, the only one characteristic layer, evaporates, consists mainly of anhydrites and gypsum. This layer has variable thickness from 0 up to 60 m and has great significance as the main correlation level and seismic marker on the scale of the entire Carpathian Foredeep (Myśliwiec 2004; Krzywiec 2001). These beds can be observed in the Kozaki 1 drill core. Drilling was done approximately 7.5 km to the south-west from the beginning of the profile. These evaporates are mainly gypsum with a thickness of 34 m and occurs at a depth of 609–643 m. This salina basin was developed during the Badenian salinity crisis in northern Central Paratethys (Bąbel 2004).
Due to the difficulty of stratigraphic recognition of sedimentary rocks on the area of the Roztocze Hills, evaporate layer is the only certain horizon which allowed to determine the relative age of the succession. The lack of evaporates layer on the Polish part of the Roztocze Hills area and different types of rocks with different thicknesses are the main problems in the deposits correlation along the north-eastern Carpathian Foredeep marginal zone.
Measurements were made between Józefów and Pardysówka quarries, which are located in the distance of about 950 m from each other, on the most south-western range of hills of the Roztocze Hills area. Both of them are characterized by Miocene complex, consisting mainly of conglomerates and limestones (Wysocka et al. 2006). The whole complex is located directly on the Cretaceous basement and is inclined slightly toward the south (Roniewicz and Wysocka 1999). In both quarries many normal and reverse faults were found, associated with post-sedimentary Miocene or younger seismic activity in the area. Their planes are parallel to the Carpathian Foredeep marginal zone (Jaroszewski 1977; Jankowski and Margielewski 2015). All observed faults are crossing all the layers at a low angle (Wysocka et al. 2006).
The seismic survey and the field works
Field measurement scheme showing four deployments (bottom) using 60 seismic stations and 80 shot positions in each. Thanks to overlaps of shots, the minimum fold was set as 22 (middle panel). The top panel shows the significant change of 22 m in the elevation along 1280 m long profile
Example of shot gathers for shots 1032 and 1069 (left panels) after diversity stack of four repeated weight drops. The middle panels show carefully selected front mute, that removes linear refraction arrivals. Right panels show the NMO effect with velocity of 1600 and 1400 m/s, respectively. Clear shallow reflections are visible at 55 ms for shot 1032, and 90 and 140 ms for shot 1069. All panels presented with AGC (100 ms) applied
Example of optimal processing for shot 1069. Raw gather after diversity stack of four repeated weight drops (a); gather after spectral whitening b shows flat spectrum up to the 180 Hz; c, d shows corresponding processing steps as in a, b with applied high pass filter above 30 Hz; inlets in each panel shows spectral content of the gather. All panels presented with AGC (100 ms) and front mute applied
Acquisition parameters
Feature | Measurement |
---|---|
Vertical stack | 4 |
Sampling interval | 2.5 ms |
Record length | 1 s |
Receivers | 4.5 Hz |
Station interval | 5 m |
Shot interval | 5 m |
Active channels | 60 |
Fold | 22–60 |
Offset | 0–345 m |
Data processing
Reflection seismic imaging
Two paths of data processing
Common processing | Deep focus (additional steps) | |
---|---|---|
1 | Data cutting | |
2 | Geometry and sorting | |
3 | Trace editing | |
4 | Elevation statics | |
5 | Refraction statics | |
6 | Front mute | |
7 | Spherical divergence with time | |
8 | Spectral whitening (10–180 Hz) | |
9 | Tail mute | |
10 | NMO | |
11 | Stack | |
12 | FK and high-pass filtering | |
13 | Depth conversion |
The processing has been performed separately for shallow area and for deep structures. For both paths the continuously recorded data has to be cut according to shot times. The next step was to add geometry and sort to shot gathers with detailed quality check. This step is time consuming but important in case of wireless stations, as mistakes in geometry are easy to make and might have a severe impact on the final quality. Further on, a trace editing was performed to exclude noisy channels resulted from poor geophone coupling. Elevation statics was estimated based on the topography and the floating datum has been set just below the lowest elevation at 240 m. Refraction statics was calculated using standard routines implemented in the commercial seismic software with limited offset of 100 m. This allows to recognize the weathering layer without taking into account large velocity variations resulted from change of topography. A next step in common processing was careful front mute to remove guided waves and preserve wide angle shallow reflections. This procedure is crucial, as explained in many papers (Buker et al. 1998; Robertsson et al. 1996), and has been manually performed for each shot. The effect of application of front mute is presented in Fig. 3 in middle panels, where clearly refracted arrivals have been removed while shallow reflections are preserved. The first recognition of near-surface velocities based on analysis of shallow reflections flattening shows variable velocities along the profile that varies between 1400 and 1600 m/s. The effect of reflection flattening is present in Fig. 3 (right panels), showing also effect of optimal combination of 70% stretch mute and the front mute.
Example shot 1069 deconvolution tests: surface consistent deconvolution (a), surface consistent spectral whitening (b), single trace deconvolution (c), spectral whitening (d). Inlets in each panel shows spectral content of the gather. Simple spectral whitening d results in the sharpest wide-angle reflections. All panels presented with AGC (100 ms), front mute and high-pass filter (> 30 Hz) applied
The shallow structure processing
Result of the first breaks travel time tomography showing the P wave velocity in the shallow structure (top). White line marks the elevation, black dashed lines mark strong dipping velocity gradients indicating the discontinuity of velocities. The middle panels shows the same result with seismic ray paths to mark the well recovered areas. The lower panel, with different vertical exaggeration (VE = 0.2) shows the velocity field based on tomographic result and the velocity analysis, that was further used to the depth conversion. Dashed rectangle marks the area with tomographic velocities
Shallow structure in the depth domain obtained with shallow reflections enhanced processing (top). The middle panel shows several discontinuities in the flat reflections (marked with colour solid lines) that do not reach the surface. Those marked with solid yellow lines are confirmed with tomography. Yellow polygon marks the area of the low reflectivity that differs from surrounding structures, and corresponds to a zone of the strong velocity gradient in tomography. Dashed rectangular marks the area as in the lower panel. The lower panel, with different vertical exaggeration (VE = 1), shows tomographic result as in Fig. 6 with an unreliable greyed area without ray coverage
The deep structure processing
Clear deep reflection (red arrow) at about 550 ms is visible at a number of shots. Example for shot 2002 shows raw data after diversity stack (left), application of front mute and high pass filter above 35 Hz (middle), and NMO with velocity of 1500 m/s and high pass filter above 50 Hz. Simple high pass filtration clearly removes surface waves and enhance reflections. Flattening of reflection for small NMO velocity shows deepening of the reflector toward NE. All panels presented with AGC (100 ms) applied. The red line marks the tail mute
Time domain stack with deep reflection enhanced processing. The data gaps at the deployment contact points are the effect of tail mute application to remove effect of surface waves. Clear deepening reflection at 450–700 ms marked with red line is visible along the profile. In shallow part between 50 and 200 ms several discontinuities of reflections are visible. Strong cut-of layering marked with black line is showing the Marginal Zone of the Carpathian Foredeep. Yellow rectangle marks the shallow area as in Fig. 7
Conclusions
Shallow seismic investigations yielded detailed images of the Carpathian Foredeep marginal zone. A large offset survey combined with modified accelerated weight drop source was specially designed to allow both high resolution reflection image and refraction tomography. Four strokes of the source give enough energy to observe refractions at all offsets up to 350 m, but also to recognize a deep structure down to 700 m. Two processing paths were used to enhance both shallow and deep structures, resulting in detailed image of the near-surface faults, but also a sharp deep reflection. The 5-m spacing used for both shots and receivers was not dense enough to clearly recognize the first 30 m. Additional information like ERT experiment or different seismic processing, e.g., MASW (Park et al. 1999), and finally joint interpretation might be used to further verify this part of the structure. Still, a clear image of a main fault in the area was presented. This weight drop seismic profile was performed to recognize near-surface structures, that is why it was surprising to observe clear reflection at 700 m (Fig. 9). For surveys of this type with limited number of stations we suggest to use different spacing for shots and receivers. For example deploying stations with 8 m spacing and shooting with 2 m spacing will result in similar fold, but would limits number of deployments, limiting number of repeated shots. Thanks to dense shooting it would be possible to recover shallower structures, but wider deployments would give important long offset refracted arrivals. In the end acquisition time should be similar with higher resolution results. To make it optimal it would be beneficial to perform test shooting and recognize the maximum offset of clear observations, and design the survey for specific environment.
The horizon marked in red in Fig. 9 refers to the evaporate layers that occurs throughout the area of the Carpathian Foredeep and represents the best reference horizon for geophysical and geological research in this area. Such a evaporate layer is observed and well documented at much shallower depths in Ukraine.
Summarizing, we show that presented type of seismic survey with weight drop source and standalone stations can lead to a clear image of the structure from 30 down to 700 m, that could be performed effectively with low costs. The key to achieve detailed result is a careful data analysis using multiple techniques, that we hope to improve even further in the future.
Notes
Acknowledgements
We would like to thank colleagues in the Imaging Department of IG PAS, as well as Editor Michał Malinowski for many suggestions that significantly improved the quality of this paper. This research was funded by National Science Centre, Poland (NCN) Grant UMO-2015/19/B/ST10/01833. Part of this work was supported within statutory activities no. 3841/E-41/S/2017 of the Ministry of Science and Higher Education of Poland.
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