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DACIA-PLAN Survey
In the year of 2001, the DACIA PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic sounding survey was performed in the south east of Romania. This experiment was part of an international collaboration between the Netherlands Research Centre for Integrated Solid Earth Science, the University of Bucharest, the Romanian National institute for Earth Physics, the University of Karlsruhe, Germany, the University of South Carolina and the University of Texas, El Paso, USA.
The seismic profile of DACIA-PLAN is approximately 140 km long, running in a WNW-ESE direction from the south-eastern Carpathian orogen to near the Danube delta, crossing the seismically active Vrancea zone and the Focsani Basin (fig 2.1). The elevation ranges from 40 m in the south-eastern part of the profile to 1240 m at the mountainous area in the north-west. The recording was carried out from west to east in three different but overlapping segments, referred to as deployment 1, 2 and 3 in the text.
The primary ambition with this extensive survey was to attain new information about the deep structure of the external Carpathian nappes, and to describe the geometry of those Tertiary/Quaternary basins evolved within and nearby the seismically active Vrancea zone, (Panea et al., 2005).
Geological Setting
The crossed part of the Carpathians is made up by the “External Moldavides System” (Panea et al., 2005), which comprises the Tarcau nappes, Marginal folds and Subcarpathian nappes (fig 2.2). The whole of the Moldavian nappe system is foundated by crystalline rock, which for the most part is covered by Palaeozoic and Mesozoic sediments (see fig 2.3 & 2.4). The depth to the basement cover is still under discussion, previous geological and geophysical studies have suggested a thickness of about 8 km for the nappe pile in the area crossed by the DACIA PLAN profile (e.g., Matenco and Bertotti., 2000). In the paper of Bocin et al., (2005) an upper crustal velocity model is presented, based on tomographic travel-time inversion of DACIA PLAN first arrivals (fig 2.7). This model showed apparent basement (material with velocities above 5.8 km/s) lying at depths as shallows as 3-4 km in the westernmost segment of the DACIA PLAN profile.
The Tarcau and Marginal Folds nappes in the west, mainly consist of Cretaceous marine basin sediment and clastic sediment deposited under Palaeogene to Neogene (Panea et al., 2005). Further to the east, the Subcarpathian nappe mostly consists of sediments deposited in a shallow marine to brackish environment, which makes it possible to find shales and marls in the area. Also small portions of evaporitic formations like gypsum and salt, formed under lower and middle Miocene, can be found here (Stefanescu et al., 2000).
The foreland of the south-eastern Carpathians in the area of the DACIA-PLAN profile consists of two stable units, with internally dissimilar characters. These units are the East European/Scythian and the Moesian platforms which are separated by the North Dobrogea orogenic zone (fig 2.6). East European and Scythian are considered to be two crustal blocks, which lie north of the Trotus Fault. The relatively thick crust (40-45 km) of the blocks has been developed below a thin nappe pile or below the foredeep sediments in the area (Bocin et al., 2005). South of the Trotus fault and west of the Pecenaga-Camena fault lies the thinner (35-40 km) Moesian block. The crustal unit is Precambrian aged, covered by up to 13 km Middle Miocene to Quaternary sediments in the Focsani Basin (Bocin et al., 2005). These sediments are in turn underlain by an up to 10 km thick Paleozoic to Paleogene sedimentary sequence (Landers at al., 2004). East of the Peceneaga- Camena fault, the North Debrogea zone separates the Scythian and Moesian units. This zone is made up of a complex and highly deformed basement, overlain by an up to 13 km thick heterogeneous Triassic-Cretaceous sedimentary layer (Bocin et al., 2005).
The foredeep, developed in front of the eastern and southern Carpathians (fig 2.6) consists of evaporates and clastic rocks (moulasse-conglomerates, shale and sandstones), that thickens closer to the thrust belt. The width ranging from about 10 km in the north to more than 100 km further south. The thickest sedimentary deposits are found in the Focsani Basin, where Miocene-Pliocene deposits can reach about 13 km in some places (Panea et al., 2005).
In the area of the DACIA-PLAN profile, the contact zone between foredeep and Carpathian orogen is considered as a blind thrust overlain by Miocene-Pliocene post tectonic cover. Studies of the position and tilting of the sediments, and notation of the eastward dip of the Upper Sarmatian unit, imply that the contact zone is a backthrust (Matenco and Bertotti, 2000).
Figure 2.3: Geological section of south-eastern Romania, comprising DACIA-PLAN region in the eastern half of the figure. See fig 2.6 for location of the geological structures shown here. Note that the maximum depth under the nappes is about 10 km, instead of 3-4 km suggested in Bocin et al., 2005. Image from Landers et al. (2004).
Figure 2.4: Geological interpretation of different depths in the region. DACIA-PLAN profile crosses right over the section in an east-west direction. Image from Landers et al. (2004)
Vrancea Zone
The Vrancea zone is one of the most seismic intense areas in Europe (Landers et al., 2004). Major earthquakes have occurred in last century (1940, 1977, 1986 and 1990), causing high death rate and large economical damage. This complex region is interesting in many aspects, and the mapping of the area is high on the scientific agenda.
Traditionally, the zone has been divided into two vertical segments located at different depths (Fig 2.5). Intermediate earthquakes define the deeper segment, with hypocenters located at 60-200 km depth. Events occurring in this zone can be high in magnitude ( 7.4 ). The other segment is characterised by shallower earthquakes at depth 20-60 km, with hypocenters shifted to the east. Those are considered to be crustal events, with a moderate magnitude ( 5.6 ). A relatively inactive seismic zone in the depth range of 40-70 km separates the two segments.
The cause of the crustal earthquakes is believed to be linked with the intermediate earthquakes in the region (Panea et al., 2005). It is not fully understood what triggers these intermediate events, but a popular hypothesis suggests that the Vrancea zone represents an isolated section of the Eurasian plate where lithospheric delamination is still going on (e.g Landers et al., 2004). The geometry for such a process is not clarified, and was a motive for those deep seismic refraction and reflection surveys that have been carried out in the region lately (see 2.4).
Previous Investigations
This part of Romania has been investigated by deep seismic reflection and refraction surveys before. The extension of some of these profiles is shown in fig 2.6. The foreland and parts of the easternmost thrust belt have also been objective for exploration by extensive reflection profiling (Panea et al., 2005).
The line XI is one of a number deep seismic sounding profiles that were collected during 1970-1974. The purpose was to image the crustal geometry in the region by recording refraction arrivals from the Conrad and Moho discontinuities as well as the sedimentary basement. The interpretations, that were done from this data (e.g. Râdulescu et al., 1976), show high consistency with more modern interpretations over the same area (e.g. Panea et al., 2005). The crustal geometry seen in the cross-section of fig 2.5 is a result from the XI profile.
VRANCEA99 is a 300 km long refraction profile, running in a NNE-SSW direction. The profile is geographically crossed by the VRANCEA2001 refraction profile that is extended in E-W direction over the whole of Romania. An intention with those surveys was to reveal new information about the geometry of the crust and upper mantel in the vicinity of the Vrancea zone. Based on data from these projects, several reports have been presented describing the subsurface of the region (e.g Hauser et al., 2002).
Figure 2.6: Tectonic map over south-eastern Europe. The DACIA-PLAN profile is the thick black line denoted DP. Also the location of the seismic refraction and reflection profiles mentioned in the text is shown, VRANCEA99 (VR99), VRANCEA2001 (VR01), deep refraction profiles X1. Image from Panea et al. (2005).
The DACIA-PLAN reflection data has been processed and interpreted before. Panea et al, (2005), describes the processing of the dataset into two stacked seismic sections. In the first stack (full stack), data from the entire survey section was processed to a depth of 20 s. The intention was to investigate the crust and upper mantle under the Vrancea zone. In the second stack (partial stack), data from deployment 3 was processed for investigation of the sedimentary and upper crustal structures in the Focsani Basin. The reason why processing was carried out as two separate jobs claimed to be that the signal-to-noise ratio was much lower in deployment 3 than in the other two. The reflected phases are well imaged from the sedimentary deposits in the basin within deployment 3, and are best seen in the partial stack (Fig 7.1 b). Moving to the west, geology becomes more complex under the sedimentary nappes of the south-eastern Carpathians, which could have contributed to the fact that quite a few well resolved reflection phases were found in deployment 1 and 2, (Fig 7.2 b).
The clear reflections from the sedimentary deposits imaged in the partial stack made the interpretation of this region straightforward, and the final result showed good correlation to previous descriptions of the basin. Despite the fairly low image quality in most of the full stack, a quite detailed interpretation of the section was presented (see Panea et al., 2005). Here, observed events in the stack were interpreted in the context of previous reports describing the region (e.g. Hauser et al., 2001).
From the DACIA-PLAN first arrival data, Bocin et al., (2004), used tomographic inversion to create a high resolution 2.5D velocity model of the upper crust along the seismic profile (Fig 2.7). Velocity structure over the Vrancea Zone suggests the pre-Tertiary basement lying at a depth less than 5 km, which is about the half of what has been suggested before (e.g., Matenco and Bertotti, 2000). In the Focsani Basin; depth to the basement, as well as the lateral structural heterogeneity at the basement level, show high correlation to previous interpretations of the basin (Bocin et al., 2004).
Filtering Domains
Thanks to the mathematical principles of the Fourier transform the procedures of designing filters and applying them on an input signal can be carried out both in time- and frequency-domain. Consequently, it might be a good idea to study the filtering process from both those viewpoints. When filtering a signal in time domain we use convolution, which is a mathematical operation that modifies an input time series xt (the seismic trace) with another time sequence ht (filter operator). The samples of the operator are often referred to as the filter coefficients. Convolution between xt and ht is written as ft = xt * ht , (2.1).
where each coefficient is given by n ftht j x j . (2.2) j0.
When filtering a seismic trace a zero- or minimum phase operator is commonly used. This is because these types of filters will minimize the phase shift of an input trace. After convolution, the output trace will only contain those frequencies that made up the filter operator.
Table of contents :
1 Introduction
2 Background
2.1 DACIA-PLAN Survey
2.2 Geological Setting
2.3 Vrancea Zone
2.4 Previous Investigations
3 Theory
3.1 Filtering
3.2 Deconvolution
3.3 Refracted and Residual Static Corrections
3.4 CMP Sorting
3.5 Procedure of Stacking
4 Prestack Processing
4.1 Data Parameters
4.2 Reduction of Data
4.3 Displaying of Data
4.4 Geometry Correction
4.5 Trace Editing
4.6 First Break Picking
4.7 Static Correction
4.8 Deconvolution and Spectral Whitening
4.9 Front Muting
5 Processing of Data
5.1 Brute Stack
5.2 First Velocity Analysis
5.3 Residual Statics
5.4 Final Stack
6 Interpretation
6.1 Partial Stack
6.2 Full Stack
6.3 Velocity Model
7 Discussion
Bibliography
