Imaging of Hidden Structures from the North Apuseni Mts, Romania, Using Narrow-Angle Seismic Reflection Data

I present results of processing and structural interpretation of narrow-angle seismic reflection data recorded over an area of 30 × 50 km located in the southern part of the North Apuseni Mts, Romania. The investigated area is characterized by complex subsurface geology and rough topography. The seismic measurements were performed along five linear profiles, P1-P5, using an active spread of 96 geophones for each shot point; geophone spacing was 25 m. The length of each acquisition line is greater than 10 km. The sig-nal-to-noise ratio of these data varies along the lines and its variation is considered to be an effect of rough topography, complex subsurface geology and varying surface conditions encountered during seismic data acquisition. The data processing was performed using a standard processing flow but with different processing parameters from one data set to another. I obtained five depth-converted migrated seismic sections after data processing. The accuracy of the depth values depends on that of the stacking velocities obtained from the velocity analyses performed on the filtered seismic data. Borehole information is not available, the investigated area belonging to the areas investigated for hydrocarbons. Each seismic section shows a different structural image of the subsurface and provides useful information about the tectonic and stratigraphic evolution of the investigated area. I obtained various structural images of the subsurface after the interpretation of the depth-converted migrated seismic sections, from a simple one with undeformed and inclined reflectors to a complex one with folded and faulted reflectors, especially the older ones. I interpreted intrusive bodies piercing through the overlying sediments, which are in good agreement with the results of older geophysical studies.


Introduction
Accurate information about the subsurface geological structure is obtained after the integrated interpretation of the seismic and the borehole data recorded in studies performed for hydrocarbon exploration. The accuracy of the structural images of the subsurface depends on the complexity of the geological structure and the errors introduced during the seismic data acquisition and processing.
Seismic reflection data can be recorded in two-or three-dimensional seismic surveys using short or long active geophone spreads for each shot point. Decades ago, on the Romanian territory, seismic reflection data were recorded using active spreads of 48 or 96 geophones for each shot point, the geophone spacing being 50 m or 25 m. For structural interpretation, seismic data processing has been done using a standard processing flow [1].
Numerous hydrocarbon-bearing structures were identified, and exploited, in areas from the Romanian segment of the Eastern Pannonian Basin after the geological interpretation of the time seismic sections and the borehole data. Because of this, little information about seismic and borehole data and their interpretation was made available for external publications. Decades ago, a small set of seismic sections and borehole data provided by industry were used to explain the tectonic and stratigraphic evolution of the northern part of the Eastern Pannonian Basin [2]. For the Beius Basin, located in the southern part of the North Apuseni Mts, a structural contour map of the basement and an isopach map of the Neogene formations were built after the geological interpretation of the seismic sections provided by industry [3]; images with interpreted time seismic sections, the location and the distribution of the seismic lines are not shown in [3].
In this study, I analyze and process vintage seismic reflection data recorded in surveys performed for hydrocarbon exploration and made available by industry for research studies. The study area is located in the southern part of the North Apuseni Mts, which includes the Beius Basin; its size is about 30 × 50 km. The aim of the study is to image the complex geological structure of the subsurface using depth-converted migrated seismic sections obtained after the processing of narrow-angle seismic reflection data.

Geological Description of the Study Area
The investigated area is located in the southern part of the North Apuseni Mts ( Figure 1). All five seismic lines, P1 -P5, are located in the area between the Padurea Craiului and Codru Moma Mts, Romania, mainly corresponding to the Beius Basin ( Figure 2).
According to [4] and [5], the Apuseni Mts were formed in Cretaceous times as     The application of refraction static corrections during the data processing will partially attenuate the spatially-aliased energy seen on the f-k amplitude spectra of the shot gathers.

Analysis and Processing of Seismic Reflection Data
The seismic data processing was done following a standard flow. For each line, the input data are represented by shot gathers saved in the SEG-Y format. The number of shot gathers in each data set varies from one data set to another. The geometry was defined for linear profiles. For all data sets, the refraction static corrections were computed for a final datum at +400 m and using replacement velocities determined after the head wave analysis. Frequency filtering was performed using f-k filters, band-pass frequency filters and predictive deconvolu-     After the application of refraction static corrections, the coherent noise was removed using the f-k filtering, the band-pass frequency filtering for 18 -66 Hz, and predictive deconvolution (Figure 6(b)). Some of the filtered shot gathers contain remaining spatially-aliased surface waves. These waves will be partially attenuated during the stacking of the filtered traces sorted after CDP numbers.
The time-to-depth conversion was done after migration and stacking of traces. The data set recorded along the line P3 contains 322 shot gathers. Most of the shot gathers are characterized by low signal-to-noise ratio as an effect of various factors, such as complex subsurface geology, rough topography and noise generated along the line during data recordings. I display in Figure 7(a) an example of good shot gather recorded in the presence of significant variations in elevation along the geophone spread. Surface waves are spatially aliased and strong in amplitude. Their removal was performed, after the application of refraction static corrections, using f-k filtering, band-pass frequency filtering for 18 -46 Hz, and predictive deconvolution. The filtered version of the record from Figure  7(a) is displayed in Figure 7(b); clear reflections are seen after the frequency filtering. All filtered records were used to obtain a depth-converted migrated seismic section.
The data set recorded along the line P4 contains 477 shot gathers, most of them being characterized by low signal-to-noise ratio. The recordings were performed over an area with rough topography (Figure 2(d)). An example of good record, before and after the frequency filtering, is displayed in Figure 8. The coherent noise was removed using f-k filtering, band-pass frequency filtering for 18 -46 Hz, and predictive deconvolution. The low quality of shot gathers reflected into the noisy image of the depth-converted migrated seismic section.

Results and Discussions
Time and depth-converted seismic sections provide accurate information about the geological structure of the subsurface. When available, velocity data measured in wells can be used to verify the accuracy of the time-to-depth conversion of the seismic data. Seismic sections obtained after processing of seismic reflection data recorded using active spreads per shot points with 48 and 96 channels were used by [2] in the analysis of the tectonic and stratigraphic evolution of the northern part of the Pannonian Basin. Additional structural information is provided by the interpretation of the vintage seismic data presented in this study.
The line P1 has a northwest to southeast orientation and a length of about 10 km. The uninterpreted depth-converted migrated seismic section is displayed in indicates an erosional surface, also identified by [2]. The upper sequence contains horizontal, parallel, thin reflectors characterized by weak amplitudes. Thin sedimentary layers can be interpreted inside this sequence. According to the geological map known for this area, these deposits might be of Upper Pannonian to Quaternary ages. The spatially-aliased surface waves remained after the frequency filtering applied during data processing interfered with the reflectors from this sequence and affected their continuity and amplitude (Figure 10(b)).
The lower sequence contains inclined reflectors with toplap and truncation terminations. During and after the Pannonian sedimentation, the area was active from a tectonic point of view being affected by erosion [2]. The amplitude reflectors vary from strong amplitude, at distances of 0 -2 km on the seismic section, to weak amplitude, at distances of 2 -10 km (Figure 10(b)). Apart of possible  The line P2 has a north to south orientation and a length of about 19 km. The uninterpreted depth-converted migrated seismic section is displayed in Figure   11  The line P3 has a northeast to southwest orientation and a length of about 17 km. The uninterpreted depth-converted migrated seismic section is displayed in Figure 12(a). A seismic sequence with undeformed and inclined parallel reflectors is interpreted on the northeastern half of the seismic section (Figure 12(b)). These sedimentary deposits might be of Pannonian to Quaternary ages, according to the geological description of the area in [2]. The thickness of this sequence is about 500 m, in its northeastern end, and decreases to less than 100 m toward southwest. The reflectors interpreted below this sequence, characterized by strong amplitudes, might indicate the presence of the Badenian-Sarmatian deposits. The interpretation of the seismic section is unclear toward southwest. What is sure is that the basement units of the Codru Moma Mts. are imaged at shallow depths. The lack of a clear reflector from the top of the basement along the entire seismic section might be considered to be an effect of the low-quality seismic data recorded along this line combined with the rough topography of the top of the basement. Intrusive bodies with the lateral parts very inclined were interpreted on distances of 8.2 -9.2 km, 11.7 -12.5 km and, probably, 14.7 -16 km (solid white lines in Figure 12(b)). Intrusive rocks outcrop within a narrow and elongated area with a northwest to southeast orientation at southeast of the P3 line. Their prolongation into the depth toward northwest was seen on the map of the vertical gradient of the magnetic anomaly presented in [3].
The line P4 has a northwest to southeast orientation and a length of about 26 km, being parallel to the southwestern border of the Padurea Craiului Mts. The uninterpreted depth-converted migrated seismic section is displayed in Figure   13(a). A seismic sequence with deformed and discontinuous thin reflectors is interpreted along the entire line. Its thickness varies from few hundreds of meters to less than 100 m (Figure 13(b)). The sedimentary deposits might be of  (Figure 14(b)). A narrow basin with sediments was interpreted at distances of 3 -4 km. The steep fault interpreted at a distance of about 7 km separates the Neogene sedimentary deposits of the Beius Basin from the basement units of the Padurea Craiului Mts, which act as a rift shoulder. A packet of reflectors can be seen on the distance interval of 7 -10 km until depths of about 1 km. Two sedimentary sequences can be interpreted, an upper one containing continuous and slightly deformed thin reflectors characterized by high amplitudes and a lower sequence in which the reflectors are characterized by weak amplitudes. The fractured top of the basement is indicated by a strong reflector (Figure 14(b)). The sedimentary deposits from the upper sequence might be of Late Miocene age, while those from the lower sequence might be of Middle Miocene age (Badenian and Sarmatian), according to [2]. The lack of reflectors at distances of 10 -12 km and depths greater than 0 m might be a result of the processing of low signal-to-noise ratio seismic data. Another explanation might be the presence of an intrusive body of Permian rocks with the lateral parts very inclined in the upper side covered by thin sedimentary deposits, probably of Late Miocene age. The interpretation of the reflectors seen on the southwestern half of the seismic section is difficult to be performed in the absence of borehole data and/or other seismic data. The thin sequence of sedimentary layers might represent sedimentary deposits of Late Miocene age. The deformed reflectors seen below the depth of 0 m might represent sedimentary deposits of Middle Miocene age [2]. Their deformation, probably, took place during the inversion from the end of Sarmatian and the Lower Pannonian rifting when strike-slip faults were developed. The presumed top of the basement used in interpretation is affected by Middle Miocene faults.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.