Design of a combined cube of seismic data acquired in areas of complicated relief by use of various seismic source types

. The paper gives brief information on the study area with attention focused mainly on complex surface seismic and geological environment, stipulating the use of 3D seismic survey in this area. As the area features complicated relief, the sources and receivers have been chosen individually for each location of the survey. Seismic energy sources generated various seismic oscillations and this complicated the design of combined 3D cube for the whole study area. By use of modeling we have reduced all oscillations to the minimal-phased form. The next stage of processing consisted in use of shifts of time sections for combination of seismic boundaries acquired for various parts of the area: onshore, offshore, swampy areas. Thus, applying the standard procedures it was possible to design combined 3D cube.


Introduction
Zykh-Hovsan area is located in Surakhany district in the south-western part of Absheron peninsula, close to the eastern part of Baku, Azerbaijan Republic (Figure 1).
The relief of the area is the coastal plain covered mainly by ancient Caspian sediments.In the north-west and in the western part the area is neighboring by the plain-shaped uplift, elongated in almost meridional direction and made of Absheron age limestone.Absolute heights of the relief vary from -22.8 m in the east to +30 m and higher in the north-western part of the area.The south part of the area is located in the coastal zone of the Caspian sea (part of Hovsan field is extended into the shelf zone).Characteristic feature of the relief is the presence of salt lakes and saline lands [1][2][3].
From the point of view of orohydrography the area is poor, no rivers are there in the area.Only artificial channels presence is observed in the central and north portions of the area.The presence of a large amount of sediment ponds and waste disposal facilities must be noted.The Zykh salt lake of 2 sq.km area is located in the Zykh field.In the southeastern bank of the lake the mud volcano of 100 m diameter is observed.The study area is featured also by presence of small salt lakes with oil presence and salty, swampy lands, which are mostly in the western part.The area abounds with many industrial facilities such as small plants, auto-service centers, electricity mains, car parking, water supply stations, oil-and water-settling facilities, etc.The industrial facilities are allocated across the whole area and concentrated mainly in its central part.In the south-eastern part of the area mostly sewage lines and waste-settling facilities are located.Both surface and subsurface electric lines, oil, gas and water pipelines are present in the area.Railway and highway networks are also dense here.The area is crossed by Baku-Hovsan and Baku-Qum-adasy highways and a number of small roads, both with paved and unpaved surfaces.Heydar Aliyev's airport is in the southwest from the work area.
Within the limits of the area, the south coastal line of Absheron peninsula creates a cape Hovsan with two wharfs on it: one is for arriving vessels and the other is for oil-loading.The wharf named "Ribniy port" is in the east.
All the described above displays the complicated surface geological setting of the area covered by 3D seismic: on land, offshore and in transition zones [4][5].
Seismic survey by use of Reflection Wave Method was applied in the area in 1937-1941, 1945-1949, 1949-1951 and in 1955, by Common Depth Point method in 1977-1978.Some lines were worked out in 1993 and 1999.In 1996 and 2003-2004 the detailed seismic survey by Common Depth Point method was carried out by "Azerneftegeofizka" (currently Exploration Geophysics Department).In 1993 the area was covered by "onshore-offshore" seismic survey, partially covering the south-eastern part of the area [4][5].
Geological and geophysical data acquired from Absheron oil and gas province -one of the oldest oil and gas provinces around the world, including also the studied Zykh-Hovsan area have been repeatedly used for generalization of data.Results were reflected in some publications and in reports of entities of State Oil Company of Azerbaijan Republic, stored currently in geological archives.
Geological and geophysical studies and deep drilling led to discovery of two fields in the study area: Zykh field -in the west (currently at the final stage of development) and Hovsan field-in the east.Both fields are under development currently.

Materials and methods
Zykh-Hovsan area was covered by seismic 3D CDP survey [15], in total, 70 sq.kmarea has been worked out.The observation system is the central symmetrical ("cross") receiver array of 14 lines with 144 active channels and 300x300m net of receiver and shot lines, which provided nominal fold of 84 and regular distribution of survey attributes.Distance between receiver and shot point was 50 m, the number of active channels 144х14=2016.In total 42 shot points were within the limits of one array.For this fixed array the shot was done on 42 points of shot line (between 4 th and 11 th of receiver line), which was located between channels 71 and 72.Then the array was displaced for 300 m along the receiver line and work out of next 42 shot points was done further until the end of the whole stripe (block).
Depending on surface geology, 3D seismic suvey in this area applied various excitation sources such as vibration, pulsed, explosion sources and airguns and appropriate seismic receivers: geophones, marshphones and hydrophones.
Graph of processing was correspondent to the final time sections keeping the same ratio of amplitudes within the maximal wide stripe of recorded frequencies and totally satisfied all major requirements of this study.Field seismic data were transferred in SEG-Y format with superimposed geometry and then reformatted into the specific Geocluster (CST) format.Errors in geometry were revealed at the stage of interactive editing while overlaying the muting line on seismic records of shot points [6][7].Control over geometry description was done through the whole data amount.Binning (25х25 м) was fulfilled by use of TDETQ module and followed by analysis of seismic data quality.
Problem Statement:  In this area we have used 4 types of seismic sources and three types of receivers.In the field works no coupling of sources was implemented, causing some difficulties while data processing.Moreover, the coupling of receivers was incomplete and this also required making some corections and preparation of initial data for further processing.
 Quality control and removal of invalid data in the process of seismic data input, checking conformity of seismic records acquired by use of various source and receiver types.
Acquired data analysis displayed that areas with various types of sources are clearly displayed on the amplitude map [6][7].The lowest amplitude values (Figure 2) from pulsed sources.Seismic records made by use of acoustic sources are also distinguishing from each other.Dispersion of dominating frequencies across the area significantly varies and this might be caused by variety of source types (Figure 3).In general, signal frequency range is 5-35 Hz with the largest part of data falling into the range of 10-20 Hz. figures 4 and 5 display fragments of characteristic field seismic records of shot points and their amplitudefrequency spectra.In general, seismic records were deteriorated by cone of surface waves of various intensity and frequency.Pulsed source was featured by high noise level and low signal/noise ratio, and use of removable charges in the lake area was inefficient as seismic records displayed surface noise and low values of useful signal [8][9][10].The most important task in data processing aiming to obtain the combined seismic cube consists in reduction of seismic signal from all types of sources to a single form, that is the minimal phase.It has been noted earlier that for this area we have used 4 types of sources and 3 types of receivers.From theoretical point of view, the signals of explosive and pulsed sources are very similar and have minimal-phased form of pulse while the vibration source is null-phased and the acoustic source is mixed-phased.To reduce the pulse from vibration source to the minimal-phased form we have applied modeling by use of available sweepsignal data (linear sweep within the frequency range 8-80 Hz, length -10sec, cone -0.5 sec) to obtain Klauder pulse, for which we further calculated the filter of reduction to the minimal-phased form (Figure 6).In the modeling process, we have obtained the reduction filter, however its further use for real data became inefficient as the filter changed the signal phase variously for various signal frequencies.However, the use of minimal-phased filter within the sweep-signal frequency range allowed to gain the better result.Therefore, we have used this filter.
Acoustic source has mixed-phase signal nature and to reduce it to the minimal-phased form we have used the statistical deconvolution with reduction of seismic airgun signature to a minimal-phased signal [11][12].The procedure has been done within the wide window W800-W2500 with 1% noise factor and operator length 400 msec.
As various types of receivers (geophones, hydrophones and receivers for transition zone) have been used in the area we applied the correction for nonidentity.For this, we have designed the maps of amplitudes and frequencies of the signal.By use of these maps and seismic records from various source types, we came to the conclusion, that signal frequency and its phase do not depend on receiver type.The only one distinction is the amplitude, which varied unevenly across the whole area [13][14] and this difference was excluded while applying surface correlated dynamic correction for sources and receivers.
One of the major and effort consuming stages of processing was the design of combined seismic data cube.For efficient stacking in the overlapping zone where various types of seismic sources are used and removal of possible time shift, we drawn preliminary stack sections for each source separately.Initial cubes were designed by use of CDP seismic records, which have been used for: restoration of amplitudes because of attenuation and spherical divergence (REFOR); spectrum filtration (FILTR В (12, 24, 48, 64)); deconvolution (DECON) and amplitude equalization (DYNQU).Velocities were chosen based on preliminary scanning, which resulted in achieving of a priory velocity rule for non dense network [10].

Results
Acquired sections were characterized by poor resolution, which complicated the analysis.The exception was a cube calculated by seismic records acquired by use of airgun.Therefore, it has been decided to apply compensation to static and kinematic corrections (PACS3D) separately for each type of source.Based on derived corrections the preliminary stacks for each source were done repeatedly [15][16].In the process of analysis it has been made clear that there is a time shift for vibration source in comparison to the pulsed and acoustic sources -30 msec (Figures 7 and 8), and to explosive source -25 msec (Figure 9).After making compensations for the source type, the calculations of kinematic and static corrections were done repeatedly for all onshore sources (Figure 10).Then corrections for receivers located on land and derived at the previous stage have been applied to the seismic records from airgun.Thus, we were able to improve tracing of horizons in transition zone (onshore-offshore).After all static and kinematic corrections, we have applied compensation for velocities on vertical spectra calculated by VESPA module, 600х600 m net (Figure 15).And finally, the compensation for static corrections has been done in three-factor model by TDSAT module, corrections were from -2 to 3 msec and this confirmed the efficiency of compensations made to static and kinematic corrections.

Discussion
The second stage of compensation for static and kinematic corrections covered the whole area, including onshore, offshore and transition zone.This stage of processing resulted in design of combined seismic cube in a limited frequency range (Figures 11 and 12) and maps of static corrections for shot points and receiver points (Figure 13).Figure 14 displays the general map of fold of combined cube.

Fig. 6 .
Fig. 6.Vibration source reduction to the minimal-phased form of pulse by use of modeling.

Fig. 7 .
Fig. 7. Initial stacked sections for pulsed and vibration sources on line 390: prior to shift; 2) after the shift.

Fig. 10 .
Fig. 10.Time sections prior to (1) and after the first compensation (2) for static and kinematic corrections on line 380.

Fig. 11 .
Fig. 11.Time sections prior to (1) and after the second correction (2) of static and kinematic corrections on line 380.

Fig. 12 .
Fig. 12. Combined seismic cube in a limited frequency range.

Fig. 13 .
Fig. 13.Static corrections calculated by PACS 3D module at the second stage of correction.