PRESURVEY STUDIES AND 2D SURVEY DESIGN

 PRESURVEY STUDIES AND 2D SURVEY DESIGN

2.1 PRESURVEY STUDIES

Pre- survey studies are essential for effective data acquisition in order to meet exploration objectives in the survey area. Before the start of any data acquisition project, a database is to be built up regarding the area.

A.    Formation of data base:

  i.      Geological and seismo-geological objectives of the survey should be clearly known.

ii.      To collect geological map of the area depicting all the relevant geological information and to study details of exposures, surface dips and surface tectonic features.

iii.      To collect satellite images of the area.

iv.      To collect time structure maps of the target zones and to study the major structural aspects, fault patterns and their position and other sub surface tectonic activity.

v.      To collect the details of wells drilled in the area and their status.

vi.      To collect the log data (sonic , density and dip meter ) of the key wells.

vii.      To collect the VSP data of the key wells falling in the area.

viii.      To collect the detailed information about acquisition and processing parameters of earlier seismic data.

ix.      To collect the information about areas of drilling difficulty, poor data quality and logistic difficulties and to mark them on the project map.

x.      To collect the key seismic sections (dip & strike) and to identify prominent reflectors in the time window of interest, details about average and rms velocity, frequency content in the zones of interest.

xi.      To collect the near surface information (weathering layer thickness and velocity, sub-weathering velocity) from earlier data.

xii.      To collect and study the topo sheets of the area to find the logistic constraints, elevation variation, rivers / back waters, approach roads and bridges etc.

xiii.      To prepare the project map in the suitable scale depicting work assigned, logistics, earlier lines, drilled wells (suitably annotated) , surface geological features and other necessary details like reserve forests, highways & rail tracks, rivers etc.

2.2 DESK COMPUTATIONS

Based on objective and study and analysis of data, the following parameters are noted down which are used for determining of survey parameters.

1.      Maximum depth of interest

2.      Minimum depth of interest

3.      Maximum dip

4.      Velocity information of target and area

5.      Maximum frequency

6.      Thickness of beds

This information is used for calculation of following parameters:

1.      CDP interval/Bin interval

2.      Near offset/Minimum offset (Xmin)

3.      Far offset/Maximum offset (Xmax)

4.      Migration Aperture

5.      Desired frequency to get required vertical resolution

6.      Record Length

Designing, Analysis and optimization of Geometry:

            Some of the parameters are calculated as above but some others parameters, and in few cases fine tuning of above parameter, required analysis and optimization. The reprocessing of previous data, if available, may be required to arrive at required nominal fold of coverage. Designing of geometry and comparison of attributes will optimize the final acquisition parameters.

Modeling Studies: 

The database gathered above is to be analyzed critically to study the nature of the area with respect to geological complexity. Areas with geological complexity like highly folded and faulted beds, thrust belts, sub-trap, sub-thrust mapping, mapping under exposed anticlines, etc need detailed modelling studies to analyze the subsurface illumination to arrive at the final acquisition parameters and design the spread configuration.

E. Interaction and Discussion:

The analysis of all aspects of the surveys including expected data quality, need to be done prior to experimental and regular seismic work.

Apart from the above, the following also need to be carried out:

·   An extensive interaction with processing geophysicists and interpreters during pre-survey studies and continuously thereafter.

·   Analysis of the problems faced while processing / interpreting the data of earlier vintages to plan the necessary strategies.

·   The quality and quantity, status/ performance, make and specifications of all available inputs like seismic instruments, ground electronics, shallow refraction/uphole survey instrument, shooting systems, geophone strings, topographic survey equipment and communication equipment need to be analysed / ascertained. Any additional requirement of equipment and accessories and their availability to be analysed.

·   After carrying out the pre-survey studies, the project reports are to be presented in a technical forum, which comprises geoscientists of the concerned Basin and RCC for improving the technologies, methodologies and refining the strategies by discussions, suggestions and constructive criticisms.

2.3 DIFFERENT ACQUISITION PARAMETERS

1. Type of spreads

            The most common and widely accepted spreads are Split spread and End on spread. If the interest is in the shallow as well as deep target, asymmetrical split spread is a better choice. Following table shows the comparison of split spread and end on configuration assuming limited numbers of channels are available and hence equal numbers of channels are used to design split spread or end-on spread.

Sl. No.	End-on Spread	Split-spread
1.	It gives a longer spread, which enables us to look deeper.	The method is suitable for shallow targets
2.	It is suitable for better multiple suppression	If random noise is the only problem and the area is free of multiples, the method is well suited.
3.	It provides a better velocity analysis.	It reduces the NMO stretch.
4.	It is convenient for field operations. Since the shot points and the corresponding spread are separated, any activity at the corresponding and successive shot points does not hinder the active spread.	Since the shot points are in the centre of the corresponding spread, any activity at the corresponding and successive shot points hinder the active spread.
5.	It may ensure up-dip shooting if dip direction is monocline and it is known.	Half of the ray paths may be in downdip direction if dip is monocline. However, it is better suited for conflicting dip or unknown dip areas.  It is also suitable in horst / graben set up. In mapping of geologically complex subsurface, it minimizes the shadow zones.

            With the increasing channel capacity of present day seismic recording instrument, there is little binding on the number of active channels laid in the spread. That is why nowadays mostly split spread configurations are used with longer arms on either side, which serves all the purpose of mapping deeper targets, velocity analysis as well as mapping geologically complex subsurface structures by minimizing shadow zones. Since number of active channels in split spread geometry is more (double) it is cost effective in land seismic survey (which drilling a shot point is costly than laying a channel) due to lesser shot density for a given fold.

2. Direction of shooting:

1.      It is the direction in which the seismic ray travels from the source to the receiver. It has significance only in case of end-on spread and to some extent in asymmetrical split spread.

2.      A wave traveling updip suffers less scattering and arrives at all the receivers within a given array at approximately the same time resulting in constructive interference especially at higher frequencies.

3.      In the case of updip shooting, the total surface coverage to map a steeply dipping reflector is less as compared to that of downdip shooting. Hence, up-dip shooting is usually preferred in 2D Surveys.

4.      In 3 D survey the direction of shooting has not much significance since the reflected energy is recorded from all direction. However the spread Geometry/direction of shooting may be fixed in such a way that majority of reflected energy is recorded from updip side.

5.      In marine surveys the direction of shooting depends on the logistics and the sub-surface geology. The longer side of the survey area is usually the direction of shooting.

3. Group Interval/CDP interval/Bin Size (Spatial Sampling):

            The horizontal resolution provided by 3D seismic image is function of the trace spacing within the 3D data volume. As the trace spacing decreases the horizontal resolution increase. The dimension of the inline and cross line spacing in a 3D data volume defines the size of the stacking bin.  As a general rule there should be a minimum of three stacking bins, preferably four bins across the narrowest stratigraphic feature that needs to be resolved.

Fresnel Zone criterion:

Trace interval= (2/3)R

where R = (V_av/2)√ (Tο/fmax), Tο = two-way-time of the shallowest target V_av = average velocity, f_max = max. Frequency, T_o = zero offset two-way-time, Ө = dip, R = radius of first Fresnel zone

Spatial aliasing criterion:

Trace interval = V_int / (4 * f_max * Sin θ)

Where, V_int is the interval velocity at target lev

els

  

          f_max is the maximum frequency

                        θ is the maximum dip in the areas. If dip is less than the 30 degrees it may be taken as 30 degrees.

4. Fold :

            The foldage optimization is done using the previously acquired data.

        i.            2D fold                 - Optimization is done from previous 2D data available sections with different possible foldage is generated from the existing data at processing center and compared.

       2D Fold= Number of Channels * Group interval / (2* Shot interval)

     ii.            3D fold                 - Optimization is done from previous 2D data foldage in the area for same signal to noise ratio

.

     

                                    Fold (3D) / Fold (2D)= 4 {(B_x * B_y tanq)/pGV} f_d

                                          where, G = CMP interval of 2D data, f_d = dominant frequency

3D fold = Inline fold * Crossline fold

·         The 3D fold needs to be optimized both in in-line and cross-line direction. Ideally the two should be same. Hence, foldage in the two directions is optimized keeping in view the subsurface mapping objective and optimal use of inputs.

·         The stacking fold is the number of traces that are summed during data processing to create the single image trace positioned at the center of that bin.

·         In a 3D context stacking fold is the product of inline fold (fold in the direction of the receiver-line) and cross-line fold (fold in the direction perpendicular to the receiver line)

·         To build a high quality 3D image, it is critical to create the proper stacking fold across the image space and also ensure that the fold has a wide range of offsets and azimuths.

5. Offsets:

            This is the distance between shot and receiver and encompasses three aspects, viz. minimum offset, maximum offset and its distribution. Near offsets are needed for data inversion, far offsets are needed for velocity analysis, multiple suppression and AVO analysis and the middle offsets are needed as link between the near and far.

·         Near Offse

t

   

                       i.      The near offset for 2D should be less than or equal to one group interval.

                       ii.      The maximum near offset for 3D should be less than 1.0 to 1.2 times the depth of the shallowest horizon to be mapped.

·         Far Offset

                          i.      The far offset should be small enough so that the shallowest reflection reaches just below the first break and avoid wide angle reflection distortion and large enough for good velocity analysis for effective multiple suppression.

                       ii.      Minimum and Maximum offsets:

1.      Differential move out (if multiples exist)

            ∆T = X²/ 2T_οV²

where, V = stacking velocity ≈ Vrms and X = offset

2.      Velocity analysis (for far trace)- X = V √ (2T_ο/f)

3.      NMO stretch criteria.        X (10%) = V T_ο √ (0.21)

                     iii.      The value of far offset is limited on the higher side by the NMO stretch criteria. And it is limited on the lower side by the differential move-out (multiple attenuation) criteria and velocity analysis criteria whichever is higher. i.e. X_m,           X_v < X_far < X_nmo

                     iv.      Rule of Thumb: X_max should be approximately the same as the primary target depth, usually expressed as X_max =Target Depth.

6. Migration aperture:

·         It is the area/distance by which the image area is to be extended to get full-migrated coverage.

·         Migration apron is normally chosen as the larger of:

                                      i.      The lateral migration movement of each dip in the expected geology,

                                   ii.      The distance required to capture diffraction energy coming upwards at a scattering angle of 30°, or

                                 iii.      The radius of the first Fresnel zone.

·         Migration Aperture = Z * tanq                                            

where Z=depth to the target and q=dip of the target reflector

·         The appropriate value of migration aperture should be decided from the above calculations based on the sub-surface complexity and imaging requirements.

·         In the emerging scenario, the requirement of Pre-Stack Time Migration (PSTM) / Pre-Stack Depth Migration (PSDM) is becoming almost a routine process; the migration aperture calculations should take into account the above processes. The calculations require generation and analysis of unit impulse response and should be done in consultation with the processors and the Client / Basin Manager.

7. Recording Parameters

·         Record Length:

                                      i.      The record length must be sufficient enough to capture target horizons, migration apron and diffraction tails.

                                   ii.      The record length must be equal to T_max= T_d+2L where T_d is the time of the deepest selection and L is the length of the longest filter in Time

·         Sampling Interval

                                      i.      Sample rate in time determines the temporal resolution. It should able to sample at least 4 samples in the time period of the highest anticipated frequency.

·         High Cut filter

                                      i.      The cut off frequency depends on the sampling interval. High cut filter is used to attenuate frequencies above the Nyquist frequency which depends upon the sampling interval) to avoid their aliasing.

                                   ii.      The High cut filter setting is generally kept at 0.5 to 0.7 of Nyquist frequency with the required slope in dB/octave.