EXPERIMENTAL WORK IN SEISMIC DATA ACQUISITION

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 EXPERIMENTAL WORK


 GUIDELINES FOR SELECTION OF EXPERIMENTAL SITE


  • Experimental site should be free from sub surface faulting
  • Continuity of reflection events must be there
  • Experimental site should represent the whole area
  • It should be free from surface undulations, industrial & cultural noise, power line etc.
  • Full suite of experiments is to be carried out at every experimental site
  • All efforts should be made to control the vehicular & other cultural noise if any
  • The experimental spreads should be oriented along the proposed profiles.

The following experimental work is to be carried prior to regular seismic work at least at one site (preferably in the centre of the area) within the survey area to fine-tune the acquisition parameters.
1.      Up-hole survey
2.      Shot hole depth and charge size optimization
3.      Noise survey
4.      Array test

 UP-HOLE SURVEYS:


Objective:
1.      To optimize the charge depth for noise survey.
2.      Input for preparation of near surface model for optimizing the depth of shot holes along the lines /swaths
3.      To compute the velocity of weathered/sub-weathered layer and thickness of weathered layer to facilitate computation of static corrections
Methodology:
1.      Shot hole should be drilled below the sub-weathered layer.
2.      Litho cutting samples to be collected from each shot hole at every 3 m. depth interval. The cutting are to be analyzed and lithology to be identified.
3.      Regular shot interval of 2m to be used from deepest interval up to the surface
4.      At least four channels with single geophone to be planted at offsets of 1m, 3m, 5m and15m from the shot hole.
5.      A uniform type of seismic source viz. detonators to be used for shooting all the levels. Number of detonators can be optimized for deepest level so as to record sufficient amplitude at the surface. It can be varied with depth such that the number of detonators is kept constant in the zone where optimum depth is expected and reproduction gain is to be kept constant.
6.      Uphole surveys should be conducted at an interval of 1-2 km interval/grid covering line crossings for 2D surveys and at an interval of 1 sq.km. For areas with fast lateral near surface velocity variation, the interval should be 1 km or even less
7.      The planned Upholes should be conducted and interpreted well before shooting the seismic line/swath
8.      The uphole data is to be recorded on permanent magnetic medium for future use.
Analysis of data:
1.      The recorded slant times are converted into vertical corrected times
2.      The time-depth (T-D) plots are generated for all the recorded offsets
3.      The velocity and depth of weathered/sub-weathered layers are then computed
4.      The amplitude of the first break in the expected zone is studied. Any increase in amplitude indicates better medium and a decrease indicates a poor medium
5.      Near Surface Models (NSM) are prepared incorporating actual surface elevation, velocity & lithology information. Appropriate velocity layering is to be marked on the model and depicted with different colours.
6.      The optimum depth (OD) will be decided taking into consideration velocity, uphole pulse shape & lithology in the order of priority. The OD along NSM will be marked in advance and the drilled depth will be marked to display the drilling performance and its relation with variations in data quality if any.
7.      Once the planned Upholes are completed in the area, depth contour map of weathered and sub-weathered layers to be prepared.
8.      NSM will be also used for computation of static corrections.

Conclusion:
1.            The optimum charge depth
2.            Near surface model
3.            Velocity and thickness of different layers.

 OPTIMIZATION OF CHARGE SIZE AND DEPTH


Objective:
1.      Charge depth optimization
2.      Charge size optimization
Methodology:
1.      The result of the Uphole survey carried out in the area of noise survey is utilized to identify probable shot hole depths for depth optimization. The charge used in previous seismic investigations is used for this experiment.
2.      Full spread length as designed in the pre-planning stage is deployed for both these experiments.
3.      Minimum near trace offset is used.
4.      The optimum shot hole depth identified from analysis of charge depth optimization experiment is used for charge size optimization.
5.      Different charge sizes (at least 2 kg. less & more than the previously used charge sizes) are tested in different shot holes of optimized depths and  at the same place (separated by 5-10 m. ) .
Data processing:
1.      Unfiltered shot gather plot for all shots
2.      Frequency spectra of near, middle and far traces for all shots

Data analysis and interpretation:

Charge depth optimization:
1.      Compare all the shot plots with different charge depths. Study the frequency and energy content and event mappability within the zone of interest. Higher frequency content with minimum noise cone is preferable
2.      Compare the amplitude spectra for all the charge depths at various offsets. Study the bandwidth at a common level; say –3, -6 and -12 dB and the peak frequency. A wider bandwidth and higher peak frequency gives better resolution and hence is preferable
3.      Select the optimum charge depth and relate to the uphole result

Charge size optimization:
1.      Compare all the shot plots with different charge size recorded at optimum depth and study the frequency and energy content within the zone of interest. Higher frequency content vis a vis sufficient energy reaching the far offsets is essential
2.      Compare the amplitude spectra for all the charge size at various offsets. Study the bandwidth at a common level; say –3 dB and the peak frequency. A wider bandwidth and higher peak frequency gives better resolution and hence is preferable
3.      Select the minimum charge size that gives the optimum result

Conclusion:
1.   The optimum charge size for the area
2.   The optimum Charge depth at the location.

 NOISE SURVEY:


Objective:
1.      To study the noise characteristics of the area
2.      To optimize the near trace offset
Methodology:
  1. Noise profile can be conducted in two ways
a.      Walk-away spread method, i.e. the shot point location is fixed and the spread is moved successively away. This method is operationally more time consuming
b.      Walk-away shot point method also known as transpose wave-test method, i.e. the spread is fixed and the shot point is moved. This method introduces some variability due to moving shots but is operationally less time consuming and is more conventionally used for noise survey
  1. In areas of varying tectonic set-up and surface conditions, noise survey should be conducted at more than one location. In case the variation is not significant, it can be conducted at some central location representing the area
  2. At each location, noise profile should be shot in both dip and strike direction
  3. The offset coverage on the ground should be slightly more than the expected far offset computed in the pre-planning stage
  4. The walk-away shot point method is conducted using a set of shot holes with shot interval equal to the spread length plus one group interval
  5. The number of shots required depends on the maximum offset to be tested
  6. The group interval for noise profile is 5 m. The noise profile should be designed based on channel capacity of the party so as to minimize the number of shots needed to cover the offset.

Analysis of data:
1.      Field monitor analysis
2.      Generate noise section for the entire length of the profile by juxtaposing the field monitors
3.      Mark the noise events with different colours.          
4.      Calculate the frequency, velocity and wavelength and tabulate.
5.      Mark the offset at which the shallowest reflector of interest is just outside the noise cone.
6.      Pick traces corresponding to multiples of channel spacing covering slightly more than the expected near offset. Compute the amplitude spectrum of traces for full time window.
7.      F-K spectrum in two windows, one shallow and one deep. Compute the noise wave characteristics, i.e. velocity, frequency and wavelength along with their amplitude strength
Interpretation:
1.      The coherent noise is essentially characterized by low velocity and low frequency.
2.      From the computed values, the range of wavelength of the noise trains are obtained which gives the first hand idea about the geophone array to be used to attenuate them
3.      The filtered output gives the idea of the noise getting attenuated by low cut filter (LCF)
4.      In case LCF is used during acquisition, noise wavelengths present in the filtered section only needs to be attenuated
5.      However, if low frequency signal is important in the area, then LCF is not used during acquisition and all the noise wavelengths present in the unfiltered section needs to be attenuated
6.      Study of the amplitude spectrum of the traces indicates the prominence of low frequency events at smaller offsets and gradual decay towards higher offset. The offset beyond which these amplitudes are minimum and do not vary appreciably gives the optimum near trace offset parameter
Conclusions:
a)   Effect of low cut filter
b)   The noise wavelength range to be attenuated by geophone array
c)   Near trace offset

 ARRAY TEST:


Objective:
To optimize the geophone array that is to be used in attenuation of the coherent noise in regular production work.

Methodology:
  1. The results of the Uphole survey, charge depth & size optimization and noise survey experiment  carried out at the experimental site  are utilized to fix the optimum charge depth for array optimization
  2. The spread is designed by folding or laying separately the required active channels in 6 or 8 numbers of mini-spreads. The numbers of mini spreads depend on the number of arrays to be tested. The group interval is taken as optimized in pre-planning stage
  3. Theoretical geophone array response curves can be generated for various geophone arrays to be tested to understand the effect of arrays on the noise trends to be tested.
  4. Each mini-spread consists of different array. Normally one spread is laid with 10/12 geophones bunched for comparison of other arrays
  5. The other arrays in the spreads are designed based on noise wavelengths obtained from the noise survey. A geophone array is characterized by number of elements n and element spacing D
  6. The total offset on the ground should cover the computed far offset.
  7. The array designing is conducted using a set of shot holes with shot interval equal to the mini-spread length plus one group interval
  8. The number of shots required depends on the maximum offset to be tested. The fold-back layout should be designed based on available channel capacity so as to minimize the number of shots needed to record upto the required maximum offset.
Data processing:
  1. Simulated unfiltered shot gather plot for each geophone array by juxtaposing the corresponding shots
2.      Simulated shot gather plot for each geophone array with LCF of 8 Hz to 12 Hz
  1. Amplitude spectra of near, middle and far traces for each array
4.      Two F-K spectra for each array plot, one inside the noise cone and one outside the noise cone

Data analysis and interpretation:
  1. Each array record is studied by correlating the events within zone of interest
  2. Mark the noise wave trends in each plot. This gives idea about the filtering effectiveness of the arrays
  3. Compare the unfiltered plots with LCF applied plots to see the effective efficiency of the geophone array in association with the LCF
  4. Compare the attenuation characteristics of the arrays with their response curves.
  5. Compare the amplitude spectra for all the arrays at near, middle and far offsets. Study the bandwidth at a common level; say –3 dB and the peak frequency. A wider bandwidth and higher peak frequency gives better resolution and hence is preferable.
  6. Compare the signal and noise level of different arrays ‘inside the noise cone’ and “outside the noise cone” from the F-K spectra respectively.
  7. The array providing better signal level and less noise level is selected as the optimum geophone array
Conclusion:
a)      The optimum geophone array length, number of elements and element spacing.
b)     For a given area of operation and the type of geometry adopted, judicious choice is to be made between bunching of geophone element at the picket and geophone array, keeping in view the strength of the ground roll and its interference with the signal at the objective level.

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