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  • Report

    Reflection Seismology Processing (ProMAX)

    Benjamin Zuercher, Noel Ammann

    June 2, 2013

  • Contents

    1 Introduction 1

    2 General info about the data 1

    3 Overview of the processing flow 3

    4 Pre-stack processing 4

    4.1 Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    4.2 Amplitude scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    4.2.1 True amplitude recovery . . . . . . . . . . . . . . . . . . . . . . . . 5

    4.2.2 Automatic gain control . . . . . . . . . . . . . . . . . . . . . . . . . 6

    4.3 Top mute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    4.4 First break picking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    4.5 Refraction statics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    4.6 Frequency filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    4.7 Deconvolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    5 Stack processing 14

    5.1 CDP sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    5.2 Velocity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    5.3 NMO correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    5.4 Stacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    5.5 Residual statics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    5.6 Iterations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    6 Poststack processing 20

    6.1 Noise reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    6.2 Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    6.3 Time to depth conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    7 Interpretation 24

    References 26

  • 1 Introduction

    In the context of the course Reflection Seismology Processing at ETH Zurich, a seismic

    dataset was given to be processed with the software ProMax 2D Version 5000.0.3.3 from

    Landmark Graphics Corporation. The used Computer runs with Linux Red Hat.

    The goal of this course was to get an insight into seismic data processing with the given

    software, learn the tools behind the processing and finally get a realistic image of the

    subsurface of an area in Northern Germany.

    To do so, velocities of the different layers have to be reconstructed as accurate as possible

    by processing the raw data through different steps. Reflections should be seen better

    after the processing due to an increased signal-to-noise ratio and an improvement of the

    resolution.

    2 General info about the data

    All important information about the geometry could be found on the recording sheet and

    had to be added to the data as a first step. The seismic survey line has a length of 14200m

    in total and a spread length of 6100m. The recording consists of 120 channels with each

    having 24 geophones coupled. A gap of 200m between channel 60 and channel 61 needed

    to be added as well to the data (see figure 1)

    Figure 1: Channel configuration for the data from Northern Germany. A total spread length of 6100meters includes 120 channels. Every 50 meters one channel is located except between channel 60 andchannel 61 where a gap of 200 meters is inserted. Every channel consists of 24 Geophones.

    The spacing between each geophone is 2m, hence one channel spacing is 50m. A group

    of geophones is always connected in the center (see figure 2).

    1

  • Figure 2: Geophone configuration of one channel. 24 geophones are coupled in the middle, each having2 meter spacing to the next one. There was not one line with 24 geophones but two lines (0 meterhorizontal spacing) with each 12 geophones and a geophone spacing of 4 meters.

    The recording has 285 stations (101-385) in total. The recording length is 6s and the

    sampling rate is 2ms. A notch out filter with 50 Hz was applied.

    The position of the channels for all shots (the whole seismic line) can be seen in figure

    3. The geophones stayed at the same place for the first few and the last few shots. The

    boundary between yellow and red represents the place where the source was. Because the

    place of the source changed and the channels stayed at the same place at the beginning,

    one can say that the source is rolling in the seismic line. The same can be said for the

    end of the seismic line (roll out).

    Figure 3: The seismic line showing the offset. Zero offset can be seen at the color boundary betweenyellow and red.

    The CDP fold is at the beginning of the seismic line very small but the value increases

    fast (roll in) and reaches then a maximum of 35. The values do not much vary in the

    middle of the seismic line, increase then once again shortly and will then decrease a lot

    to the end of the seismic line (roll out).

    2

  • Figure 4: Fold vs Common Depth Point

    3 Overview of the processing flow

    We can split our processing into four main steps. When the geometry is correctly set

    up, first corrections for all the shots can be done. After a deconvolution is done, it will

    be stacked, analyzed and improved and finally a migration is applied before it will be

    converted from time into depth. The following points summarizes all the processing flows

    we used:

    Pre-stack processing

    Editing (Kill traces)

    Amplitude scaling (Correct for attenuation)

    Top mute (Get rid of insignificant waves)

    First break picking (Gets the information for a good velocity model)

    Refraction statics (Correct for weathered layer and topography)

    Frequency filtering (Get rid of ambient noise)

    Deconvolution (Improve resolution)

    Stack processing

    CDP sort (Reflections are sorted into a CDP gather)

    3

  • Velocity analysis (Picking velocities at recognisable layers)

    NMO correction (Correct reflection arrival times)

    Stacking (Summarizing into a single output)

    Residual statistics (Velocity corrections in the shallower part)

    Iterations (Iteration of the whole stack processing flow to improve the stack)

    Post-stack processing

    Noise Reduction / Image enhancement (Using a filter to reduce noise)

    Migration (Convert the reflections into a more realistic geological image)

    Time to depth conversion (Convert the time-axis to a depth-axis)

    Interpretation

    4 Pre-stack processing

    Before we start with processing, an example shot gather is shown in figure 5.

    Figure 5: Example shot number 45 before processing. The shot contains a lot of noise and bad coupledtraces, hence the resolution is quite bad.

    4

  • 4.1 Editing

    Bad traces were killed. They were good recognizable, because of their high noises before

    the first breaks. If we not have them removed, the results from the first break picking

    would have been random and incorrect at these traces. The high noise is probably the

    cause from bad coupled geophones or an ambient noise close to this geophone.

    4.2 Amplitude scaling

    4.2.1 True amplitude recovery

    We need to apply an amplitude recovery due to attenuation and wavefront spreading

    effects [Yilmaz, 2001]. We use a mathematical function for this true amplitude recovery:

    A(t) = A0(t) tn, where A(t) is the output, A0(t) is the initial amplitude, t is the traveltime and n is the exponential term which we will vary until we have a suitable result. We

    tested values for n between 1.5 and 2.2 and found the best value to be 1.6. This value

    was chosen, because the reflections are now much better recognizable and if the n value is

    too high, the noise will be increased in the deeper parts and the upper reflections are less

    clearer recognizable and we dont want that. The maximum application time was chosen

    to be 2800ms, because no more reflections can be seen beneath this value.

    Figure 6: Shot number 45 after applying true amplitude recovery. The inserted exponential term has thevalue 1.6. Reflections are much more visible after this processing step.

    5

  • 4.2.2 Automatic gain control

    Automatic Gain control is a similar operator like the one described before, because is tries

    to compensate the attenuation of a waves which are propagating trough a medium. But

    it only will be applied in a certain time gate. This time is defined by an operator length

    and is now to be found. The operator length was tested between 500ms and 1700ms and

    the optimal value for our data is 1500ms. A higher value will strengthen the reflections

    and decrease the noise. Too high values will cause that deeper reflections vanish again in

    the noise.

    Figure 7: Shot number 45 after applying automatic gain control. An operator length of 1500ms was usedand so this flow caused that the reflections are now more highlighted than before.

    4.3 Top mute

    Basically, we are only interested in the reflection waves and therefore first arrival waves

    with high amplitudes can be removed from the screen with a top mute. The information

    will not be deleted, it just does not appear anymore on the screen when applying the top

    mute [ProMAX, 1999]. An example of a top mute is shown in figure 8.

    6

  • Figure 8: Shot number 45 with a top mute. The green line is the boundary where all data above wasremoved.

    4.4 First break picking

    First breaks give us helpful informa