Shallow reflection pitfalls

1
Shallow reflection pitfalls

• Acquisition, processing, and interpretation
• Requires high frequencies
• Ground roll aliassing
• Air-waves
• Refractions
• Processing artifacts

From Steeples and Miller, Geophysics, July 1998
2
Acquisition

• Conduct noise test (walkaway) move geophones
  progressively farther from shot point
• Use half spacing than planned for data
• Test longer offsets
• Try at least two types of sources, sizes, and
  sweeps (weight drops, explosives, projectiles,
  Vibroseis)
• Usually use 40 Hz geophones - may need 100 Hz
• Amplitude of reflections to ground roll depends
  on how well coupled geophones are good coupling
  decreases ground roll relative to reflections
• filter (frequency or fk) ground roll (surface
  waves) as they are usually half the frequency of
  reflections
• data frequency too low direct waves
  contaminate shallow reflections hard to filter
  so use high frequencies.

3
Aliassing
Temporal alliassing occurs when frequency is too
high for sample rate (Nyquist) - Can also have
spatial aliassing when geophone interval is
longer than wavelength (especially with ground
roll) - aliassed ground roll may have moveout
similar to reflections - moving shotpoint 1
interval should not affect reflections but will
affect spatial aliassed waves - decreasing
geophone interval substantially improves
reflections but remove spatial alliasing -
frequency content of apparent reflectors is
much lower then that of first arrivals. - air
wave produced by source about 330 m/s
4
Air wave
Ground roll
Interference from direct waves
Spatially aliassed ground roll and air wave
5
Air waves - difficult to frequency or fk
filter - spatially aliassed and appears lower -
try surgical mute - signal/noise ratio may
decrease with fold
Stacked air waves Not a reflection!
6
Refractions
- frequency similar to reflections - wide
angle refractions and shallow reflections may
interfere - look for rapid decrease of frequency
with depth - application of NMO and stack may
greatly decrease frequency of reflections due to
misalignment compare with gathers - dip
refraction may produce shingled effect -
misidentifying refraction as reflection is common
problem - major weakness of shallow reflection
is distinguishing reflections from refractions
7
Top events lt 40 ms are refractions
Effect of a dipping reflector
8
Processing
As with all reflection work, can produce
artifacts Deconvolution may not help Reflections
should be visible on gathers Electronic noise
AGC affects appearance
9

• Interpretation
• depends on processing
• use raw data (gathers) as well

Excessive AGC
Fault produced by incorrect processing (top is
wrong)
10
Overprocessing
Before (random data in middle)
After - apparent bed
From Hill (1999)
11
Top events lt 40 ms are refractions
Effect of a dipping reflector
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Chirp seismic
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Chirp seismic

• High-resolution water profilers
• Vertical resolution less than 1 m
• Penetration 10s m
• Applications
• Sediment classification
• Pipeline laying
• Platform evaluation
• Differs from pinger in that chirp sends out a
  range of frequencies
• (this allows better resolution)
• 6. Differs from seismic in that it produces a
  zero-offset section
• with a fold of one (so does not pick up very weak
  signals)

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Processing

• Source is computer generated pulses with
  amplitude
• and phase control (like land vibroseis)
• 2. Recorded signal in then cross-correlated with
  source to get wavelet.

15
After correlation, this results in a Klauder
wavelet for each reflection.
Correlation is essentially the same as
convolution, but with a a reversed signal.
16
http//woodshole.er.usgs.gov/operations/sfmapping/
seissoftware.htm
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What it looks like
18
pinger
chirp
seismic
Side-scan
http//www.meridata.fi/mddssindepth.htm