Transcript New ProMAX modules for reflectivity calibration and noise

New ProMAX modules for
reflectivity calibration and noise
attenuation
David C. Henley
New ProMAX modules
• Introduction
• 1) The spectral ratio reflectivity calibration
method
• SPECRAT module examples
• 2) The spectral clipping attenuation
method
• CLIPPER module examples
• Commentary
Introduction
• Spectral ratio technique delivers r(t,a) for
sea floor P-P reflections, over some range of
a. Useful for OBS, OBC data inversion?
• Spectral clipping provides non-linear
attenuation of monochromatic noise
components (e.g. 60 Hz), and reduction of
reverbs and other periodic phenomena.
Spectral ratio technique
• Limited use in some form for ~25 years.
• Used in experimental work by Shell in Gulf
of Mexico (W.L. Walters, 1975) and
offshore Canada (D.C. Henley, 1984).
• Applicable only to marine hydrophone
streamer data.
• Often (incorrectly) applied to X-T domain.
Source
Sea surface
R
a
Hydrophones
R
Sea floor
Primary and multiple sea floor reflection observed along
a single raypath
Mathematics 1
Primary sea floor reflection
P(t )  (1 / 2 R) W (t )  r (t )
Single bounce multiple
M (t )  (1)  (1 / 4R) W (t )  r (t )  r (t )
Mathematics 2
P ( f )  (1 / 2R) W ( f )  r ( f )
M ( f )  (1)  (1 / 4R) W ( f )  r ( f )  r ( f )
Mathematics 3
Calibrated reflectivity spectrum
(2)  M ( f )  P ( f )
r( f ) 

P( f ) P ( f )  

Scaled wavelet spectrum
( R)  P ( f )  P ( f )  M ( f )
W(f)

M ( f )M ( f )  

Source
Sea surface
R
a
Hydrophones
R
Sea floor
Primary and multiple sea floor reflection observed along
a single raypath (more than one hydrophone required)
Source
Sea surface
Hydrophone
R2
R1
a
b
Sea floor
Primary and multiple sea floor reflection observed at a
single offset (only one hydrophone)
Geometry issues
• Acquisition geometry does not match
single-raypath geometry implied by math.
Approximate solution--NMO and stack.
• Exact single-raypath geometry simulated
by Radial Trace transform of shot gather.
• Sea floor primary and multiple usually
spatially aliased on marine shot gathers,
complicating R-T transform.
Model
0
100 ms
200 ms
Gated primary reflection from White Rose model
Timing line spacing = 20 ms.
Model
0
100 ms
200 ms
Gated first order sea floor multiple from White
Rose model. Timing line spacing = 20 ms.
0
Model
100 ms
200 ms
Calibrated reflectivity functions. Trace
spacing = 0.2, timing line spacing = 20 ms
0
Model
100 ms
200 ms
Estimated source wavelet. Timing line spacing = 20 ms.
Real data
0
100 ms
200 ms
Primary sea floor reflection from R-T transform of
real marine shot gather. Timing line spacing = 20 ms.
Real data
0
100 ms
200 ms
First order sea floor multiple gated from R-T
transform of real marine shot gather. Timing
line spacing = 20 ms.
Real data
0
100 ms
200 ms
Calibrated sea floor reflectivity functions. Trace
spacing = 0.2, Timing line spacing = 20 ms.
Real data
0
100 ms
200 ms
Estimated source wavelets. Timing line
spacing = 20 ms.
Comments--spectral ratio method
• Technique can be used on other
primary/multiple pairs, but with lower
resolution and accuracy.
• Higher order sea floor multiples can be
used in the formulae, as long as scale factor
is adjusted for ratio of path lengths.
Resulting reflectivity bandwidth is
narrower the higher the multiple order.
Spectral clipping technique
• Practical (but non-linear) solution to a
difficult noise problem
• Well suited to strong monochromatic
noises
• Handles all monochromatic noises in single
pass
• Very effective in R-T domain against
dispersive noise
Spectral clipping concept
A
t
F.T.
f
+
f
-
Seismic trace (left) transforms to
amplitude and phase spectrum (right)
Spectral clipping concept
Monochromatic
noise
Upper threshold
A
(dB)
Wings
Peak width
Median
spectrum
Lower threshold
f
Running median spectrum computed from trace
amplitude spectrum, thresholds equidistant above
and below median spectrum.
Spectral clipping concept
Edited region-peak + wings
A
(dB)
Median spectrum
f
Raw spectrum edited by replacement with
median spectral values
Spectral clipping concept
A
t
F.T.
+
f
f
-
Edited amplitude spectrum and unchanged
phase spectrum transform to noise-free trace
0m
0
Example 1
875 m
1.0 s
2.0 s
Raw shot gathers from Okotoks field school line
0m
Example 1
875 m
0
1.0 s
2.0 s
Okotoks shots after 60 Hz notch filter
0m
Example 1
875 m
0
1.0 s
2.0 s
Okotoks shots after spectral clipping.
-435 m
0
Example 2
400 m
1.0 s
2.0 s
Raw shot gathers from Okotoks field school line
-435 m
0
Example 2
400 m
1.0 s
2.0 s
Shot gathers after 60 Hz notch filter
-435 m
0
Example 2
400 m
1.0 s
2.0 s
Okotoks shot gathers after spectral clipping
Comments--spectral clipping
• Spectral clipping is most reliable on the
strongest noise
• Phase is undisturbed by spectral clipping
• R-T transform of dispersed linear noise
yields traces with strong single-frequency
noise that can be greatly attenuated with
spectral clipping.
Acknowledgements
• Sponsors of CREWES
• CREWES staff