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 Hydrophones Sea surface R R a Sea floor Primary and multiple sea floor reflection observed along a single raypath

Mathematics 1 Primary sea floor reflection Single bounce multiple

Mathematics 2

Mathematics 3 Calibrated reflectivity spectrum Scaled wavelet spectrum

Source Hydrophones Sea surface R R a Sea floor Primary and multiple sea floor reflection observed along a single raypath (more than one hydrophone required)

Source Hydrophone Sea surface R R a b Sea floor 2 R 1 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 Gated primary reflection from White Rose model Timing line spacing = 20 ms.

Model Gated first order sea floor multiple from White Rose model. Timing line spacing = 20 ms.

Model Calibrated reflectivity functions. Trace spacing = 0.2, timing line spacing = 20 ms

Model Estimated source wavelet. Timing line spacing = 20 ms.

Real data Primary sea floor reflection from R-T transform of real marine shot gather. Timing line spacing = 20 ms.

Real data First order sea floor multiple gated from R-T transform of real marine shot gather. Timing line spacing = 20 ms.

Real data Calibrated sea floor reflectivity functions. Trace spacing = 0.2, Timing line spacing = 20 ms.

Real data 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 F.T. f + f - Seismic trace (left) transforms to amplitude and phase spectrum (right)

Spectral clipping concept Monochromatic noise Upper threshold A Wings (dB) 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 F.T. f + f - Edited amplitude spectrum and unchanged phase spectrum transform to noise-free trace

Example 1 Raw shot gathers from Okotoks field school line

Example 1 Okotoks shots after 60 Hz notch filter

Example 1 Okotoks shots after spectral clipping.

Example 2 Raw shot gathers from Okotoks field school line

Example 2 Shot gathers after 60 Hz notch filter

Example 2 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