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SAGEEP 2014 Refraction/Reflection Session Optimized interpretation of SAGEEP 2011 blind refraction data with Fresnel Volume Tomography and Plus-Minus refraction. Siegfried R.C. Rohdewald Intelligent Resources Inc. Vancouver, British Columbia Canada.

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Siegfried R.C. Rohdewald Intelligent Resources Inc. Vancouver, British Columbia Canada

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SAGEEP 2014 Refraction/Reflection SessionOptimized interpretation of SAGEEP 2011 blind refraction data with Fresnel Volume Tomography and Plus-Minus refraction

Siegfried R.C. Rohdewald

Intelligent Resources Inc.

Vancouver, British Columbia

Canada


Smooth Inversion = 1D gradient initial model +2D WET Wavepath Eikonal Traveltime tomography

Get minimum-structure 1D gradient initial model :

Top : pseudo-2D DeltatV display

  • 1D DeltatV velocity-depth profile below each station

  • 1D Newton search for each layer

  • velocity too low below anticlines

  • velocity too high below synclines

  • based on synthetic times for Broad Epikarst model (Sheehan, 2005a, Fig. 1).

    Bottom : 1D-gradient initial model

  • generated from top by lateral averaging of velocities

  • minimum-structure initial model

  • DeltatV artefacts are completely removed


2D WET Wavepath Eikonal Traveltime inversion

  • rays that arrive within half period of fastest ray : tSP + tPR – tSR <= 1 / 2f (Sheehan, 2005a, Fig. 2)

  • nonlinear 2D optimization with steepest descent, to determine model update for one wavepath

  • SIRT back-projection step, along wave paths instead of rays

  • natural WET smoothing with wave paths (Schuster 1993, Watanabe 1999)

  • partial modeling of finite frequency wave propagation

  • partial modeling of diffraction, around low-velocity areas

  • WET parameters sometimes need to be adjusted, to avoid artefacts

  • see RAYFRACT.HLP help file

Fresnel volume or wave path approach :


Interpretation with Plus-Minus refraction method (PMR)

  • Assign traces to 3 layers : weathering layer, overburden layer, basement. See next slide.

  • Velocity and thickness of weathering layer determined with slope-intercept, between adjacent shot points. Model can vary laterally.

  • Velocity of overburden layer also determined with slope-intercept method

  • Bottom of overburden layer (top of basement) determined with Plus-Minus refraction (PMR)

  • Velocity of basement from PMR


Semi-automatic mapping of traces to refractors

  • yellow is weathering layer, red is overburden layer, green is basement

  • mapping is not required for Smooth inversion. Station spacing is 3m.

  • specify 1D velocity model : upper velocity limits for weathering, 1st refractor

  • specify lateral and vertical smoothing of CMP-sorted traveltime field

  • map traces to refractors by matching apparent velocity to 1D velocity model

  • lateral smoothing of crossover distance, after mapping to refractors


Plus-Minus refraction interpretation

  • basement velocity dips to below 2,000 m/s at station no. 63

  • this hints at a basement fault zone, dip of fault is not visible

  • lateral smoothing of refractors, for Plus-Minus method (Hagedoorn, 1959)

  • overburden refractor colored blue, basement refractor colored black


1D initial model : smooth DeltatV inversion


WET with Ricker wavelet weighting

Wavepath width 30%, 100 Iterations

Wavepath width 10%, 100 Iterations

Wavepath width 5%, 100 Iterations

Wavepath width 3%, 100 Iterations

e) 7th run, 6th run as starting model, wavepath width 3%, 100 WET iterations, RMS error 0.6%

Figure 1: WET with wavepath velocity update weighted with a Ricker wavelet (a) 1D-gradient starting model, (b) velocity tomogram obtained with wavepath width 30%, (c) 10%, (d) 5%, (e) 3%. Color scale is velocity in m/s. Contour interval is 250 m/s. Horizontal axis is offset from first profile receiver, in m. Vertical axis is elevation in m. Overburden Plus-Minus refractor is colored cyan, basement refractor colored orange. These two refractors are the same in (b) through (e). See Figure 4.


WET with Gaussian weighting

Wavepath width 30%, 100 Iterations

Wavepath width 10%, 100 Iterations

Wavepath width 5%, 100 Iterations

Wavepath width 3%, 100 Iterations


Wavepath coverage for Ricker weighting

Wavepath width 30%, 100 Iterations

Wavepath width 10%, 100 Iterations

Wavepath width 5%, 100 Iterations

Wavepath width 3%, 100 Iterations


Discussion

  • Wide wavepaths (low frequency) make WET inversion less dependent on the starting model and more robust but produce a smooth tomogram

  • Narrow wavepaths can give a sharper tomogram, but WET becomes more dependent on the starting model (previous run) and less robust

  • WET images the dipping low-velocity fault zone (Zelt et al., 2013) more realistically with iteratively decreasing wavepath width

  • Contours for velocity 2,500 m/s and higher velocities become more parallel to the fault zone, which dips down to the right (towards offset 250m at elevation of -80m).

  • Thin wavepaths make WET tomography more prone to generating artefacts, especially with bad or noisy first break picks and strong refractor curvature.


Conclusions

  • Plotting 1.5D Plus-Minus refractors on the WET tomogram allows interactive adaptation of parameters, until the layered analysis matches the 2D velocity tomogram

  • Layered refraction modeling is non-unique and subjective due to mapping of traces to assumed refractors and lateral smoothing, necessary for refractor velocity estimation and time-to-depth conversion

  • WET interpretation depends on the maximum allowed basement velocity, which may not be well-constrained by the first break picks

  • Gaussian weighting can produce more focused tomograms for wide wavepaths than Ricker weighting

  • Tomograms obtained with an iterative approach of wavepath adjustment show improvement compared to the standard ray-based approach.


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