Month: March 2015

AVO of methane hydrates

March 10, 2015 Documentation No comments

Another old paper is added to the collection of reproducible documents:
Seismic AVO analysis of methane hydrate structures

Marine seismic data from the Blake Outer Ridge offshore Florida show strong “bottom simulating reflections” (BSR) associated with methane hydrate occurence in deep marine sediments. We use a detailed amplitude versus offset (AVO) analysis of these data to explore the validity of models which might explain the origin of the bottom simulating reflector. After careful preprocessing steps, we determine a BSR model which can successfully reproduce the observed AVO responses. The P- and S-velocity behavior predicted by the forward modeling is further investigated by estimating the P- and S-impedance contrasts at all subsurface positions. Our results indicate that the Blake Outer Ridge BSR is compatible with a model of methane hydrate in sediment, overlaying a layer of free methane gas-saturated sediment. The hydrate-bearing sediments seem to be characterized by a high P-wave velocity of approximately 2.5 km/s, an anomalously low S-wave velocity of approximately 0.5 km/s, and a thickness of around 190 meters. The underlaying gas-saturated sediments have a P-wave velocity of 1.6 km/s, an S-wave velocity of 1.1 km/s, and a thickness of approximately 250 meters.

Tutorial on tuning and AVO

March 9, 2015 Examples No comments

The example in rsf/tutorials/tuning reproduces the tutorial from Wes Hamlyn on thin-bed tuning and AVO analysis in seismic interpretation. The tutorial was published in the December 2014 issue of The Leading Edge. Madagascar users are encouraged to try improving the results.

See also:

Program of the month: sfgrey

March 4, 2015 Programs 2 comments

sfgrey is the most widely used program in Madagascar. It is used for plotting multidimensional images with grayscale or pseudocolor.

sfgrey shares many of its options with other plotting programs, such as sfgraph, sfwiggle, and sfcontour. You can look for common options by running sfdoc stdplot or checking out stdplot documentation online.

Parameters that control the range of data to be displayed are clip=, pclip=, bias=, allpos=, mean=. The default behavior is pclip=99, which means that data values get clipped to the 99-nth percentile. To display values without clipping, use pclip=100. Setting the clip value with clip= takes the precedence over setting the percentage clip with pclip=. The bias= defines the data value for the middle of the color scale range, the default (appropriate for seismic data) is bias=0. When displaying values that are all larger than the bias value, set allpos=y (all positive). To set the bias to the mean value of the data without specifying it explicitly, use mean=y. The following example from trip/asg/project uses mean=y to display a synthetic model.

The gpow= parameter applies a nonlinear scaling by taking the image to the corresponding power. If the value of gpow is less than zero, the appropriate value is estimated from the data. The following example from gee/pch/ida uses the value of gpow=0.25.

If the input is a 3-D cube, sfgrey can use a particular panel (2-D slice) to estimate clip or glow. The panel is specified by gainpanel= and set by default to the first non-zero panel. To estimate clip using the whole cube, specify gainpanel=all. To clip each panel individually, use gainpanel=each. To add a scale bar, specify scalebar=y. By default, the scale bar is vertical. You can make it horizontal by using bartype=h. To set the minimum and maximum values on the scalebar, use minval= and maxval=. To make the scale bar run in reverse, use barreverse=y. The following example from tccs/optapert/sigsbee uses bartype=h minval=0 maxval=1.

By default, sfgrey displays the first axis running vertical from top to bottom, which corresponds to transp=y yreverse=y and is a common way to display seismic data. For other kinds of data, you can modify the default behavior by setting transp=, xreverse=, and yreverse=. The following example from geo391/hw5/pocs displays a seismic horizon using transp=y yreverse=n.

For an explanation of different color schemes (specified with color= parameter), please refer to previous posts:

10 previous programs of the month:

CiSE Paper on Madagascar Community

March 3, 2015 Links No comments

The paper Reproducible Research as a Community Effort: Lessons from the Madagascar Project was published in the January/February 2015 issue of Computing in Science and Engineering, a special issue on Scientific Software Communities.

Reproducible research is the discipline of attaching software code and data to publications, which enables the reader to reproduce, verify, and extend published computational experiments. Instead of being the responsibility of an individual author, computational reproducibility should become the responsibility of open source scientific-software communities. A dedicated community effort can keep a body of computational research alive by actively maintaining its reproducibility. The Madagascar open source software project offers an example of such a community.

Program of the month: sfhistogram

March 1, 2015 Programs No comments

sfthistogram computes a histogram for distribution of values in the input dataset.

The following example from rsf/rsf/sfnoise plots the histogram of a normally-distributed random noise:

The output of sfhistogram contains integer values arranged in a one-dimensional array. The sampling is specified by n1=, d1=, and o1= parameters.

10 previous programs of the month: