Seismic Reflection Lab

SO461 Lab, Spring 2013

 

Reduced view of an entire marine seismic record. Note:
  • Location map of this and other lines in the project, in the lower right corner.
  • Processing information along the upper right side.
  • The data on the left portion of the page.

 

This shows the field recording parameters for the survey. In particular note the diagram of the ship, with the energy source towed behind, and the long streamer array towed behind that.

 

Scales on the two axes.
  • The primary scale on the horizontal axes are the shot points, or distance along the profile.
  • The scale on the vertical axis is in TWTT (two way travel time), in seconds. Within the water column there is no problem converting this to depth (1500 m/sec), but once into the sediment conversion requires knowledge of the sound velocity. In water saturated sediments at the surface this is close to the speed in water, but with depth it approaches the value for solid rock which is several times faster. Processing of the record is designed to create a correctly scaled plot with depth instead of time.
  • Drilling results can give actual results for the sound velocity, either using the recovered core to see what the rock looks like, or using geophysical logs which measure the sound velocity in the hole.

 

This shows an artefact that is common in reflection profiling, a "bow tie". Note the explanation on the left.

 

This record shows annotations. The strongest reflectors will be sedimentary layers, which should be relatively horizontal, and faults, which will tend to dip much more steeply.

The red line marks a major change in the layer visible in the image. Below that point the sedimentary layering appears much less obvious, and it is likely the bottom might be another kind of rock.

The light blue lines trace some layers in the bedded sedimentary layers. The law of superposition can be used to show that there are older rocks on the right side that are not present on the left side.

 

Annotated record tracing two layers. Note that this divides the record into three sequences, and that one pinches out.

 

The green line marks a likely fault surface. Note the strong sedimentary layering to the left and the lack of corresponding layering to the right.
Note the two reflections near the water surface, and angular unconformity below them with beds dipping gently to the right.  Older rocks are just under the surface on the left, with progressively younger rocks to the right.

 

Location map for our survey data, which was supplied by the Minerals Management Service.

Note that the locations of two exploratory oil wells are indicated.  Wells are important because the provide "ground truth": layer thicknesses, rock types, and sound velocities of each rock type.
Create a 3D fence diagram
  • •Show seismic line at defined zoom level in Adobe  (must be the same for both lines)
  • •Find the intersection with the second line (“crossover”) , shown along the top margin.
  • •Screen capture into Paint
  • •Cut at the crossover
  • •Repeat for the second line, in a second Paint window
  • •Skew the two images (Image, Stretch/Skew), about 30° for a starting value, one positive and the other negative
  • •Paste the two images together in Paint
    The reflections should match in the two lines, and you should be able to see the dip of the bedding.

We have the following four profiles that you should look at:

Your requirements.

Pacific OCS Region, Minerals Management Service, 1997, Oceanside seismic data set: POCS-CD97-01.