Seismic Reflection Examples

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.

 

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.  At the ends of the line (here the left side) you do not have coverage at the surface, but get increasing coverage with depth.  The "cliff" at the bottom is not real.  The record at the surface starts with the first shot point labeled.
  • 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.

On the top of the diagram, numbers show:

  • Water depth in feet
  • Common Depth Point, for the processing of the data
  • Shot Point

At selected points on the top, you get

  • Block boundaries
  • Crossovers with other seismic lines.

Computing depth

  • On left edge, the  sediment/rock starts at 1 sec TWTT.  This would be a depth of 750 m:  (1500 m/sec)  * (1 sec) / 2
  • There is a strong reflector at 1.3 sec TWTT.  This is 0.3 sec into the sediment; the true depth would depend on the sound velocity.  If the sound velocity in the sediment were 2000 m/sec, the thickness of sediment down to that point would be  300 m = (2000 m/sec) *  (0.3 sec) / 2.  If the velocity were larger, the depth would be more.

 

Common artefact in reflection profiling, a "bow tie". Note the explanation on the left.

 

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

The red line marks a major change in the layers 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 in the center 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.

Apparent dip.  On a single line, you see the apparent dip.  This is the true dip only if the seismic line parallels the strike; otherwise the apparent dip will be less than the true dip.  In the extreme case, when the line is perpendicular to the strike, the apparent dip will be zero.

Create a 3D fence diagram
  • Show seismic line at defined zoom level in Adobe  (must be the same zoom level  for both lines)
  • Find the intersection with the second line (“crossover”) , shown along the top margin of the profile, which will indicate the shot point in the second profile.


  • Screen capture into Paint
  • Cut at the crossover
  • Repeat for the second line, in a second Paint window.  Insure that the images have the same scale, and match up at the top and the bottom.
  • 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
  • Reflections should match in the two lines, and you should be able to see the dip of the bedding.

Last revision 12/7/2014