Single fold vs. Multi-fold CMP seismic profiling Geology 493M 3D Seismic Workshop Single fold vs. Multi-fold CMP seismic profiling tom.h.wilson wilson@geo.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV

source receiver layouts used to acquire common midpoint data. Shooting Geometries source receiver layouts used to acquire common midpoint data. Very often these layouts are asymmetrical

We now have a nice geologic looking seismic section. Here’s an example of such a data set; the shot locations are obvious aren’t they. We now have a nice geologic looking seismic section. Can you spot the inversion structure? This data set is referred to as being single fold - that is there is only one record per reflection point.

The preceding process works well for horizontal or gently dipping strata, but when layers dip, the t-x relationship is asymmetrical and the NMO correction more difficult to make. Pull out a sheet of paper and ….

Common Midpoint Data Reflections share the same midpoint and in this case (horizontal reflector) they also share a common reflection point or depth point We have multiple records from the same reflection point. These data are 3 fold.

Three traces in this case (a three fold data set) get summed together to yield one stack trace. + + = The Stack Trace

What will this CMP reflection event look like in the t-x plot?

Even when the layer is dipping, the apex of a reflection hyperbola in the CMP gather (below) is located at x = 0 or at the common midpoint location between all the sources and receivers.

The seismic data you see most often will be of this type The seismic data you see most often will be of this type. The individual traces shown in most seismic displays will appear to have been collected with coincident source and receiver and, in this format, all the ray paths recorded on the receiver are normal incident on reflection surfaces.

Why go to all the trouble of collecting duplicate data?

A fairly noisy shot record from an area in central West Virginia. Greenbrier Ls. Onondaga/Oriskany Trenton Acoustic Basement ?

Stack Data High signal-to- noise ratio

Common midpoint sorting looks easy on paper

In reality (in West Virginia) straight roads are hard to find.

Bring the traces inside the circle together for stacking. Along crooked survey lines, the common midpoint gather includes all records whose midpoints fall within a certain radius of some point

CMP stack traces Noisy single-fold data

Normal incident coincident source-receiver records

Some inherent difficulties

The resulting stack trace

This data format can lead to some very unusual features in a seismic section. Consider, for example, a survey across a syncline. Consider the distribution of normal incidence ray-paths.

Consider what happens across the axis of the syncline and the relation of recording points to reflection points.

The record of reflection travel time to the various points in the subsurface contains dramatic image distortions - instead of a syncline we have an anticline

If you think this is only a theoretical construct - think again Pity the poor souls that keeping drilling these “anticlines!”

Which structure has the greater reserves?

How will the reflections from an isolated point reflector appear in the stack (normal incidence) section. Point reflector

Two-D seismic interpretation exercise - You’ll be working with a 3D data base from the Gulf Coast area. So the structures you’ll see will be down-to-basin faults accommodating gravity slide of large portions of the sedimentary section downhill.

Assignment Continue your interpretation of the 4 seismic lines from the 3D survey area. For our next class: 1) mark travel time values to the reflector along each line shown on the basemap. Do this often enough to define the major faults and general structure of the 1.3 second horizon 2) contour the travel time data 3) bring your results to class for discussion on Tuesday.