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Environmental and Exploration Geophysics II

Normal Incidence (coincident source-receiver) Ray-Tracing

tom.h.wilson

[email protected]

Department of Geology and Geography West Virginia University Morgantown, WV

Tom Wilson, Department of Geology and Geography

Off-end 12 geophone source receiver layout Tom Wilson, Department of Geology and Geography

Drag receiver string to the right -> Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Don’t forget to turn in today

Moveout in the common midpoint gather is hyperbolic For the flat layer there is no difference!

Tom Wilson, Department of Geology and Geography

regardless of whether the layer is flat lying or dipping.

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography Crooked Line Effects Along crooked survey lines, the common midpoint gather includes all records whose midpoints fall within a certain radius of some point

Tom Wilson, Department of Geology and Geography

NMO corrections to the arrivals in a common midpoint gather yield the same coincidence of sources and receivers, but in this case all sources and receivers relocate to the same midpoint. Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Reflection coefficients with wavelets Let’s assume that the traces shown at right are nmo corrected traces in a common midpoint gather.

They are all identical.

However, the real world doesn’t work this way. We always have noise in our data.

Tom Wilson, Department of Geology and Geography Source Receiver Offset -> Pure signal

Here’s the same data set with a lot of noise thrown in.

Note so easy to see the signal in this case is it?

Tom Wilson, Department of Geology and Geography Pure signal Noisy signal

Greenbrier Huron Onondaga Tom Wilson, Department of Geology and Geography 

i n

  1

a ij

Where

is the trace number and j is a specific time

If we sum all the noisy traces together - sample by sample - we get the trace plotted in the gap at right. This summation of all 16 traces is referred to as a stack trace.

Note that the stack trace compares quite well with the pure signal.

Tom Wilson, Department of Geology and Geography Stack Trace

Pure signal

Greenbrier Huron Onondaga

CMP sorting, NMO correction and stacking gives us higher signal to noise ratio and resolution of detail Tom Wilson, Department of Geology and Geography

Have a look at Ray-Trace Exercise IV The simulated normal incidence ray-paths have the potential to provide a more accurate visual image of the subsurface.

Tom Wilson, Department of Geology and Geography

A shot record view. Compare to the appropriate equations Tom Wilson, Department of Geology and Geography

Thus when you look at data of the type we are interpreting in our projects the individual traces shown in that section are equivalent to coincident source-receiver recordings and the ray paths are normal incident.

Tom Wilson, Department of Geology and Geography

We have an easy-to-interpret image as long as the reflectors are flat and horizontal, but as soon as we introduce structure into the reflector geometry the records become complex in appearance.

Consider, for example, a survey across a syncline. Consider the distribution of normal incidence ray-paths. Tom Wilson, Department of Geology and Geography

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

Tom Wilson, Department of Geology and Geography

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 Tom Wilson, Department of Geology and Geography

If you think this is only a theoretical construct - think again Pity the poor souls that keeping drilling these “anticlines!” Tom Wilson, Department of Geology and Geography

Reflection events are recorded down dip of the subsurface reflection points.

We will introduce the refraction response later.

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography Synclinal contraction

We have intentionally not included diffraction events.

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography Anticlines from points The Diffraction Point Response

Tom Wilson, Department of Geology and Geography

Diffractor Diffraction Bring questions to class this Wednesday.

Tom Wilson, Department of Geology and Geography

Common Midpoint (CMP) gather, also often referred to as Common Depth Point (CDP)

Tom Wilson, Department of Geology and Geography

Common Midpoint Gather Doug Smith’s seismic Data Processing site http://www-geo.phys.ualberta.ca/~doug/G438/Assignments07/Lectures/Common_Midpoint.pdf

Tom Wilson, Department of Geology and Geography

Velocity Analysis

Discussion of problems 4.1

Read over and think about how you are going to solve problems 4.5 and 4.8

Tom Wilson, Department of Geology and Geography

Dix Interval Velocities Read over discussions of Dix Interval Velocities (pages 170 -181)

The Dix interval velocity assumes V NMO velocities are equivalent to RMS velocities. Starting with the basic definition of the RMS velocity we derive the interval velocity V n .

V RMS n



i n

  1

V i

t i i n

  1

t i V n

2 

RMS n t



RMS n

 1

t n

Tom Wilson, Department of Geology and Geography

• Turn in 12 phone stacking chart • Problem 4.1 is due next Monday • Familiarize yourself with the Dix interval velocity computation. •This coming Wednesday, we will discuss the multilayer reflection problem and assign problems 4.4 and 4.8. Look over these problems prior to Wednesday. Be prepared to ask questions. • Look over Exercises IV-V. Due dates will be assigned later. Tom Wilson, Department of Geology and Geography