Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Normal Incidence (coincident source-receiver) Ray-Tracing

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

Don’t forget to turn in today

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

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

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. Source Receiver Offset -> Reflection coefficients with wavelets Pure signal

Tom Wilson, Department of Geology and Geography 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? Pure signalNoisy signal

Tom Wilson, Department of Geology and Geography Greenbrier Huron Onondaga Where i is the trace number and j is a specific time

Tom Wilson, Department of Geology and Geography 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. Stack Trace Pure signal Greenbrier Huron Onondaga

Tom Wilson, Department of Geology and Geography 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 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 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 Synclinal contraction

Tom Wilson, Department of Geology and Geography We have intentionally not included diffraction events.

Tom Wilson, Department of Geology and Geography

Anticlines from points- The Diffraction Point Response

Tom Wilson, Department of Geology and Geography

Diffraction Diffractor 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 Doug Smith’s seismic Data Processing site Common Midpoint Gather

Tom Wilson, Department of Geology and Geography Velocity Analysis

Tom Wilson, Department of Geology and Geography 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 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. Read over discussions of Dix Interval Velocities (pages )

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.