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 Moveout and Coincident Source-Receiver Concepts & 3D Seismic Interpretation

Tom Wilson, Department of Geology and Geography Just a reminder: pages 149 to 164 in Chapter 4 were on your reading list. Continue reading Chapter 4. We will be addressing some issues in a different sequence than in the text. 1)Review T NMO relation on page 160 2)Understand the t 2 -x 2 transformation (p ) 3)We will come back to a discussion of determining velocities, thicknesses.. (p 170 – 180). Review for now. 4)You will be expected to understand how to apply relationships associated with the dipping interface problem (p ). You will encounter additional discussion of moveout in these pages.

Tom Wilson, Department of Geology and Geography 5) Read about multiple reflections and diffractions (p ). 6) Familiarize yourself with the common depth point concept (p ). 7) Correcting for normal moveout (p ).

Tom Wilson, Department of Geology and Geography Objectives for the day Normal moveout ( NMO ) and its elimination ( NMO Correction ) What do dipping layer reflections look like in the shot record? Quantitative relationships for the dipping layer reflection The problem posed by dipping layers Common midpoint sorting & CMP gathers Transformation of the dipping layer reflection in the CMP gather.

Tom Wilson, Department of Geology and Geography Here is some shot data collected in Marshall Co. WV We’d like to turn this into geology. Why do the amplitudes drop off below 200ms? Enhanced display How do we get here?

Tom Wilson, Department of Geology and Geography How do we get from the shot data to the data you’ve been interpreting in the Gulf – or

Tom Wilson, Department of Geology and Geography this data from the North Sea ….

Tom Wilson, Department of Geology and Geography The effective source receiver geometry for the records shown at right across the east margin of the Rome Trough is corrected so that the source and receivers share the same surface location. Note that critical refractions point to individual source points. The short story or Appalachians

Tom Wilson, Department of Geology and Geography At this point it is apparent that something has to be done to flatten out the hyperbolas to make them look more like continuous geologic horizons Flatten in time

Tom Wilson, Department of Geology and Geography Note that the reflection point coverage spans half the distance between the source and receiver Off-end Split spread The split spread provides symmetrical coverage about the source

Tom Wilson, Department of Geology and Geography Moveout and the moveout correction

Tom Wilson, Department of Geology and Geography Redefine the reflection time equal to the 0-offset arrival time (t 0 ) plus the  t (drop from t 0 or “moveout”).

Tom Wilson, Department of Geology and Geography  t is the normal moveout correction Assume  t 2 is small relative to other terms and can be ignored to approximate the moveout

Tom Wilson, Department of Geology and Geography Look at the reflection time distance relationship in terms of t 2 versus x 2 Square both sides of this equation

Tom Wilson, Department of Geology and Geography The hyperbola becomes a straight line

Tom Wilson, Department of Geology and Geography In the t 2 -x 2 form, the slope is 1/V 2

Tom Wilson, Department of Geology and Geography V is derived from the slope of the reflection event as portrayed in the t 2 -x 2 plot. The derived velocity is referred to as the Normal Moveout Velocity, NMO velocity, or, just V NMO. The normal moveout velocity - V NMO

Tom Wilson, Department of Geology and Geography The V NMO is used as a correction velocity If the velocity is accurately determined the corrected time  equals t 0

Tom Wilson, Department of Geology and Geography hyperbolas or ellipses

Tom Wilson, Department of Geology and Geography If the correction velocity (V NMO ) is too high then the correction is too small and we still have a hyperbola

Tom Wilson, Department of Geology and Geography And we have an ellipse

Tom Wilson, Department of Geology and Geography Roll-along split-spread shooting geometry

Tom Wilson, Department of Geology and Geography NMO correction of the reflection events appearing in the shot records across relatively horizontal strata yields a more accurate image of subsurface geology. NMO corrected reflections

Tom Wilson, Department of Geology and Geography

Dipping Layer Reflection Event has Offset Apex. How do you find depth h, velocity V and dip  ?

Tom Wilson, Department of Geology and Geography If you could not see the direct arrival then you could solve for V using either expressions for t 0 or t apex. Features of the reflection from a dipping interface as observed in the shot record.

Tom Wilson, Department of Geology and Geography The NMO correction is symmetrical about the zero offset or source point. The dipping layer reflection event is not.

Tom Wilson, Department of Geology and Geography Reflection points

Tom Wilson, Department of Geology and Geography Following the distribution of common reflection points

Tom Wilson, Department of Geology and Geography This is referred to as a stacking chart. The significance of the name will become apparent later on. Different source receiver combinations provide information from the same reflection point

Tom Wilson, Department of Geology and Geography For next week at this time construct a stacking chart for a symmetrical split spread consisting of 12 geophones arranged 6 on each side of the source. Bring questions to class on Tuesday

Tom Wilson, Department of Geology and Geography

Time-distance relationship for reflections in a CMP gather are identical to those in the shot record.

Tom Wilson, Department of Geology and Geography Definition - CMP Gather: A collection of traces sharing a common midpoint.

Tom Wilson, Department of Geology and Geography Raypaths in the CMP Gather don’t necessarily provide information from the same reflection point!

Tom Wilson, Department of Geology and Geography But reflection events in a CMP gather have a special property

Tom Wilson, Department of Geology and Geography Even when the layer dips they remain hyperbolic

Tom Wilson, Department of Geology and Geography

And they are symmetrical

Tom Wilson, Department of Geology and Geography The effect of the moveout correction on the traces in the common midpoint (CMP) gather is to create a composite normal incidence trace that effectively shares a coincident source and receiver at the midpoint shared by all the traces in the gather. We’ll discuss CMP data in more detail in a couple lectures. Symmetrical hyperbola are easy to NMO correct!

Tom Wilson, Department of Geology and Geography Construct a stacking chart for a symmetrical split spread consisting of 12 geophones arranged 6 on each side of the source (see handout). Bring questions to class This Wednesday. The chart is due next Monday. Complete your reading of Chapter 4. Dipping layer reflection events are covered on pages , with additional discussion on pages The idea of common depth point sorting is discussed on pages We’ve talked tangentially about resolution ( ) and velocity analysis ( ). We will be talking about stacking of CDP gathers ( ) and migration ( ). Discussions of migration will come later but it is helpful to be aware of the issues early on. Look over problems 4.1, 4.4 and 4.8.