Environmental and Exploration Geophysics II

Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics II

[email protected]

Department of Geology and GeographyWest Virginia University

Morgantown, WV

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

InterpretationInterpretation


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 TNMO relation on page 160

2) Understand the t2-x2 transformation (p165-167)

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 192- 199). You will encounter additional discussion of moveout in these pages.


5) Read about multiple reflections and diffractions (p206-217).

6) Familiarize yourself with the common depth point concept (p 225-229).

7) Correcting for normal moveout (p232- 241).


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.


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?


How do we get from the shot data to the data you’ve been interpreting in the Gulf – or


this data from the North Sea ….


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 storyor Appalachians


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


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


t

Moveout and the moveout correction


Redefine the reflection time equal to the 0-offset arrival time (t0) plus the t (drop from t0 or “moveout”).


t is the normal moveout correction

Assume t2 is small relative to other terms and can be ignored to approximate the moveout


Look at the reflection time distance relationship in terms of t2 versus x2

Square both sides of this

equation


The hyperbola becomes a straight line


In the t2-x2 form, the slope is 1/V2


V is derived from the slope of the reflection event as portrayed in the t2-x2 plot. The derived

velocity is referred to as the

Normal Moveout Velocity, NMO velocity, or, just VNMO.

The normal moveout velocity - VNMO


The VNMO is used as a correction velocity

22 2

2NMO

xtV

If the velocity is accurately determined the corrected time equals t0


hyperbolas or ellipses


If the correction velocity (VNMO) is too high then the correction is too small and we still have a hyperbola


NMO

2 22 2

1 1If V V then 1NMOV V

And we have an ellipse


Roll-along split-spread shooting geometry


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


Vh

Vh

ttapex

2

cos2

0

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

cos0

ttapex

0

1costtapex


sin2hxapex

sin2apexx

h

If you could not see the direct arrival then you could solve for V using either expressions for t0 or tapex.

Features of the reflection from a dipping interface as observed in the shot record.


The NMO correction is symmetrical about the zero offset or source point. The dipping layer reflection event is not.


Reflection points


Following the distribution of common reflection points


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


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


Split Spread Shooting Geometry (12 phones)

-6 -5 -4 -3 -2 -1 1 2 3 4 5 6

02

4

6

8

10

Stacking Chart

Reflection Points

shot 1


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


Definition - CMP Gather: A collection of traces sharing a common midpoint.


Raypaths in the CMP Gather don’t necessarily provide information from the same reflection point!


But reflection events in a CMP gather have a special property


Even when the layer dips they remain hyperbolic


And they are symmetrical


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!


• 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 183-186, with additional discussion on pages 186-196. The idea of common depth point sorting is discussed on pages 225 -229. We’ve talked tangentially about resolution (217-219) and velocity analysis (233-238). We will be talking about stacking of CDP gathers (238- 241) and migration (241-244). 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.

Environmental and Exploration Geophysics II

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