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EARS2232 Exploration Seismics Practical: Reflection Survey Processing

Velocity measurement from seismic reflection surveys

Hand-in: 20.11.2006 (box in front of undergraduate office)

Name: ______________________________ ID #:__________-___________-_________ Background Measurement of velocity is extremely important for reflection seismic processing. All the filtering, static corrections and waveform processing that precedes it, is just done to make the results of velocity analysis more reliable. Those results then get used to make the simple travel-time image into a geometrically correct time or depth image. This last process is very sensitive to velocity errors, so the velocities must be right. This practical has two parts – first some measurements made directly from a CMP gather, to reinforce the basic travel-time relationships, and second, some measurements made using the industry standard method of semblance analysis. You are required to annotate some answers on the figures provided. Please mark and label your answers clearly. Include this question paper and the associated figures, as well as any separate sheets that you use, when you submit your assignment for marking, so that I have a chance to understand what you did. Show how you got to your results!!

Q1.1a Q1.1B Q1.2a Q1.2b Q1.3 Q1.4 Q1.5 Q1.6 Q1.7 Q1.8

3

3 8 5 5 20 15 8 4 4

Q2.1a Q2.1b Q2.2 Q2.3 Total

12

6 3 4 100

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2-layer case

The travel time T of a reflected wave, from a boundary at depth z at the base of a single overburden layer, thickness z and wavespeed V, recorded at source-receiver offset x is

2

22

0

2

V

XTT +=

where T0 is the travel time measured at zero offset, and T0 = 2z/V. This is an exact formula. It is a hyperbola, which if x«z can be roughly approximated to a parabola as

0

2

2

02 TV

XTT +≈

Multi-layer case

When the reflected wave comes from a boundary at the base of n layers, when the velocity contrasts at boundaries are small, and x<z, an equation of the same form as (1) still holds,

2

22

0

2

rmsV

XTT +=

except ‘velocity’ is now the RMS velocity, a ‘root-mean-square’ velocity, where the velocity in each layer (the ‘interval velocity’ Vinti) is weighted by the travel time t through each layer

( )

∑

∑=

j

i

rmst

VtV i

2

int2

Hence, reflections still appear as hyperbolae. Linear regression with x2 as “x” and T2 as “y” gives Vrms and T0 by manipulating slope and intercept respectively. Given measured velocity for reflections from the top and base of an interval, then Equation (3) can be applied to each, and re-arranged to give Dix’s Equation. This allows the actual velocity in an interval to be computed.

( )

topbase

rmstoprmsbase

tt

VtVtV topbase

−

−=

22

2

int

To get approximate depths, break down the T0 values into two-, then one-way times in each interval, and combine them with Vint values to give thicknesses.

Comment: dips and offsets

This analysis can be extended to gently-dipping layers, again giving a similar result, but the velocity in all of the equations above can be replaced by

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( )dip

VV

cos=

Hence dip, whether positive or negative, always increases the inferred velocity.

However, the constraint “x<z” to get from a hyperbola to a parabola for the traveltime of reflections is quite serious and often violated in practice. The consequence is that the travel-time variation at the longest offsets is not hyperbolic. Because of this ‘non-hyperbolic moveout’, applying the first hyperbolic equation to measure velocity will give a different answer when different ranges of offsets are used. The true RMS velocity is only found from near-offset data. Hence, in general, we call the velocity that we see in data simply “Vnmo” or moveout velocity, and we have to use it in Dix’s Equation regardless.

A yet further cause of non-hyperbolic moveout is anisotropy (a gradual variation of velocity between horizontal to vertical raypaths.

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Part 1 – direct measurement using T2-X2 approach (75%)

Question 1 uses a marine CMP gather shown as Figure 1. This figure is also enlarged in 3 separate segments, to make manual measurements of arrival times and offsets easier. Read the caption to Figure 1 before starting to make any measurements from the data. Q1.1. (a) What is the typical speed of a P-wave in water? Given that value, and the offset of the first trace, identify the direct P-wave through the water on the first trace, and follow it across the whole gather. Highlight it carefully on the Figure. (b) Why does the direct wave appear to retain its high frequency content? Q1.2. (a) Identify the sea-bed refraction: measure its velocity and intercept time, and hence compute the water depth. (b) State two reasons why the sea-bed refraction is a low-amplitude, low-frequency, arrival. Q1.3. Can you recognise the sea-bed reflection? Highlight it carefully on the Figure, in a different colour to your answer for Q1.1a. [Hint: it becomes asymptotic to an arrival that you have already identified. As a help draw the traveltime distance plot for a direct, refracted and reflected arrival for a 2-layer horizontal model]. Q1.4. On Figure 1, 5 reflections labeled A-D are indicated. For each of the 4 reflections (perhaps starting with C, as it’s the easiest)

• highlight it across as much of the gather as you can

• pick 4 or more arrival times across the gather at the widest possible range of offsets

• for each time T, measure its associated offset distance X

• tabulate these values, and convert each value of T and X to T2 and X2

• plot the T2 and X2 values and using linear regression on T2 and X2 values, find the slope and intercept of the best-fitting straight line to the T2 and X2 values

• find the moveout velocity Vnmo and zero-offset two-way time, T0 Q1.5. Reflector A is the top of Unit A, reflector B the top of Unit B, and so on for C and D. Use Dix’s equation with your results from Q1.4 to compute velocities and thicknesses for intervals A to D. Q1.6. Can you see any evidence for non-hyperbolic moveout in reflections A-D? Q1.7. Some other arrivals in this gather are not refractions, primary reflections. Highlight them on the Figure, describe them, and comment on what they might be. Q1.8. Collate your results for Q1.2a and Q1.5 into a table of velocity and thickness for all layers: draw them as a velocity versus depth profile. Suggest a geological interpretation of these results.

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Part 2 – Semblance analysis (25%)

Figures 2-4 show velocity spectrum displays, of the semblance statistic, from 3 CMP gathers from a profile across the Wairarapa Basin, New Zealand. Figure 5 shows the geological setting of the profile. CMP 160 and CMP 2240 are about 10 km apart, and CMP 2240 and CMP 3125 are about 5 km apart (the CMP spacing is 5m). Q2.1. (a) Pick stacking velocity functions for each of these three CMP’s. Alongside each semblance display is a small (20 CMP wide) section of the stacked section, which you can use as a guide to your picking: high amplitude events on the stack should give the more accurate velocity values. If you are uncertain about the correctness of a pick, use Dix’ equation to compute the interval velocity implied by your pick. Is the value realistic? (b) Can you see any multiple arrivals on the semblance display? Q2.2. Plot the stacking velocity functions together on a single plot. Use surface velocities of 1500 ms-1 for CMPs 2240 and 3125, and 1000 ms-1 for CMP 160. Q2.3. Discuss how the velocity functions vary across the Basin in relation to the geological setting of the survey.

Figure 5

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Figure 1 (enlargement 1 of 3)

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Figure 1 (enlargement 2 of 3)

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Figure 1 (enlargement 3 of 3)

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Q1.1. (a) Typical speed of a P-wave in water? m s-1 Q1.2. (a) Sea-bed refraction velocity m s-1

Sea-bed refraction intercept time seconds Water depth m

A B C D E

Zero-offset TWT (s)

Vnmo (ms-1)

Q1.5 – 1.6 Thickness (m) Velocity (ms-1) Water

Sea-bed sediments

Unit A

Unit B

Unit C

Unit D

(END)