Geophysics:Seismic Reflection Data

Geol493K- West Virginia University

How to image the subsurface? Drilling

Use surface geology

Seismic Reflection Imaging

• Send sound into the rock, and collect the echoes

Gravity

Magnetics

Seismic AcquisitionSend sound waves, collect the echoes

Steps in the Seismic Process

1. Data Acquisition – done by a service company

2. Data processing – done by a service company

3. Interpretation – done by in-house geologists or geophysicists

Basic Principles Seismic velocity is dependent mostly on rock stiffness

Usually: Denser= Faster

Abrupt change in stiffness causes:

Reflected energy

Refracted energyFaster rockDenser

Slowerrock

•Reflection

•Refraction

Snell’s Law

Reflected energy

Refracted energyV2

V1a1 a1

a2

Sin a1 Sin a2

V1 V2=

At some point, if V1>V2, a2 = 90 and all the energy goes along the contact (a1= critical angle)

What is a Reflector?

Major changes in properties usually produce strong, continuous

Boundary between beds with different properties.

Bed 1

Bed 2

lower velocity

higher velocity

energy source

signal receiver

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One Shot- Reflections to all geophones Wave SpreadingSeismic waves propagate as Wave Fronts, not just rays

•Energy spreads out as (distance)2

•One shot causes reflections all along the reflector

Most Seismic Surveys use P-Waves

You need 3-component geophones for S-waves

V1

V2

V3

V1= (k1 + 4/3 h1)/ ρ1

V1= stiffness / ρ1

V= acoustic velocityk = bulk modulush = shear modulusρ= density

Depth vs Time

V1

V2

V3

Dep

th

echo1

echo3

echo2T

ime

Reflection Coefficients

What is he magnitude of each reflection?

Acoustic impedance = Z1= v1ρ1

Reflection coefficient = (Z2- Z1 )/(Z2+ Z1)

Fraction of reflected energy

Typically -0.1 to 0.1

Shale

Sand

Velocity = 2000 m/sDensity = 1.7 gm/ccI = 2000 * 1.7 = 3400

Velocity = 2400 m/sDensity = 1.8 gm/ccI = 2400 * 1.8 = 4320

ReflectionCoefficient =

4320 - 3400

4300 + 3400=

I below – I above

I below + I above= 0.119

Of the incident energy, 12% is reflected, 88% is transmitted

Seismic Interface Example

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Shale

Carbonate

Velocity = 2000 m/sDensity = 1.7 gm/ccI = 2000 * 1.7 = 3400

Velocity = 2600 m/sDensity = 2.1 gm/ccI = 2600 * 2.1 = 5460

ReflectionCoefficient =

5460 - 3400

5460 + 3400=

I below – I above

I below + I above= 0.232

Of the incident energy, 23% is reflected, 77% is transmitted

Seismic Interface Example

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Reflection seismogram viewed as the convolved output of a reflectivity function with an input pulse

Marine Seismic Survey

Marine Swath Data for a 3D

Streamers

Diverter

Vibroseis trucks

Seismic Acquisition - Onshore

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Drilling holes for dynamite source

80 kg shot for deep seismicRussia Laying out Geophones

Seismic Acquisition - Onshore

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Seismic Acquisition - Onshore

Geophones

Seismic Acquisition - Onshore

Laying out Geophones in Mountainous Areas 24

Seismic Processing

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Velocity model

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Seismic Processing

Field Record(marine)

Data ProcessingStream

Subsurface ‘Image’

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Common Depth Point

S are shots

D are receivers

Single-ended spread

12 channels

6-fold redundancy

Sort the shot-receiver pairs that collect data from the same point

Shot Gather

•All traces from a common mid point(redundant data)

•Hyperbolas get flatter with increasing V

Seismic Processing: Geometry of Seismic Acquisition

ΔT= normal moveout

t= travel time

t= (1/V)(x2 + 4z2)1/2

•A hyperbola

•t, x are known

•V unknown, but can be found graphically using the seismic data

•z calculated (V, t)

Time-distance plot

Geometry

Velocity Analysis: find the velocity function that will flatten the hyperbolas

Velocity function UncorrectedGather

Corrected

Stacking – Noise reductionGather After Stack

Sum these

Stacked, Unmigrated Line: has issues

Stacking Assumes Layer-Cake Structure

AB

Stacking assumes layer-cake structure

In a tight syncline, 3 reflections originating from different spotsmay focus at the receiver

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Migration – Correcting for Location

Sweep Ellipse

S RUnmigrated energy on single trace...

...spread to all possible locations of origin

S R

Sweep Ellipse

S R

Sweep Ellipse

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Migrated (corrected) line after processing

Fault Diffractions:

Seismic energy is scattered at sharp edges

Diffractions

Raw data Migrated data

L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Migration

Unmigrated ImageUnmigrated Image

Migrated ImageMigrated Image

Positioning Problems ‘Blur’ the

Image

Migration ReducesPositioning

Problems, which Improves the Image

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Seismic Multiples: Ghosts of the seabed

Ghost 2Ghost 1

Multiples in Marine Data

Butler et al. 2015

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The Ideal Seismic Response

Increase in Impedance Decrease in Impedance

Able to resolve boundaries of beds a few meters thick

1 meter

43 ExxonMobil

Seismic - Units 10’s of Meters Thick

10 m

Predominantly

Shale

Predominantly

Shale

Predominantly

Sand

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Vertical resolution of seismic is depends on frequency and velocity–

f= 30Hz , v= 6000 ft/sec

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Scale for Seismic Data

Although seismic data can not image small-scale stratal units, it can image mid- to large-scale units

Parasequences

Bed Sets

Parasequence Sets

Sequences

Beds

Lamina Sets

Lamina

Sequence Sets

The big advantage of seismic data is areal coverage

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Time versus Depth

0.58 sec

m

1865

926

288

926 m

Two way time (TWT) does not equate directly to depth

Depth of a specific reflector can be determined using wells

47 ExxonMobil

Seismic Shot

Borehole Geophone

De

pth

Check shots measure the

vertical one-way time

from surface to various

depths within the well

Used to calibrate well

depths and times from a

sonic log

Time to Depth - Check Shot Data

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Check Shot survey example

Interval velocity

IntegratedSonic log

Acoustic impedancefrom Sonic log

SeismicReflectivity

Wavelet

SyntheticSeismic

Creating synthetic seismic data:

Convolution

• Use the sonic and density logs to compute an impedance ‘log’• Calculate the reflection coefficients• Convolve our pulse with the seismic reflectivity• Sum the individual wavelets to get the synthetic seismic trace

Seismic Ties

Compare well data to seismic data

Relate horizon tops in a well with specific reflections

ReferenceLog

Synthetic(+)

Synthetic(-) Tops Take Home Ideas

Seismic Data Requires CarefulAcquisitionProcessing

Power of Stacking and MigrationImproves Signal to Noise

A Seismic Reflector is Boundary Between Beds of Different PropertiesRelates to Lithology – Density and Velocity

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