10. Seismic Interpretation

1

Seismic Interpretation and Subsurface Mapping

2

1. Introduction

• Seismic interpretation and subsurface mapping are key skills that are used commonly in the oil industry

• This teaching resource introduces the basic principles of seismic interpretation and then, if time permits, they can be applied in a practical exercise

• The resource dovetails with the A level Geology specifications

3

2. Basic principles

• Seismic acquisition• Seismic processing• Understanding the data• Seismic interpretation

Seismic ExplorationFigure 13.1

Reflection Seismic Method

• Waves reflected back directly from subsurface rock interfaces

• Shorter distance from explosion to the detectors

• Basic Principles– Seismic waves travel at known velocities

through rock materials– Vary with type of rock, shale = 3.6 km/s;

sandstone = 4.2 km/s; limestone = 5.0 km/s– Shotpoint – origin of waves (explosives,

vibrations, sound)– Geophones - detectors

Figure 13.2Variable-Density Mode, waves of certain amplitude shaded black, other light colored

7

Seismic acquisition offshore

• An air gun towed behind the survey ship transmits sound waves through the water column and into the subsurface

• Changes in rock type or fluid content reflect the sound waves towards the surface

• Receivers towed behind the vessel record how long it takes for the sound waves to return to the surface

• Sound waves reflected by different boundaries arrive at different times.

• The same principles apply to onshore acquisition

8

• Onshore seismic acquisition requires an energy input from a “thumper” truck. Geophones arrayed in a line behind the truck record the returning seismic signal.

Sub-horizontal beds

Unconformity

Dipping beds

Seismic acquisition onshore (1)

Geophones (receivers)

Vibrator (source)

9

Seismic acquisition onshore (2)

Lithology change

Angular unconformity

Lithology change

• Seismic horizons represent changes in density and allow the subsurface geology to be interpreted.

10

• Wiggle trace to CDP gather• Normal move out correction• Stacking• What is a reflector?

Seismic processing

11

Wiggle trace to CDP gather

Graphs of intensity of sound as received by the recorders

Graphs of intensity for one location collected into groups and shown in a sequence.

Wiggle traces

CDP gather

12

Normal move out correction

Data for one point from different signals to different receivers

1. More time needed to reach distant receivers so the data look like a curve.

2. Correcting for normal move out restores the curve to a near horizontal display.

Change in lithology from mud to sand so sound is reflected back to surface

Sound receiversR3 R2 R1

Sound wave in

CDP

CMP

Fas

test

Slowes

t

Sound sources S1 S2 S3

Original CDP gather …

corrected for normal move out

21

Wav

e re

flect

ed

13

Stacking

Next, take all the sound traces for that one place and stack them on top of each other

First, gather sound data for one location and correct for delayed arrival (normal move out)

Finally, place stacks for adjacent locations side by side to produce a seismic line

14

What is a reflector?There are many reflectors on a seismic section. Major changes in properties usually produce strong, continuous reflectors as shown by the arrow.

A seismic reflector is a boundary between beds with different properties. There may be a change of lithology or fluid fill from Bed 1 to Bed 2. These property changes cause some sound waves to be reflected towards the surface.

Bed 1

Bed 2

Incoming ray Reflec

ted

ray

Refracted ray

lower velocity

higher velocity

energy source

signal receiver

15

Understanding the data

• Common Depth Points (CDPs)• Floating datum• Two way time (TWT)• Time versus depth

16

Common Depth Points

CDPs are defined as ‘the common reflecting point at depth on a reflector or the halfway point when a wave travels from a source to a reflector to a receiver’.

Common midpoint above

CDP

Change in lithology = reflecting horizon

Common reflecting point or common depth point (CDP)

Sound sources S1 S2 S3

Sound receiversR3 R2 R1

Sound wave in Sound

wav

e

refle

cted

17

The floating datum line represents travel time between the recording surface and the zero line (generally sea level). This travel time depends on rock type, how weathered the rock is, and other factors.

The topographic elevation is the height above sea level of the surface along which the seismic data were acquired.

Floating datum

18

0.25 seconds

Two way time (TWT) indicates the time required for the seismic wave to travel from a source to some point below the surface and back up to a receiver.

In this example the TWT is 0.5 seconds.

0.25 seconds

0

0.5

TWT

seco

nds

surface

Two way time (TWT)

19

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

boreholes• For example, 926 m depth = 0.58 sec. TWT

20

Seismic interpretation

• Check line scale and orientation.

• Work from the top of the section, where clarity is usually best, towards the bottom.

• Distinguish the major reflectors and geometries of seismic sequences.

21

Scale and orientation

• Use the scale bar to estimate the length of the line

• Use CDPs to check theorientation of the line on the accompanying map

22

Top down approach

first

second

third

• Start at the top of the section, where definition is usually best

• Work down the section toward the zone where the signal to noise ratio is reduced and the reflector definition is less clear

23

Continuous reflector truncating short ones

Reflector character and geometry

Reflectors onlapping continuous one

Next continuous reflector

Seismic reflection configuration and reflection

continuity Figure 13.4

Primary depositional conditions – Parallel, divergent, prograding

Undaform, Clinoform, Fondoform

Figure 13.5

Depositional Environments in relationship to wave base.

GEOL 553 Lecture 3; Subsurface Analysis

Seismic

Define geometries of genetic reflection Define geometries of genetic reflection packages that envelope seismic packages that envelope seismic sequences and systems tractssequences and systems tracts

Identify bounding discontinuities on Identify bounding discontinuities on basis of reflection termination patterns basis of reflection termination patterns and continuityand continuity

Seismic stratigraphic interpretation Seismic stratigraphic interpretation used to:used to:


Seismic Boundaries

Toplap termination Toplap termination Truncation of sediment surface Truncation of sediment surface Often channel bottomOften channel bottom

Termination below discontinuity, or Termination below discontinuity, or upper sequence boundaryupper sequence boundary ::

Onlap over surfaceOnlap over surface Downlap surfaceDownlap surface

Above a discontinuity defining lower Above a discontinuity defining lower sequence boundary:sequence boundary:

Relationships to strata

Figure 13.7Non-depositional

Seismic reflection patterns

Figure 13.8

Erosional truncation Toplap

onlap

Downlap

Nondepositional hiatus

Relationships that define Unconformable boundaries

Figure 13.9Mapping unconformities key to seismic sequence analysis

Sequence Boundaries, Downlap, Reflection

terminations

Figure 13.10

Above discontinuities – onlap, downlap

Below discontinuities – truncation, toplap, apparent truncation


Seismic Boundaries

Below Boundary - Toplap terminationBelow Boundary - Toplap termination


Seismic Boundaries

Below Boundary -Below Boundary - Truncation of Truncation of surfacesurface


Seismic Boundaries

ChanneledChanneled

Surface Surface

– – Below Below BoundaryBoundary


Seismic Boundaries

Over Boundary - Onlap onto surfaceOver Boundary - Onlap onto surface


Seismic Boundaries

Over Boundary- Downlap onto Over Boundary- Downlap onto surfacesurface





41


Sequence Stratigraphy

• Surfaces of erosion & non-deposition (sequence boundaries)

• Flooding (trangressive surfaces [TS] &/or maximum flooding surfaces [mfs]) & high stand condensed surfaces

Subdivision & interpretation of Subdivision & interpretation of sedimentary record using a framework sedimentary record using a framework surfaces seen in outcrops, surfaces seen in outcrops, well logs, & 2-well logs, & 2-D and 3-D seismicD and 3-D seismic. . Include:Include:

This framework used to predict the This framework used to predict the extent of sedimentary facies geometry, extent of sedimentary facies geometry, lithologic character, grain size, sorting & lithologic character, grain size, sorting & reservoir qualityreservoir quality


Tools Define Bounding Surfaces

• Relative time framework for sedimentary succession

• Better understanding of inter-relationship of depositional settings & their lateral correlation

These surfaces subdivide These surfaces subdivide sedimentary rocksedimentary rock &provide:-provide:-

Conceptual models follow that link the processes that formed the sediments and enable the prediction of their gross geometries


• Sequence geometries are subdivided and defined by– Maximum Flooding Surfaces (mfs) – Transgressive Surfaces (TS) – Sequence Boundaries (SB)

• Define how vertical succession or stacking patterns of unconfined sheets are arranged – Prograde (step seaward) – Retrograde (step landward) – Aggrade (build vertically)

• Sheets and unconfined lobes may contain– Non-amalgamated bodies

• Amalgamated, multi-storied bodies– Incised topographic fill of valleys– Unconfined but localized lobes from point & multiple up dip

sources– Unconfined but localized build ups (carbonates)

Hierarchy of Geometries






• Sequence geometries are subdivided and defined by– Maximum Flooding Surfaces (mfs) – Transgressive Surfaces (TS) – Sequence Boundaries (SB)

• Define how vertical succession or stacking patterns of unconfined sheets are arranged – Prograde (step seaward) – Retrograde (step landward) – Aggrade (build vertically)

• Sheets and unconfined lobes may contain– Non-amalgamated bodies

• Amalgamated, multi-storied bodies– Incised topographic fill of valleys– Unconfined but localized lobes from point & multiple

up dip sources– Unconfined but localized build ups (carbonates)

Hierarchy of Geometries


Ebb Ooid Delta - UAE


Delta Mouth Bar - Kentucky

Note Incised SurfaceNote Incised Surface


Channel – Gulf Coast



Flood Deltas & Channels - Kty


TidalChannel

sKhor

alBazam

-UAE


Tidal, Storm or Tsunami Channel



Tsunami Load & Drape - Kty

Note Uniform Thickness of LayerNote Uniform Thickness of Layer


Clastic Sequence Stratigraphic Hierarchies


Channels & Shelves

Chann

el

Chann

elShelfShelf

Both have unique Both have unique processes & structures that processes & structures that

can be used to identify can be used to identify their settingtheir setting


Tools Enable Sequence Stratigraphic Analysis

• Subdivision of section into sequences, parasequences and beds.

• Link conceptual models with mix of components of the individual sequence, parasequence or beds

• Use these to explain the depositional setting in terms of their lithology, grain size, sedimentary structures, contacts character (gradational, abrupt) etc

This analysis involvesThis analysis involves


SequenceStratigraphi

cAnalysis

End of the Lecture

Can it be supper time?




Unconfined Flow - Not in a Channel

• Unique Processes– Flow is in all directions – No lateral boundaries, only upper and lower

boundaries – Velocity changes: high to low

• Sediment responses– Decrease in grain size: Fining outward (coarse

to fine)– Erosional/sharp/gradational contacts – Accretion: Downstream, upstream and vertical – Decrease in sedimentary structures away from

source • Geometries

– Sheets – Thin in direction of flow

Documents

10. Seismic Interpretation