Download Lecture 5 the Seismic method and more Lecture notes Geophysics in PDF only on Docsity!

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Lecture 5 Lecture 5

Mitchum et al., 1977b AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Basic Exploration Workflow

To D/P Drop Prospect Drill Wildcats Confirmation Well Identify Opportunities Process Seismic Data Capture Prime Areas Interpret Seismic Data Acquire Seismic Data Success Success Failure Uneconomic Economic Analysis Assess Prospects

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Raw Seismic Data

Device

Device

For the explosion we just considered.

Listening device #1 records a reflection starting at 0.4 seconds Listening device #2 records a reflection starting at 0.8 seconds

To Image the Subsurface, We Use Many Shots (explosions)

and Many Receivers (listening devices)

Arranged in Lines either on Land or Offshore

Time

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Seismic Acquisition

• (^) A 3D survey is designed based on:
  • (^) Imaging Objectives: image area, target depth, dips,

velocity, size/thickness of bodies to be imaged, etc.

• (^) Survey Parameters: survey area, fold, offsets,

sampling, shooting direction, etc.

• (^) Balance between Data Quality & $$$$$

Land Operations

Vibrators Generate a Disturbance Geophones Detect Motion

Marine Operations

Air Guns Generate a Disturbance Hydrophones Detect Pressure

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Seismic Processing

Field Record (marine)

Data Processing

Stream

Subsurface ‘Image’

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Shot Gather

Source Receivers R1 R2 R3 R4 R S Direct Arrival Reflections 2 Way Travel Time Offset (Distance) R1 R2 R3 R4 R Direct Arrival Reflection For each shot, reflections are recorded in 5 receivers There are 5 ‘bounce’ points along interface 3 1 2 3

For Shot 1

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

CMP Gather

The travel times differ

since the path for a near

offset trace

is less than the path for a

far offset trace

With the correct velocity,

we can correct for the

difference in travel time

for each trace.

The curvature of this hyperbola is a function of the average velocity down to the depth of the reflection CMP Gather Offset Distance

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method CMP Gather Offset Distance

With Correct Velocity, Gather is Flat

Velocity Too Slow Velocity Correct Velocity Too Fast Flat Curves Down Curves Up

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Positioning Problems

Energy Source

The seismic ray hits an

inclined surface at 90º and

reflects back

0.2 s down 0.2 s up 0.4 s -

The reflection is

displayed beneath the

source-receiver

midpoint

Bounce Point

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Time for an Exercise

Where would the reflection lie?

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Time for an Exercise

Where would the reflection lie?

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Exercise Answer

The reflection is downdip and its

dip is less than the interface

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Migration – Power of Correlation

Two reflections on unmigrated data After spreading to all possible locations Reflections are not positioned in the subsurface correctly since they have dip Constructive interference occurs where the reflections are properly positioned Destructive interference dominates where the reflections are NOT properly positioned

F W Schroeder Courtesy of ExxonMobil L 5 – Seismic Method

Seismic Migration

Unmigrated ImageUnmigrated Image Unmigrated ImageUnmigrated Image Migrated ImageMigrated ImageMigrated ImageMigrated Image

Positioning

Problems ‘Blur’ the

Image

Migration Reduces

Positioning

Problems, which

Improves the

Image