Refraction Methods

cplin@mail.nctu.edu.tw

Principle of Seismic Refraction Analysis

Principle of Seismic Refraction Analysis

Snells Law

211

sinsinsinvvv

tri ==

V1

V2

i r

t

Refraction Basics

Body Wave in Layered MediaRefraction Basics

Head Wave

Refraction Basics

Ray Path

pvvvv n

n ===== sinsinsinsin3

3

2

2

1

1 L

pvv nn

n ==

1sin

1

1

Refraction Basics

Ray Slowness Decomposition

xv

t

= 1

pxzv

xv

zv

t iii +

=

+

= cossincos

Refraction Basics

Travel Time Curve

Refraction Basics

Picking First Arrival TimeRefraction Basics

Low Velocity Over High Velocity Halfspace

Horizontal two-layer model

Refraction Basics

Determining Earth Structure From Travel Time Curve

Refraction Basics

Determining Earth Structure From Travel Time Curve

Refraction Basics

Derivation of Travel Time Equation

Refraction Basics

High Velocity Over Low Velocity Halfspace

Refraction Basics

2

2

1

1 sinsinvv =

Multiple Subsurface Model

Refraction Basics

Multiple Subsurface Model

Refraction Basics

Hidden Layers

Low Velocity Layer

Refraction Basics

Hidden Layers

Thin, large velocitycontrast layer

Refraction Basics

Dipping Layer (Shooting Down Dip)

Refraction Basics

Dipping Layer (Shooting Up Dip)

Refraction Basics

Recognizing Dipping Layers

Refraction Basics

Interpretation of Dipping Layers

Refraction Basics

Interpretation Methods

Intercept Time Method

The Reciprocal Method

Generalized Reciprocal Method

Tomographic Method

Interpretation Method

Intercept Time Method

The intercept time method requires a mathematical model in which each layer has a constant velocity and is bounded by planedipping interfaces.

Gives the exact solution (depth, dip, and velocity of each layer) by using the apparent velocities and the intercept time measured on the travel time curves.

Interpretation Method

The Reciprocal MethodIrregular layer boundary

Main consideration is to do the computation at everyreceiver station (not only at the shot location)

Interpretation Method

The Reciprocal Method

Interpretation Method

The Reciprocal Method

Interpretation Method

The Reciprocal Method

Refraction Basics

The Reciprocal Method

The Reciprocal Method

Generalized Reciprocal Method

The reciprocal method can treat major refractor velocity changes and simple refractor irregularities. It essentially extends intercept time method computations from the shot points to each detector. However, there are some problems if the refractors are too irregular.

GRM is similar to the conventional reciprocal method, but include the principle of migrationby considering the offset distance XY.

Interpretation Method

Tomographic Analysis

Interpretation Method

Tomographic Analysis

Interpretation Method

Refraction Method Procedure

Data Acquisition Traveltime Curve

Tomography Inversion GRM Analysis

Seismic Equipment

Data AcquisitionSeismic Equipment

Seismic SourcesImpact sources Example: Sledgehammer Advantages Low cost Simple to operate and maintain

Disadvantage Difficult to assure repeatabilityManually strenuous Energy small Low frequency (i.e. this source

generate a lot of surface waves)

Data AcquisitionSeismic Equipment

Seismic SourcesGun Sources Advantages Highly repeatable Energy greater than

sledgehammer Higher-frequency energy

Disadvantage SafetyMore bulky and expensiveGetting permission difficult

Data AcquisitionSeismic Equipment

Seismic SourcesExplosive Sources AdvantagesMost amount of seismic energyVery high frequency

DisadvantageSafetyPermittingData acquisition slowerExpensive to acquire and maintain

Data AcquisitionSeismic Equipment

GeophoneData Acquisition

Seismic Equipment

Seismic Recording System

Traditional Distributed

Geometrics StrataVisor Seistronix RAS-24OYO McSeis_SX

Data AcquisitionSeismic Equipment

Seismic Recording System

Data AcquisitionSeismic Equipment

Seismic Recording System

Data AcquisitionSeismic Equipment

Design a survey line

Data AcquisitionDesign a survey line

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Survey line direction Receiver interval and location Usually use 5 meter for interval

Source location 7 points method 9 points method

source receiver / geophoneuse 1/3 or line length

Data AcquisitionData Acquisition

Field Testing

Sources of Noise

Uncontrolled Ground Motion

Electronic Noise

Geologic Noise

Data Acquisition

Travetime curve

Picking first arrival

Checking travel time curves

Parallelism Reciprocal traveltime Critical distance Intercept time

Completed traveltime curves

Travetime curve

Checking traveltime curves

Parallelism

Traveltime difference dose not increase

Travetime curve

Reciprocal traveltimeChecking traveltime curves

Reciprocal traveltime should be same

Travetime curve

Critical distance (layer assignment)

Layer assignment should be same in reciprocal traveltimes

Checking traveltime curvesTravetime curve

Intercept TimeChecking traveltime curves

Intercept time from the both sides of traveltime curves should be same

Travetime curve

Analysis Method

Intercept Time Method The Reciprocal Method Generalized Reciprocal Method Tomographic Method

Ground Layers Analysis