Determination of depth to permafrost by geophysical methods

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Determination of depth to permafrost by geophysical methods

NATIONAL RESEARCH COUNCIL

CANADA

D I V I S I O N O F B U I L D I N G R E S M R C H

DETERMINATION OF DEPZH iPO PERMAFROST BY GEOPHYSICAL METHODS

by

I?,

A, Zelonka

I n t e r n a l Report No.

193

o f t h e

Division of Building Research

O T U W A A p r i l 1960

PREFACE

There i s a n u r g e n t need f o r improved technology i n d e a l i n g w i t h b u i l d i n g s and o t h e r c o n s t r u c t i o n s which, i n n o r t h e r n a r e a s of Canada, have t o be l o c a t e d on o r i n permanently f r o z e n ground. Most c o n s t r u c t i o n s l e a d t o a d i s t u r b a n c e of t h e d e l i c a t e b a l a n c e of ground t e m p e r a t u r e s o t h a t s o i l s o r i g i n a l l y f r o z e n a r e caused t o thaw, w i t h many a t t e n d a n t d i f f i c u l t i e s . While much e f f o r t i s b e i n g

put i n t o t h e a n a l y t i c a l approach t o t h e p r e d i c t i o n of changes

i n t h e thermal and m o i s t u r e regime

i n

t h e ground, it i s

q u i t e c l e a r t h a t t h i s w i l l only be developed t o an a d e q u a t e l e v e l by a s u s t a i n e d e f f o r t , and i s l i k e l y t o be e l a b o r a t e f o r many e n g i n e e r i n g purposes. !There i s much t o commend a d i r e c t e m p i r i c a l approach based on f i e l d o b s e r v a t i o n s , which can provide i n t e r i m i n f o r m a t i o n of d i r e c t a p p l i c a t i o n t o s i t u a t i o n s s i m i l a r t o t h o s e i n v e s t i g a t e d , and can a l s o r e i n - f o r c e t h e a n a l y t i c a l approach.

!&ere a r e many d i f f i c u l t i e s i n o b t a i n i n g sound f i e l d i n f o r m a t i o n on ground c o n d i t i o n s i n Northern a r e a s , b o t h f o r assessment of performance as w e l l a s f o r s i t e e x p l o r a t i o n p r i o r t o c o n s t r u c t i o n . D i r e c t sampling by d r i l l i n g , d i g g i n g , o r rodding i s a d e s t r u c t i v e method and can b e c o s t l y . No r e a l l y s a t i s f a c t o r y i n s t r u m e n t a l method of d e t e r m i n i n g by n o n - d e s t r u c t i v e means whether t h e s u b s o i l i s f r o z e n o r thawed h a s y e t been developed. Great i n t e r e s t i n t h e p o s s i b i l i t i e s of g e o p h y s i c a l methods wzs s t i m u l a t e d by t h e appearance on t h e market of a s m a l l p o r t a b l e shallow r e f r a c t i o n seismograph. The s e r v i c e s o f t h e a u t h o r of t h i s r e p o r t , a g e o p h y s i c i s t ,

were o b t a i n e d f o r a summer p r o j e c t t o examine t h e p o s s i b i l i t i e s of t h i s i n s t r u m e n t i n n o r t h e r n e x p l o r a t i o n . The work a l s o

i n c l u d e d a n e v a l u a t i o n of a n e ~ r S h r e s i ~ t ~ v i t y measuring

d e v i c e which rvns '<i??ll$ 1 oaned t o t h e D i v i s i o n 5y t h e I m p e r i a l

t h e summer~s work c a r r i e d o u t a t O t t a w a , London, Norman Wells, Fort Simpson, and Inuvik i s now r e p o r t e d .

!i!he a u t h o r , a graduate i n Physics, w i t h s p e c i a l t r a i n i n g and experience i n geophysical work, a c c e p t e d a n i n t e r i m summer appointment w i t h t h e Division i n o r d e r t o c a r r y o u t t h i s study. Ottawa A p r i l 1960 *

N. B e

Hutcheon A s s i s t a n t D i r e c t o r

TABLE OF CONTENTS

I. RESEXRCH PROGRAM

...

Page 1

I1

.

SEISMIC REFRACTION METHOD

...

General 3

...

Absorption of Seismic Waves 4

...

Optical Ray 13neory

5

...

Seismograph 8

I11

.

SEISMIC OBSERVATIOTJS

AND

DISCUSSIOIV OF RESULTS

...

Previous Work 9

Laboratory Velocity Measurements

...

11

Preliminary Field Trials

...

13

Northern Investigations

...

16

I V

.

RESISTIVITY METHOD General

...

19

Resfstivity Apparatus

...

20

Discussion of Resistivity Results

...

21

DETEG!?dINATIOB OF DEPTH TO PERMAFROST

BY GEOPHYSIZAL METHODS

Geophysical methods have l o n g been u s e d w i t h a

g r e a t d e a l o f s u c c e s s , by t h e o i l and mining i n d u s t r i e s i n e x p l o r i n g f o r m i n e r a l d e p o s i t s w i t h i n t h e e a r t h , T h i s s u c c e s s l e d t o t h e s u g g e s t i o n t h a t t h e same t e c h n i q u e s might be a p p l i e d t o s h a l l o w d e p t h e x p l o r a t i o n of e n g i n e e r i n g

i n t e r e s t . Work a l o n g t h i s l i n e h a s been hampered

i n

t h e p a s t because. of t h e expense and u n w i e l d i n e s s of t h e equipment. Recent a d v a n c e s i n i n s t r u m e n t a t i o n , however, have produced equipment designed e s p e c i a l l y f o r s h a l l o w d e p t h i n v e s t i - g a t i o n s .

A r e s e a r c h program w a s t h e r e f o r e i n i t i a t e d by t h e

DBR

t o e v a l u a t e t h e f i e l d performance o f two s u c h i n s t r u - ments, a small p o r t a b l e r e f r a c t i o n seismograph and a n e a r t h r e s i s t i v i t y measuring d e v i c e , t o determine t h e a p p l i c a t i o n of t h e s e g e o p h y s i c a l methods t o t h e mapping of p e r m a f r o s t i n Northern Canada.

I , RESFARCH PROGMI

The r e f r a c t i o n seismograph and r e s i s t i v i t y a p p a r a t u s were s u b j e c t e d t o a p r e l i m i n a r y t e s t i n g p e r i o d a t t h e DBR

l a b o r a t o r i e s a t O t t a w a b e f o r e b e i n g u s e d i n f i e l d s t u d i e s of p e r m a f r o s t a t Nol-man Wells, F o r t Sirnpson, and I n u v i k i n t h e Northwest T e r r i t o r i e s ,

The Ottawa phase of t h e program w a s d i v i d e d i n t o t h r e e p a r t s . F i r s t l y , p a p e r s p e r t a i n i n g t o e n g i n e e r i n g a p p l i c a t i o n s o f r e s i s t i v i t y and s e i s m i c g e o p h y s i c a l methods were reviewed t o g e t h e r w i t h l i t e r a t u r e c o n c e r n i n g A r c t i c t e r r a i n i n g e n e r a l and p e r m a f r o s t i n p a r t i c u l a r . Secondly, t h e i n s t r u m e n t s were f i e l d t e s t e d i n t h e O t t a w a a r e a t o g a i n f a m i l i a r i t y w i t h t h e i r u s e and a l s o t o a s s e s s t h e i r v a l u e

and l i m i t a t i o n s . T h i r d l y , " s o n i s c o p e " l a b o r a t o r y t e s t s on e l a s t i c wave v e l o c i t i e s i n f r o z e n and thawed m a t e r i a l s were conducted. These t e s t s were made i n a n a t t e m p t t o answer some q u e s t i o n s r e s u l t i n g from t h e proposed a p p l i c a - t i o n of r e f r a c t i o n seismology t o p e r m a f r o s t .

F i e l d i n s t r u m e n t t e s t s were conducted p r i m a r i l y a t t h e i n t e r c e p t o r and o u t f a l l sewer s i t e i n E a s t Ot-tawa.

This a r e a was chosen s i n c e a mapped p r o f i l e w a s a v a i l a b l e from d r i l l b o r i n g s . I n a d d i t i o n v e l o c i t y measurements were made a t a number of l o c a t i o n s t o e s t a b l i s h a b a s i s of

i d e n t i f i c a t i o n of v e l o c i t i e s w i t h roclr t y p e s , An e a r t h s l i d e w a s a l s o i n v e s t i g a t e d by means o f t h e seismograph t o d e t e r m i n e d e p t h t o bedrock. Some work was a l s o done a t London, O n t a r i o ,

Following t h e Ottawa phase of t h e program, r e s e a r c h a c t i v i t i e s were t r a n s f e r r e d t o Norman Wells. Most of t h e t i m e w a s s p e n t i n c o n d u c t i n g t e s t s a t t h e N o n a n Wells

a i r p o r t . 'This p a r t i c u l a r s i t e w a s i d e a l from t h e v i e w p o i n t of u n i f o m i t y s i n c e s u r f a c e e f f e c t s were n e g l i g i b l e ; it w a s e q u a l l y exposed t o t h e sun and was w e l l d r a i n e d . Other s t u d i e s were made o v e r wooded and moss-covered ground, Surveys were a l s o made n e a r b u i l d i n g s w i t h t h e i n t e n t of n o t i n g t h e e f f e c t of h e a t i n g on t h e p e r m a f r o s t l e v e l . F i n a l l y , t h e e f f e c t was i n v e s t i g a t e d of i r r e g u l a r i t y of

t h e p e r m a f r o s t t a b l e on s e i s m i c a r r i v a l t i m e s a c r o s s a boun- d a r y d i v i d i n g o r g a n i c - c o v e r e d s o i l and exposed m i n e r a l s o i l .

A f t e r c o m p l e t i o n of t h e i n i t i a l n o r t h e r n s t u d i e s a t Norman Viells, work w a s begun a t F o r t Simpson. It was

a n t i c i p a t e d t h a t t h e p e n l a f r o s t t a b l e t h e r e would be much more i r r e g u l a r than a t Nomian Wells due t o t h e more s o u t h e r l y l a t i t u d e of F o r t Simpson and t h e p e r i o d i c s t r i p p i n g of

f o r e s t growth which h a d t a k e n p l a c e o v e r t h e pas$ 1 0 0 y e a r s , Die s e i s m i c a n d r e s i s t i v i t y s t u d i e s a t F o r t Simpsoil were conducted c o n c u r r e n t l y w i t h a d r i l l i n g program b e i n g c a r r i e d o u t i n t h a t a r e a by

DBR p e r s o n n e l .

m e f i n a l a r e a s t u d i e d was I n t h e v i c i n i t y of I n u v i k . S e i s m i c i n v e s t i g a t i o n s were conducted o v e r s e l e c t e d g r a v e l d e p o s i t s , c e r t a i n l o o s e g r a v e l p a d s , a slumping embankment n e a r t h e wharf, a n d t h e a i r p o r t runway. lChe r e s i s t i v i t y a p p a r a t u s was n o t u s e d i n t h e I n u v i k s t u d i e s . '

11. SEISIJlIC REPRACTIOIT

METHOD

General

The p h y s i c a l t h e o r y u n d e r l y i n g s e i s m i c wave propaga- t i o n i s p r i m a r i l y a n e l a s t i c i t y problem. A f o r c e a p p l i e d t o t h e s h r f a c e of a n e l a s t i c body c a u s e s d e f o r m a t i o n of t h e body i n p r o p o r t i o n t o t h e a p p l i e d f o r c e . Assuming t h e f o r c e i s a p p l i e d b r i e f l y ( a n i m p u l s e ) , p o t e n t i a l energy i s i m p a r t e d t o t h e p a r t i c l e s i n v o l v e d i n t h e deformation. If t h e medium i s f r i c t i o n l e s s and p e r f e c t l y e l a s t i c , t h e s e p a r t i c l e s , i n t h e a b s e n c e of e x t e r n a l f o r c e s , o s c i l l a t e w i t h s i m p l e h a r - monic motion. S u b s e q u e n t l y , a d j a c e n t p a r t i c l e s o s c i l l a t e because o f t h e d i s t u r b i n g e f f e c t of t h e i n i t i a l p a r t i c l e s . m e r e s u l t i s t h e p r o p a g a t i o n of

an

e l a s t i c wave t h r o u g h t h e medium. The complete mathematical d e r i v a t i o n of t h e e l a s t i c e q u a t i o n s of motion i s e a s i l y o b t a i n e d from w e l l - known t e x t s ( 1 , 2 , 3 ) s o t h e method o f development w i l l n o t be g i v e n h e r e . For a homogeneous, i s o t r o p i c , p e r f e c t l y e l a s t i c , s e m i - i n f i n i t e body i n which t h e r e s t o r i n g f o r c e on t h e composite p a r t i c l e s i s p r o p o r t i o n a l t o t h e i r d i s p l a c e m e n t t h e l o n g i t u d i n a l o r

P

wave i s governed by t h e e q u a t i o n : where

A =

d i l a t i o n o r i n c r e a s e i n volume p e r u n i t volume K

=

b u l k modulus A= r i g i d i t y ' = d e n s i t y v2 = L a p l a c i a n o p e r a t o r

Those who a r e f a m i l i a r w i t h t h e g e n e r a l mathematical wave e q u a t i o n w i l l r e a d i l y i d e n t i f y t h e wave v e l o c i t y a s :

The wave e q u a t i o n f o r I! wave p r o p a g a t i o n i s o b t a i n e d

by a s s i g n i n g c e r t a i n boundary c o n d i t i o n s on p a r t i c l e motion.

By s p e c i f y i n g v a r i o u s boundary c o n d i t i o n s o t h e r wave t y p e s and e x p r e s s i o n s f o r v e l o c i t y a r e o b t a i n e d . Employing r o t a - t i o n a l components o f s t r e s s , t h e wave e q u a t i o n f o r t h e s h e a r o r S wave i s d e r i v e d . The v e l o c i t y of t h i s wave

i s g i v e n by:

I

-

It i s n o t e d t h a t A = 0 f o r f l u i d s s o t h a t S waves c a n n o t be propagated i n them. It a l s o f o l l o w s t h a t t h e P wave v e l o c i t y i n a s o l i d i s g r e a t e r t h a n t h e S wave v e l o c i t y .

A t h i r d wave t y p e of some importance i s t h e Rayleigh wave. It i s d e r i v e d by s p e c i f y i n g t h a t p a r t i c l e motion i s

c o n f i n e d t o t h e f r e e s u r f a c e of a s e m i - i n f i n i t e s o l i d . lZle p a r t i c l e motion i s t h a t o f a r e t r o g r a d e e l l i p s e w i t h t h e v e r t i c a l displacement a t t h e s u r f a c e e q u a l t o a p p r o x i m a t e l y

1& t i m e s t h e h o r i z o n t a l d i s p l a c e m e n t . The v e l o c i t y of t h e Rayleigh wave i s a b o u t 0.9 t h a t of t h e S wave

i n t h e same

medium ( 3 , p.31-33).

Absorption of Seismic Waves

I n p r a c t i c e t h e e a r t h imposes l i m i t a t i o n s on t h e s e i n i t i a l assumptions. m e e a r t h i s n o t p e r f e c t l y e l a s t i c ; it i s n o t homogeneous; it i s n o t i s o t r o p i c ; t h e r e i s con- s i d e r a b l e l a y e r i n g o f m a t e r i a l s w i t h d i f f e r e n t e l a s t i c p r o p e r t i e s and a l s o changes of e l a s t i c p r o p e r t i e s w i t h i n t h e same m a t e r i a l . An i m p o r t a n t f a c t o r i n s e i s m i c wave p r o p a g a t i o n i s t h e a t t e n u a t i o n of energy r e s u l t i n g from f r i c t i o n a l l o s s e s

spreading out of the wavefront. The geometric spreading

obeys the inverse square law and the absorption of energy

due to frictional losses may

be

written as a decaying

exponential. Since the amplitude is proportional to the

square root of the energy, the expression for the seismic

wave amplitude at

any

distance from the source may be written

( 4 ) :

where

A

= amplitude at distance r

= amplitude at distance r =

0

a

=

attenuation coefficient

r

= distance from source

The seismic wave amplitude decreases with distanoe

and at some point the ambient noise level and sensitivity

of the seismograph determine a limit on the source-detector

separation.

Optical

Ray

Theory

The application of boundary conditions to the elastic

wave equations in terms of particle displacements, stresses,

and strains can become exceedingly difficult. Therefore,

in dealing with problems of transmission, reflection,

and

refraction it is usual to treat elastic waves analogously

to electromagnetic waves and use optical ray theory to des-

cribe these phenomena.

Die

essential features of this

theory are outlined in Pig. 1.

In the ssismic refraction method, rays are drawn in

the direction of wave propagation perpendicular to the

wavefront (Fig.

2).

If the distance between detector and source is

varied the travel times of the wave may be measured and

then plotted to give a time-distance graph,

Fig.

3.

Knowing t h e t i m e , t h e v e l o c i t i e s , and d i a t a n o e i t i s o n l y a m a t t e r o f geometry t o o b t a i n t h e d e p t h t o t h e v2 l a y e r whicli i s g i v e n by:

I n m u l t i l a y e r problems, v e l o c i t i e s , a d d i t i o n a l t o and v2, a p p e a r and t h e d e p t h s t o and t h i c i m e s s of t h e s e l a y e r s may be c a l c u l a t e d by e x t e n d i n g t h e procedure f o r t h e t w o - l a y e r c a s e . For example i n t h e t h r e e - l a y e r problem t h e t h i c k n e s s of t h e middle l a y e r i s g i v e n by: where h 2

=

t h i c k n e s s of v2 l a y e r hl

=

t h i c k n e s s of vl l a y e r t i 2

=

v2 i n t e r c e p t time ( F i g .

3 ) .

lhis e x p r e s s i o n a p p l i e s o n l y when vl

c.

v 2 4 v

3'

If v2 i s l e s s t h a n vl, and v

₃

i s g r e a t e r t h a n vl, t h e n a c c o r d i n g t o F e r m a t r s p r i n c i p l e t h e i n t e r m e d i a t e l a y e r w i l l n o t be observed and t h e t i m e - d i s t a n c e c u r v e w i l l i n d i c a t e an a p p a r e n t t w o - l a y e r c a s e . Sometimes t h e r e f r a c t i n g l a y e r s do n o t l i e h o r i z o n t a l l y ( P i g . 4 ) . T h i s s i t u a t i o n t e n d s t o produce a n a p p a r e n t v e l o c i t y vZU t h a t i s g r e a t e r t h a n t h e a c t u a l v e l o c i t y when t h e s o u r c e i s moved "up d i p f t from t h e f i x e d d e t e c t o r

and a s m a l l e r a p p a r e n t v e l o c i t y vPd, when i t i s moved

" b o r n dip". The d i p a n g l e i s g i v e n by:

-1 -1 v1\

It may be shoru~i t h a t t h e a c t u a l v e l o c i t y o f t h e second l a y e r i s : It a l s o f o l l o w s t h a t V1 Tid C O S M

-

h d

-

_{2 c o s} ic v _T c o s d 1

i u

\

= 2 c o s ic where I n e n g i n e e r i n g a p p l i c a t i o n s it i s p o s s i b l e t o e n c o u n t e r a fill of low v e l o c i t y m a t e r i a l such a s g r a v e l o r sand a d j a c e n t t o a n a r e a o f compacted s i l t s o r c l a y s of h i g h e r v e l o c i t y . T h i s c o n d i t i o n y i e l d s a v e r t i c a l v e l o c i t y d i s c o n t i n u i t y . The t y p i c a l t i m e - d i s t a n c e p l o t f o r t h i s c a s e is s h o r n i n Pig.

5.

V e l o c i t y d i s c o n t i n u i t i e s on a s m a l l s c a l e o f t e n o c c u r n e a r t h e s u r f a c e and c o n t r i b u t e t o t h e s c a t t e r o f a r r i v a l time p l o t s on t h e t i m e - d i s t a n c e curve. The

o p e r a t o r c a n soon r e l a t e t h e s e small t i m e d i f f e r e n c e s t o t h e p e c u l i a r i t i e s o f t h e t e r r a i n . Sometimes t h e o v e r l y i n g t e r r a i n c a u s e s s u c h l a r g e t i m e d i s c r e p a n c i e s t h a t a r e l i a b l e s u b s u r f a c e p i c t u r e c a n n o t be o b t a i n e d . Numerous o t h e r s p e c i a l c a s e s t h a t i n f l u e n c e t h e t i m e - d i s t a n c e c u r v e g r e a t l y , such a s f a u l t s and e s c a r p m e n t s , may be c i t e d b u t it i s n o t t h e purpose of t h i s r e p o r t t o d w e l l upon s p e c i a l c a s e s b u t r a t h e r o n l y t o g i v e t h e r e a d e r

an i d e a o f some o f t h e problems t h a t may be encountered. For d e t a i l e d t r e a t m e n t t h e r e a d e r i s r e f e r r e d t o H e i l a n d r s work

( 5 ) .

Although a p p l i c a t i o n o f o p t i c a l ray t h e o r y t o s e i s m i c wave p r o p a g a t i o n h a s prod-uced many u s e f u l r e s u l t s i t s h o u l d be p o i n t e d o u t t h a t it f a i l s t o e x p l a i n t h e complex e l a s t i c p r o c e s s i n v o l v e d a t a boundary between two media. For

example a wave of one t y p e may be p a r t i a l l y t r a n s f o r m e d i n t o a wave of a n o t h e r t y p e . T h e r e f o r e t h e o p t i c a l r a y method of t r e a t m e n t i s o n l y a n a p p r o x i m a t i o n with c e r t a i n l i m i t a t i o n s which w i l l be s e e n l a t e r . Seismograph The r e f r a c t i o n seismograph u s e d i s e s s e n t i a l l y a t i m e measuring d e v i c e w i t h no means of o b s e r v i n g t h e vraveform of t h e d e t e c t e d energy. A s l e d g e hammer s t r u c k

on a p l a t e on t h e ground s e r v e s a s t h e s e i s m i c wave s o u r c e

and a moving c o i l t y p e t r a n s d u c e r (geophone) s e r v e s as t h e energy d e t e c t o r . The e s s e n t i a l components a r e shown i n Fig. 6. An i m p o r t a n t f e a t u r e of t h i s i n s t r u m e n t i s i t s l i g h t w e i g h t (16 l b ) making it r e a d i l y p o r t a b l e . T h i s h a s been accomplished by i n c o r p o r a t i n g t r a n s i s t o r s a n d c r y s t a l d i o d e s i n i t s c i r c u i t r y i n s t e a d of vacuum t u b e s . The c i r c u i t e l e m e n t s of t h e seismograph a r e s h o r n i n b l o c k diagram form i n Pig.

7.

O p e r a t i o n i s such t h a t a

4000-cps c r y s t a l - c o n t r o l l e d o s c i l l a t o r f e e d s i t s o u t p u t

i n t o a group of 1 0 b i n a r y c o u n t e r s whose i n p u t i s c o n t r o l l e d by t h e s w i t c h i n g of t h e g a t e c i r c u i t . The first b i n a r y

c o u n t s e v e r y o s c i l l a t o r c y c l e ; t h e second one c o u n t s e v e r y trio c y c l e s ; t h e t h i r d c o u n t s every f o u r c y c l e s and s o on.

I 1

m e t i m e s a s s o c i a t e d w i t h t h e s e a r e g , 2, 1, 2 m i l l i s e c o n d s , up t o 128 m i l l i s e c o n d s .

'Phe g a t e i s a n a m p l i f i e r which, when o p e r a t i n g a t s u f f i c i e n t l y h i g h g a i n , a l l o w s t h e o s c i l l a t o r o u t p u t t o

be f e d i n t o t h e b i n a r i e s . T h e g a i n of t h e a m p l i f i e r i s

governed by t h e g a t e c o n t r ~ l . The g a t e c o n t r o l i s a b i s t a b l e m u l t i v i b r a t o r t h a t e i t h e r a p p l i e s a b i a s t o t h e g a t e o r

During o p e r a t i o n , when t h e hammer i s s t r u c k on t h e ground it t r i g g e r s t h e g a t e c o n t r o l t o i n c r e a s e t h e g a i n of t h e g a t e a m p l i f i e r which a l l o w s t h e b i n a r i e s t o b e g i n c o u n t i n g , ?The s e i s m i c wave g e n e r a t e d by t h e hammer blow t r a v e l s t h r o u g h t h e ground, i s d e t e c t e d by t h e geophone and a m p l i f i e d . The a m p l i f i e d o u t p u t i s f e d i n t o t h e g a t e c o n t r o l where it a g a i n t r i g g e r s t h e c o n t r o l ; t h i s t i m e i t p r o v i d e s a h i g h g a t e b i a s t o s t o p t h e c o u n t i n g . !The t i m e between impact and d e t e c t i o n

i s r e a d d i r e c t l y from neon i n d i c a t o r l i g h t s i n t h e b i n a r y

c i r c u i t .

111. SEIShIIC OBSERVATIONS AI(ID DISCUSSIOIT OF RESULTS P r e v i o u s mork

Most o f t h e s e i s m i c e x p l o r a t i o n c a r r i e d o u t i n perma- f r o s t a r e a s h a s been i n c o n n e c t i o n w i t h t h e s e a r c h f o r o i l . The e f f e c t o f p e r m a f r o s t on t h e r e f r a c t i o n method h a s been i n c i d e n t a l t o t h e main problem. Then t o o , t h e u s e of t h e seismograph a s a n e n g i n e e r i n g t o o l f o r shallom g e o l o g i c a l i n v e s t i g a t i o n s h a s o n l y been o u r s o r i l y e x p l o r e d , A s a r e s u l t , l i t e r a t u r e on t h e a p p l i c a t i o n of s e i s m i c t e c h n i q u e s t o t h e mapping of p e r m a f r o s t regime i s v i r t u a l l y n o n e x i s t e n t . P a r t of t h e r e a s o n i n f a i l i n g t o i n v e s t i g a t e t h e e n g i n e e r i n g a s p e c t may be a t t r i b u t e d t o t h e l i m i t a t i o n s of t h e i n s t x ~ l m e n t s . I n t h e p a s t , seismographs have been b u l k y and d i f f i c u l t t o t r a n s p o r t . With r e c e n t advances i n e l e c - t r o n i c t e c h n o l o g y , s i m p l e , l i g h t , r e a d i l y p o r t a b l e machines have b e e n developed, This f e a t u r e , t o g e t h e r w i t h i n c r e a s e d

s e n s i t i v i t y , a l l o w i n g t h e u s e of a hammer blow i n s t e a d o f a n e x p l o s i v e c h a r g e a s a s e i s m i c wave s o u r c e , h a s g i v e n m e r i t t o t h e c o n s i d e r a t i o n of t h e r e f r a c t i o n seismograph a s an e n g i n e e r i n g t o o l .

A primary s t e p i n a p p l y i n g t h i s method was t o accumu- l a t e v e l o c i t y d a t a f o r t h e n e a r s u r f a c e l a y e r s i n o r d e r t o i d e n t i f y r e f r a c t o r v e l o c i t i e s w i t h e a r t h l a y e r s . Table I

g i v e s v e l o c i t i e s of e a r t h m a t e r i a l s o f some e n g i n e e r i n g i n t e r e s t .

TABLE I

VELOCITIES OF SOME EARm MATERIALS

It i s r e a d i l y s e e n t h a t e a r t h m a t e r i a l s a r e c h a r a c - M a t e r i a l Weathered s u r f a c e l a y e r Sands and g r a v e l Alluvium Clay S h a l e Sandstone Lime s t o n e G r a n i t e a n d i g n e o u s r o c k s Water I c e t e r i z e d by c e r t a i n . v e l o c i t y d i s t r i b u t i o n s . It i s a l s o s e e n L o n g i t u d i n a l Wave V e l o c i t y ( f t / s e c ) 500

-

1500 1100

-

3200 1640

-

6600 3200

-

9200 7500

-

1 5 , 4 0 0 4600

-

1 4 , 1 0 0 1 3 , 5 0 0

-

20,000 1 3 , 1 0 0

-

20,400 4900 1 2 , 6 0 0 * t h a t t h e s e d i s t r i b u t i o n s a r e , i n g e n e r a l , q u i t e wide. mow- l e d g e i s t h e r e f o r e r e q u i r e d of t h e v e l o c i t y c h a r a c t e r i s t i c s o f a p a r t i c u l a r a r e a t o o b t a i n optimum r e s u l t s from t h e method. A q u e s t i o n o f fundamental i n t e r e s t r e g a r d i n g t h e a p p l i c a b i l i t y o f t h e s e i s m i c r e f r a c t i o n method t o t h e d e t e c - t i o n o f p e r m a f r o s t w a s t h a t of t h e v e l o c i t y c o n t r a s t between f r o z e n and u n f r o z e n m a t e r i a l . Some g e n e r a l l i m i t s on

v e l o c i t i e s i n p e r m a f r o s t a r e a s have been compiled by J o e s t i n g ( 6 ) , Table 11.

TABLE I1

LONGITUDIlJAL WAVE VELOCITIES I N PE;RIfiFROST AREAS ( a f t e r J o e s t i n g )

L a b o r a t o r y V e l o c i t y Measurements

I n view of t h e wide l i m i t s on t h e v e l o c i t i e s of some of t h e m a t e r i a l s it was d e c i d e d t o conduct l a b o r a t o r y measure- ments of s e i s m i c wave v e l o c i t i e s t h r o u g h f r o z e n and thawed m a t e r i a l s u n d e r c o n d i t i o n s of v a r y i n g com,paction, m o i s t u r e c o n t e n t , a n d t e m p e r a t u r e . The r e s u l t s a r e g i v e n i n Table 111. M a t e r i a l L i g h t mack Thawed muck Frozen muck lhawed g r a v e l Frozen g r a v e l Bedrock ( s c h i s t ) L TABLE 111

EFFECT

OF FREEZING OK LONGITUDINAL ViAVE VELOCITIES L o n g i t u d i n a l Wave V e l o c i t y ( f t / s e c ) 1000

-

1650 1800

-

4000 4250

-

1 0 , 0 0 0 2000

-

1 0 , 0 0 0 1 3 , 0 0 0

-

1 5 , 2 5 0 7500

-

20,000 X a t e r i a l S i l t - s a n d - o r g a n i c mLxture l o o s e and d r y s l u r r y Conpacted c l a y Longi- t u d i n a l wave v e l o c i t y a t 70°F ( f t / s e c ) 3000 4000 4000

-

4400 Longi- t u d i n a l wave v e l o c i t y a t -5OF ( f t / s e c ) P(,. 3800 1 3 , 2 0 0 8700

-

9700

-

Longi

-

t u d i n a l wave v e l o c i t y a f t e r 50 m i n u t e s o f thawing 3800 11, GOO 7600

It may be s e e n t h a t t h e m o i s t u r e c o n t e n t of t h e m a t e r i a l h a s a marked e f f e c t on t h e s e i s m i c mave v e l o c i t i e s of f r o z e n m a t e r i a l . Although c o n t r o l of t h e t e m p e r a t u r e was n o t maintained d u r i n g t h e course of f r e e z i n g , it was observed t h a t t h e wave v e l o c i t y decreased when t h e f r o z e n samples were s u b j e c t e d t o thawing t e m p e r a t u r e s . I t i s n o t p o s s i b l e t o s t a t e c o n c l u s i v e l y whether t h i s was due t o change of t h e e l a s t i c p r o p e r t i e s w i t h temperature of t h e f r o z e n m a t e r i a l o r due t o thawing of t h e sample a l o n g i t s p e r i p h e r y . During t h e 50-ninute thawing p e r i o d o n l y a l i t t l e of t h e o u t e r

m a t e r i a l melted s o i t a p p e a r s l i k e l y t h a t t h e temperature increas; of t h e sample had a n e f f e c t i n reducing t h e e l a s t i c mave v e l o c i t y through it.

An e x p l a n a t i o n of t h e dependence of t h e s e i s m i c wave

v e l o c i t i e s on temperature can be deduced based upon t h e experimental work of Demenitskaia and Tsytovich (7,8),

F i r s t it i s n o t e d t h a t t h e P wave v e l o c i t y i s given

where K = bulk modulus

A = r i g i d i t y modulus

(3 = d e n s i t y

Since permafrost m a t e r i a l s f r e q u e n t l y have a h i g h w a t e r c o n t e n t , t h e t r a n s m i s s i o n of a l o n g i t u d i n a l wave through them i s a f u n c t i o n of t h e e l a s t i c p r o p e r t i e s of b o t h t h e s o i l g r a i n s and t h e i n t e r s t i t i a l w a t e r . An unfrozen s o i l of h i g h w a t e r c o n t e n t w i l l behave e s s e n t i a l l y a s a l i q u i d . I t w i l l e x h i b i t no s h e a r s t r e n g t h ( A = 0 ) and t h e v e l o c i t y i s simply,

I'fiicr, t h e same m a t e r i a l i s f r o z e n / assumes a f i n i t e v a l u e , t h e d e n s i t y d e c r e a s e s by a small amount and t h e

v e l o c i t y i s i n c r e a s e d . Hov~ever, a l l t h e w a t e r does n o t f r e e z e at once and hence t h e r e i s a dependence of _A on temperature ( 9 ) . Dernenitskaia o b t a i n s an a c t u a l e x p r e s s i o n f o r t h e r e l a t i o n of Young's modulus t o temperature under f r e e z i n g .

P r e l i m i n a r y F i e l d T r i a l s

The seismograph was found r e l a t i v e l y simple t o

o p e r a t e . A gain c o n t r o l on t h e i n s t r u m e n t allowed t h e o b s e r v e r

t o a d j u s t t h e s e n s i t i v i t y of t h e d e t e c t o r a m p l i f i e r c i r c u i t t o compensate f o r reduced s i g n a l s t r e n g t h a s t h e source- d e t e c t o r d i s t a n c e was i n c r e a s e d . Ground and d e t e c t o r motion due t o moving v e h i c l e s , a i r p l a n e s , machinery, and wind

p l a c e d a l i m i t on t h e maximum g a i n l e v e l t h a t could be used i n any a r e a a t any time. This, of course, had a d i r e c t i n f l u e n c e on t h e depth range of t h e method.

Becaude of i t s c o n s t r u c t i o n t h e geophone was respon- s i v e t o t h e v e l o c i t y of t h e ground motion. Ground motion v e l o c i t y f o r t h e d e s i r e d f i r s t a r r i v a l i s u s u a l l y of t h e

waveform shobwl i n F i g . 8 and t h e i n p u t v o l t a g e t o t h e d e t e c t o r a m p l i f i e r i s , t h e r e f o r e , of t h e same form.

If t h e a n p l i t u d e of t h e i n i t i a l v o l t a g e p u l s e i s i n s u f f i c i e n t t o t r i g g e r t h e g a t e c o n t r o l b u t t h e amplitude of t h e succeeding p u l s e i s s u f f i c i e n t t h e n t h e b i n a r i e s w i l l i n d i c a t e a time l a g . A s l o n g as t h e o p e r a t o r i s aware t h a t

t h i s s i t u a t i o n i s occurring t h e n he can compensate f o r t h e e f f e c t during i n t e r p r e t a t i o n of t h e r e s u l t s . I n t h e f i e l d t r i a l s t i m e - l e g i n t e r v a l i n g e n e r a l remained n e a r l y c o n s t a n t ; v a r i a t i o n s t h a t did occur were probably due t o s e l e c t i v e

Sometimes t h e f i r s t a r r i v a l p u l s e was n o t observed a t a l l and lower v e l o c i t y waves, probably Rayleigh waves,

were n o t i c e d . These of course were u s e l e s s t o t h e a p p l i c a t i o n of t h e s e i s m i c method and were t h e r e f o r e d i s c a r d e d . T h e i r presence was u s u a l l y i n d i c a t e d by abnormally low v e l o c i t i e s on t h e t i m e - d i s t a n c e graph a t l a r g e s o u r c e - d e t e c t o r d i s t a n c e s .

I n t h e a c t u a l s e i s m i c f i e l d t e s t s , c o n d i t i o n s v a r i e d from good t o poor and a n example of a s e i s m i c survey conducted under optimum c o n d i t i o n s w i l l be d i s c u s s e d f i r s t . This survey

was c a r r i e d o u t on t h e campus of t h e U n i v e r s i t y of Western Ontario i n London.

The v e l o c i t i e s of t h e d i f f e r e n t e a r t h l a y e r s a r e shown by t h e s l o p e s of t h e t i m e - d i s t a n c e graph ( P i g . 9).

Reduction of t h e observed r e c o r d s y i e l d e d t h e s e i s m i c s e c t i o n shomm

i n Fig. 10.

Since a d r i l l c o r e was o b t a i n e d o n l y f o r t h e f i r s t 4 f e e t

it was n o t p o s s i b l e t o i d e n t i f y any of t h e

e a r t h l a y e r s a s s o c i a t e d w i t h t h e observed v e l o c i t i e s , except f o r t h e first two v e l o c i t i e s . These were i d e n t i f i e d a s :

S i l t - s a n d - o r g a n i c mixture

-

1200 f t p e r s e c Brown c l a y

-

3600 f t p e r sec.

The d i f f e r e n c e of 8 i n . between t h e d r i l l c o r e and s e i s m i c l o g i s n o t of any p a r t i c u l a r s i g n i f i c a n c e because t h e topographic v a r i a t i o n s were even more t h a n t h i s and t h e

depth c a l c u l a t i o n s were c a r r i e d o u t only t o t h e n e a r e s t f o o t . I t i s n o t e d , however, t h a t t h e c a l c u l a t e d r e f r a c t o r i s deeper t h a n t h e brown c l a y - g r a v e l i n t e r f a c e .

The r e s u l t s of t h e work a t a sewer s i t e e a s t of Ottawa

d i d n o t produce r e s u l t s a s w e l l d e f i n e d a s t h e example j u s t mentioned. The s u r f a c e t e r r a i n was much more i r r e g u l a r , t h e

s e i s m i c energy was r a p i d l y a t t e n u a t e d , and t h e a r e a w a s n o i s y . I n a s e c t i o n of h e a v i l y v e g e t a t e d , peat-covered ground, it

w a s n e c e s s a r y t o remove t h e o r g a n i c cover s o t h a t t h e p l a t e could make c o n t a c t w i t h t h e mineralm s o i l and t h u s produce s e i s m i c waves of s u f f i c i e n t l y h i g h i n t e n s i t y .

I n g e n e r a l t h r e e d i f f e r e n t v e l o c i t i e s were observed i n t h i s area:

Tne t h i c k n e s s of vl was i n t h e o r d e r of 4 t o 6 f t . !&is l a y e r mas thought t o r e p r e s e n t t h e a e r a t e d weathered zone. The

t h i c k n e s s of v v a r i e d between 50 and 70 f t and i t i s reason-

2

a b l e t o assume t h a t t h i s v e l o c i t y r e p r e s e n t e d t h s s t i f f c l a y l a y e r s o v e r l y i n g g l a c i a l till. I n some i n s t a n c e s d i f f e r e n t

v3 v e l o c i t i e s were o b t a i n e d i n r e v e r s e s h o t s t h a t could n o t ,

be r e c o n c i l e d on t h e b a s i s of d i p p i n g beds. These were a t t r i - buted t o d i f f e r e n t v r e f r a c t o r s . I n most c a s e s bedrock w a s

3

n o t i n d i c a t e d on t h e t r a v e l - t i m e curve probably because t h e s o u r c e - d e t e c t o r s e p a r a t i o n was l i m i t e d by t h e source energy. I n o n l y one c a s e was bedrock evidenced (Pig, 11). I n t h i s c a s e , t h e i n s t r u m e n t was l a i d o u t over a road of compacted g r a v e l and t h e 18,000 f t / s e c . v e l o c i t y corresponds w i t h t h e expected v e l o c i t y of t h e limestone bedrock. The c a l c u l a t e d depth of 90 f t compares w i t h a d r i l l l o g depth of 88 f t .

Velocity measuremento were a l s o made i n two a r e a s of exposed r o c k s and i t was expected t o o b t a i n a c o n s t a n t

v e l o c i t y c h a r a c t e r i s t i c of t h e rock. Observations i n d i c a t e d , however, t h a t t h e r e were a c t u a l l y two v e l o c i t y l a y e r s a s

shown i n Table I V .

OUTCROP SHALE VELOCITIES

1~4aterial S h a l e , S t . l j u r e n t Blvd. Snale, Hogsback v1 4200 f t / s e c 4000 f t / s e c v ₂ 13,800 f t / s e c 11,500 f t / s e c

F r a c t u r i n g and e r o s i o n probaSly caused t h e lower v e l o c i t y i n both c a s e s ,

Rn

_{a r e a of some i n t e r e s t was t h e Green Creek l a n d s l i d e .} This f e a t u r e has been under a c t i v e i n v e s t i g a t i o n by t h e

D i v i s i o n and a s e i s m i c survey was c a r r i e d o u t o v e r a p o r t i o n of t h e a r e a i n an a t t e m p t t o determine depth t o bedrock, Bedrock was n o t d e t e c t e d during t h e t e s t s b u t t h e s e i s m i c r e c o r d s make it p o s s i b l e t o s a y t h a t bedrock i s g r e a t e r t h a n

95

f t

i n

depth.

Such knomledge i s u s e f u l f o r c e r t a i n e n g i n e e r i n g purposes such a s e s t i m a t i n g t h e c o s t of e x c a v a t i o n s , sewer and u t i l i t y d i t c h e s when i t i s a c t u a l l y n o t n e c e s s a r y t o knori t h e depth t o rocl: b u t o n l y whether rock o c c u r s w i t h i n a l i m i t e d depth benea-tin t h e s u r f a c e .

Nonn~n Wells.- The work a t t h e Norman Wells a i r s t r i p i n d i c a t e d

a permafrost v e l o c i t y i n t h e o r d e r of 9000 f t / s e c . !This con- t r a s t e d w i t h a s u r f a c e l a y e r v e l o c i t y of 1200 f t / s e c . !The a i r p o r t p r o f i l e sl~o~ved a r e f r a c t o r t h a t w a s i n g e n e r a l some

3 f t deeper t h a n t h e t o p of t h e f r o s t l e v e l a s shown by a u g e r b o r i n g s , The seisinic d e p t h s were c o n s i s t e n t w i t h i n t h e m s e l v e s , however, n o t v a r y i n g more t h a n 0.5 f t .

One e x p l a n a t i o n f o r t h i s discrepancy may be t h e i n a b i l i t y of o p t i c a l r a y t h e o r y t o r e p r e s e n t a c c u r a t e l y e l a s t i c wave propagation between media of two d i f f e r e n t v e l o c i t i e s , p a r t i c u l a r l y mhere t h e r e f r a c t i n g l a y e r s a r e s m a l l conpared t o t h e wavelength of t h e s e i s m i c waves (11). A second c o n s i d e r a t i o n may be t h e e f f e c t on wave v e l o c i t y of v a r i a b l e i c e l e n s i n g

i n t h e permafrost zo3e.

This may pro- duce a c o n d i t i o n s i m i l a r t o v e l o c i t y a n i s o t r o p y i n r o c k s which h a s been d i s c u s s e d by Uhrig and Van Nelle ( 1 2 ) 1955 and by Postma (13) 1955. A t h i r d p o s s i b i l i t y i s t h e presence of a

v e l o c i t y t r a n s i t i o n . zone i n t h e u p p e r l a y e r o f p e r m a f r o s t r e s u l t i n g from a g r a d u a l change i n t h e p h y s i c a l p r o p e r t i e s of t h e s o i l from a thawed t o a f r o z e n condition.. If t h e t r a n s i t i o n zone i s s u c h t h a t i t does n o t r e f r a c t a d e q u a t e energy t o t h e s e i s m i c d e t e c t o r , t h e n i t s p r e s e n c e w i l l n o t b e i n d i c a t e d by t h e t i m e - d i s t a n c e c u r v e and t h e wave w i l l b e r e f r a c t e d a t some d e e p e r d e p t h w i t h i n t h e p e r m a f r o s t .

The d i s c r e p a n c y i n t h e boundary between p e r m a f r o s t a n d thawed ground, however, does n o t r e n d e r t h e method

u s e l e s s . An a d j u s t m e n t can b e made on an e m p i r i c a l b a s i s t o r e c o n c i l e t h e s e i s m i c r e f r a c t o r w i t h t h e p e r m a f r o s t t a b l e .

A few a u g e r b o r i n g s o r d r i l l h o l e s p l a c e d a t a p p r o p r i a t e l o c a t i o n s c o u l d p r o v i d e s u i t a b l e c o n t r o l f o r t h e r e f r a c t i o n d a t a .

T e s t s made at Norman Wells a t v e r y s h a l l o w d e p t h s u n d e r c o n d i t i o n s of an i r r e g u l a r p e r m a f r o s t t a b l e produced

a t i m e - d i s t a n c e g r a p h which e x a g g e r a t e d t h e i r r e g u l a r i t y ( P i g . 1 2 ) . T h i s w a s an extreme c a s e of t h e u s e of t h e

seismograph f o r t h e d e t e c t i o n of p e r m a f r o s t b e c a u s e t h e a c t i v e l a y e r was s o shallow. S i m i l a r t e s t s were made n e a r b u i l d i n g s and o t h e r a r e a s t o determine t h e l o c a l p e r m a f r o s t t a b l e .

lZlese o n l y confirmed t h e e f f e c t n o t i c e d i n P i g . 12. A l - though it i s d i f f i c u l t t o o b t a i n q u a n t i t a t i v e d e p t h v a l u e s

i n t h e s e i n s t a n c e s , a q u a l i t a t i v e p i c t u r e i s g i v e n .

P o r t Simpson.

-

A t F o r t Simpson where t h e p e r m a f r o s t w a s found t o be h i g h l y v a r i a b l e , a c a r e f u l a u g e r b o r i n g program w a s

m a i n t a i n e d i n c o n j u n c t i o n w i t h t h e s e i s m i c r e f r a c t i o n o b s e r - v a t i o n s . P o r t Simpson i s l o c a t e d on a n i s l a n d 2.; m i l e s

l o n g by 2/'3 m i l e wide a t t n e c o n f l u e n c e of t h e l b c k e n z i e and

L i a r d R i v e r s . The i s l a n d h a s b e e n formed by a l l u v i a l d e p o s i - t i o n and i s composed of l a y e r s of c l a y , s i l t , sand, a n d

g r a v e l randomly a r r a n g e d . Consequently, a wide r a n g e o f v e l o c i t i e s w a s e n c o u n t e r e d a p p a r e n t l y due t o t h e e l a s t i c

c h a r a c t e r i s t i c s of t h e i n d i v i d u a l r e f r a c t i n g l a y e r s , A t

t i m e s this caused some ambiguity a s t o which v e l o c i t y should be a s c r i b e d t o t h e p e r m a f r o s t r e f r a c t o r . The u s e of r e v e r s e s p r e a d s , however, removed t h e ambiguity i n . a l m o s t e v e r y c a s e , Probably t h e most a r e a >.;as i n t h e west p a r t of t h e i s l a n d where t h e v e l o c i t y of water-satuxaated s i l t y f i n e sand

(7000 f t / s e c ) w a s of t h e same o r d e r a s t h e v e l o c i t y a t t r i b u t e d

t o permafrost. Then such a s i t u a t i o n is encountered it i s

e s t r e m e l y d i f f i c u l t t o d i f f e r e n t i a t e between t h e two.

A s e i s m i c p r o f i l e a c r o s s t h e i s l a n d i s g i v e n i n Fig. 1 3 a l o n g w i t h a u g e r b o r i n g s and d r i l l c o r e r e s u l t s , I t i s e v i - d e n t from t h e r e s u l t s t h a t t h e seismograph was a b l e t o f u r -

n i s h a r e a s o n a b l y a c c u r a t e p i c t u r e of t h e p e r m a f r o s t t a b l e i n t h i s a r e a , a l t h o u g h t h e r e l i a b i l i t y of t h e s e i s m i c t e c h - nique became l e s s w i t h d e c r e a s i ~ g depth t o p e r m a f r o s t . The p o s s i b l e r e a s o n s f o r t h i s a r e t h e same a s t h o s e which have been s u g g e s t e d f o r t h e Norman Wells r e s u l t s . P a r t i c u l a r n o t i c e should be t a k e n of t h e i n s t a n c e s when t h e d r i l l d i d

n o t c o n t a c t f r o s t because of b e i n g stopped by g r a v e l . D r i l l i n g t h r o u g h g r a v e l i s a slow and c o s t l y procedure w i t h t h e r e s u l t t h a t depth t o permafrost i n t'nose 1ocat;ions could n o t be

determined by t h i s means. I n such c a s e s , t h e use of t h e s e i s m i c method h a s obvious advantages.

An example of t h e d e t e c t i o n of a small d i s c o n t i n u i t y i n t h e permafrost by t'ne seismograph i s w e l l i l l u s t r a t e d by t h e work a t P o r t Simpson. A l i n e w a s

run

a c r o s s a d i s t a n c e of 110 f t from a base l i n e t o t i e i n t o a d r i l l h o l e . A

discrepancy

on

t h e t i m e - d i s t a n c e curve showed a p a r t i c u l a r nonconformity t h a t extended some 30 f t i n l a t e r a l e x t e n t t h a t could o n l y be e x p l a i n e d by t h e presence of a n e s c a r p - ment i n t h e p e r m a f r o s t t a b l e o r a pocket o f low v e l o c i t y m a t e r i a l , Borings made on t h e b a s i s of t h e s e i s m i c evidence

shoned a l e a k y c e s s p o o l which covered t h e a r e a i n q u e s t i o n . Knowledge of any such s u b s u r f a c e s t r u c t u r e s would be v a l u a b l e t o a s i t e p l a n n e r .

1nuvik.- The t e s t s i n a g r a v e l p i t a t I n u v i k d e t e c t e d perma- f r o s t a t a s h a l l o w d e p t h i n s p i t e of t h e i r r e g u l a r s u r f a c e t e r r a i n . The v a l u e of t h i s means of d e t e c t i n g p e r m a f r o s t i n an a r e a of s h a l l o w d e p t h may b e q u e s t i o n e d from t h e s t a n d p o i n t of e f f i c i e n c y and it may b e j u s t a s e x p e d i e n t t o d i g t e s t p i t s i n t h e s e c i r c u m s t a n c e s . However, w i t h i n c r e a s i n g d e p t h of thaw it would be more a d v a n t a g e o u s t o u t i l i z e t h e s e i s m i c method t h a n t o d i g p i t s . I n v e s t i g a t i o n s o v e r pads of l o o s e g r a v e l showed h i g h a t t e n u a t i o n of s e i s m i c energy. a?is i s c h a r a c t e r i s t i c of u n c o n s o l i d a t e d m a t e r i a l s and o f t e n i s a l i m i t i n g f a c t o r i n t h e u s e of t h e method. Compacted r o c k f i l l a t t h e a i r p o r t runway was r e l a t i v e l y e f f i c i e n t i n t r a n s m i t t i n g s e i s m i c energy. A v e l o c i t y o f a p p r o x i m a t e l y 2000 f t / s e c was n o t i c e d f o r the compacted m a t e r i a l as c o n t r a s t e d w i t h a n a p p r o x i m a t e v e l o c i t y of 1200 f t / s e c for. l o o s e g r a v e l . A r e f r a c t o r w i t h a v e l o c i t y c o r r e s p o n d i n g t o t h a t o f p e r m a f r o s t was r e a d i l y i d e n t i f i e d a t a d e p t h o f

13

f t . T h i s compares t o a d e p t h o f 10.5 f t t o t h e t o p o f p e r m a f r o s t a t t h a t l o c a t i o n a s g i v e n by b o r e h o l e t e m p e r a t u r e r e a d i n g s .

Ground f i s s u r e s which were common i n t h e i n v e s t i g a t e d s i t e s a t I n u v i k produced a marked d e l a y i n t r a v e l t i m e s .

In a h e a v i l y eroded embankment, f o r example, t h e t i m e r e a d i n g s were s o e r r a t i c t h a t t h e r e f r a c t i n g l a y e r c o u l d n o t b e mapped.

I V . RESISTIVITY METH-OD G e n e r a l

The r e s i s t i v i t y method of p r o s p e c t i n g i s b a s e d upon t h e e l e c t r i c a l p r o p e r t i e s o f t h e e a r t h . The t h e o r y i s o u t l i n e d b r i e f l y using t h e f o u r e l e c t r o d e Vienner c o n f i g u r a t i o n a s an

example ( F i g . 1 4 ) .

If t h e e a r t h i s composed of l a y e r s w i t h d i f f e r e n t r e s i s t i v i t i e s t h e n t h e o b s e r v e d r e s i s t i v i t y w i l l d e p a r t from

t h i s t h e o r e t i c a l expression. The d e p a r t u r e of t h e t h e o r e t i c a l r e s i s t i v i t y from t n e observed r e s i s t i v i t y i s dependent upon t h e d i f f e r e n c e i n t h e r e s i s t i v i t y of t h e second l a y e r and

i t s depth from t h e f i r s t l a y e r .

The ,Tagg method of i n t e r p r e t a t i o n r e q u i r e s a n i n d i r e c t com2arison of t h e observed r e s u l t s w i t h a f a m i l y of c u r v e s r e p r e s e n t i n g d i f f e r e n t r e s i s t i v i t y r a t i o s and d e p t h s . The method i s s l o ~ and t e d i o u s and w i l l n o t be d e s c r i b e d h e r e .

A complete d e s c r i p t i o n of t h e method i s g i v e n by Heiland

( 5 ) .

There a r e o t h e r methods of i n t e r p r e t a t i o n t h a t u t i l i z e empiri- c a l curve comparisons. m e y a r e u s u a l l y r a p i d b u t r e q u i r e supplementary i n f o r m a t i o n t o c o r r e l a t e t h e c u r v e s w i t h depths.

iChe Tagg method i s r a r e l y at-t;empted f o r a n y t h i n g more complex t h a n a two-layer case. I n f a c t t h e r e s i s t i v i t y

method i s u s u a l l y c o n f i n e d t o mapping a s i n g l e h o r i z o n o r i n d e t e c t i n g l a t e r a l v a r i a t i o n s i n r e s i s t i v i t y .

R e s i s t i v i t y Apparztus

me f o u r - p o i n t Wenner c o n f i g u r a t i o n w a s used i n a l l c a s e s . Figure

15

shov~s t h e a p p a r a t u s and t h e f i e l d l a y o u t .

The r e s i s t a n c e , R, may be r e a d d i r e c t l y on t h e i n s t r u m e n t and knowing t h e e l e c t r o d e s e p a r a t i o n , a , t h e a p p a r e n t r e s i s t i v i t y may be c a l c u l a t e d from:

R e s i s t a n c e measurement i s accomplished by means of a n a.c. impedance b r i d g e . This i s a n u l l method and does n o t draw any c u r r e n t i n t h e measuring galvanometer c i r c u i t . A

v i b r a t o r c o n v e r t s t h e d.c. o u t p u t from t h e b a t t e r y power

pack i n t o a l t e r n a t i n g c u r r e n t which i s n e c e s s a r y t o e l i m i n a t e p o l a r i z a t i o n e f f e c t s . The weight of t h e i n s t r u m e n t i s

approxinlately 20 l b n o t i n c l u d i n g c a b l e s and probes which weigh 1 0 l b . A survey may be c a r r i e d o u t by one man w i t h o u t

Discussion of R e s i s t i v i t y R e s u l t s

!he r e s i s t i v i t y a p p a r a t u s was f i r s t t e s t e d i n t h e O t t a w a a r e a u s i n g t h e Four-electrode Wenner c o n f i g u r a t i o n . I n t e r p r e t a t i o n was based on t h e Tagg curve method of s o l u t i o n

(14) and t h e r e s u l t s i n d i c a t e d a r e s i s t i v i t y change a t a depth of 4 f t which corresponded t o t h e n e a r s u r f a c e l a y e r observed s e i s m i c a l l y and by d r i l l i n g , The average s u r f a c e r e s i s t i v i t y of t h e s o i l w a s 1.7

x

lo3

ohm-cm.

Probe s e p a r a t i o n d i s t a n c e s up t o 30 f t were used. Lack of s e n s i t i v i t y on t h e p a r t of t h e i n s t r u m e n t prevented g r e a t e r probe s e p a r a t i o n s . These p r e l i m i n a r y t e s t s suggested t h a t t h i s p a r t i c u l a r i n s t r u m e n t was more s u i t a b l e f o r shallow depth d e t e r m i n a t i o n s t h a n f o r deeper ones.

A t t h e Norman Wells a i r p o r t t h e e l e c t r i c a l charac- t e r i s t i c s of t h e ground were such t h a t t h e r e s u l t s i n d i c a t e d

a

g e n e r a l i n c r e a s e i n r e s i s t i v i t y of t h e s o i l extending

d o m from t h e s u r f a c e (Pig. 1 6 ) . The a i r p o r t i s on a r i d g e t h a t i s w e l l ' d r a i n e d and t h i s probably caused t h e m o i s t u r e c o n t e n t of t h e s o i l t o be g r e a t e s t n e a r t h e s u r f a c e and t o decrease w i t h depth. This s i t u a t i o n could account f o r t h e

i n c r e a s e of r e s i s t i v i t y w i t h depth. The i n v e s t i g a t i o n s i n c l u d e d measurements of some r e s i s t i v i t i e s of t h e a c t i v e l a y e r and t h e

permafrost. These a r e given i n Table V and show a wide v a r i a - t i o n i n v a l u e s . The s i g n i f i c a n t f a c t o r , however, i s t h e much g r e a t e r r e s i s t i v i t y of f r o z e n s o i l s a s compared w i t h unfrozen ones.

W L E V

MEASURE3 ilESISTIVITY OF

SONE EARTH FIIATERIALS

M a t e r i a l m R e s i s t i v i t y ohm-cm Wet g r e y s i l t y c l a y , Brown s i l t Permafrost ( s i l t ) 6.1

x

l o 3

3.1

x

l o 3

8 . 2

lo4

With t h e a p p r e c i a b l e c o n t r a s t i n r e s i s t i v i t y between f r o z e n a n d u n f r o z e n m a t e r i a l s t h i s method s h o u l d t h e r e f o r e be e f f e c t i v e iz d e t e c t i n g p e r m a f r o s t . A s was p o i n t e d o u t i n t h e t h e o r y , however, t h e c o n d i t i o n s become c o m p l i c a t e d when t h e a n a l y s i s d e p a r t s from t h e c a s e of l a y e r s of c l e a r l y d e f i n e d r e s i s t i v i t y .

The F o r t Simpson work y i e l d e d more p o s i t i v e r e s u l t s ( F i g . 1 7 ) , t h a n t h e work a t Norman Wells. Once a g a i n t h e

Tagg c u r v e method of i n t e r p r e t a t i o n w a s used. An a p p r e c i a b l e v a r i a t i o n of s u r f a c e r e s i s t i v i t y c r e a t e d some d i f f i c u l t y i n i n t e r p r e t a t i o n . Because t h e Tagg method i s dependent upon t h e r a t i o of r e s i s t i v i t i e s a d e g r e e of u n c e r t a i n t y i s i n t r o - duced i n t o d e p t h d e t e r m i n a t i o n s . Skipp a n d G r i f f i t h s

( 1 5 )

e n c o u n t e r e d t h e same problem and m o d i f i e d t h e Tagg method i n t h e i r i n t e r p r e t i n g procedure.

In

some a r e a s t h e r e s i s t i v i t y method r e a d i l y d e t e c t s t h e p e r m a f r o s t t a b l e . The Tagg c u r v e i n t e r p r e t a t i o n method i s t e d i o u s b u t w i t h t h e accumulation of s u f f i c i e n t r e c o r d s from a s p e c i f i c a r e a it i s p o s s i b l e t h a t a l e s s time-consuming

c u r v e - f i t t i n g method c o u l d be a p p l i e d .

T h e o r e t i c a l l y , t h e r e s i s t i v i t y method i s more a c c u r a t e

a t s h a l l o w e r d e p t h s . This was confirmed by experiment.

The e l e c t r i c a l r e s i s t i v i t y method however, s u f f e r s from

l a t e r a l v a r i a t i o n s o f s u r f a c e s o i l r e s i s t i v i t y and problems

.

more complex t h a n t h e t w o - l a y e r c a s e become e x c e e d i n g l y

d i f f i c u l t t o i n t e r p r e t w i t h c o n f i d e n c e .

It i s s u g g e s t e d t h a t t h i s method c o u l d be a p p l i e d t o t h e d e t e c t i o n o f d e p t h of thaw i n a permanent i n s t a l - l a t i o n . 'Ibis c o u l d be accornplished by p l a c i n g a s e r i e s o f p r o b e s i n a f i x e d pat-tern whose s p a c i n g c o r r e s p o n d e d t o t h a t used f o r a r e s i s t i v i t y d e p t h s u r v e y . These would remain i n

p o s i t i o n f o r one s e a s o n a n d measurements would b e made p e r i o d i c a l l y . Local e f f e c t s c o u l d be e l i m i n a t e d w i t h t h i s p r o c e d u r e . The a c c u r a c y o f t h i s method i s n o t known b u t i t

would c e r t a i n l y m e r i t i n v e s t i g a t i o n . This method could be used under b u i l d i n g s where i t rrould be d i f f i c u l t t o measure thaw d e p t h s by any o t h e r means. The metnod c i t e d does n o t d i s t u r b t h e s u b s u r f a c e and a s such i s a n o n d e s t r u c t i v e method of e x p l o r a t i o n .

V. PRACTICAL COITSIDERATIONS I N GEOPHYSICAL METHODS

The p o r t a b l e seisniograph r e q u i r e s two men f o r f i e l d o p e r a t i o n , a n o b s e r v e r , and a man t o use t h e s l e d g e hammer. N e i t h e r man need have a p r o f e s s i o n a l background and a

competent o p e r a t o r could be t r a i n e d w i t h i n a m a t t e r of days. The e f f e c t i v e n e s s o f t h e seisrnograph a s a n e n g i n e e r i n g a i d i s r e l a t e d t o t h e experience of t h e i n t e r p r e t e r .

Under average c o n d i t i o n s , between 1000 and 1500 f t of p r o f i l e l e n g t h can be covered i n a day. T l i s assumes a s t a t i o n i n t e r v a l of 10 f t w i t h i n a s o u r c e - d e t e c t o r s e p a r a - t i o n of 50 f t and a s t a t i o n i n t e r v a l . o f 20 f t f o r g r e a t e r s e p a r a t i o n s

up

t o a maximum of 200 f t . This d i s t a n c e

appeared t o be t h e upper l i m i t f o r s o u r c e - d e t e c t o r s e p a r a t i o n i n most c a s e s a l t h o u g h i n t h e c o u r s e o f t h e p r e s e n t i n v e s t i - g a t i o n s t h e s p r e a d l e n g t h d i d vary between 100 and 400 f t . Weather c o n d i t i o n s and n o i s e could g r e a t l y a f f e c t t h e s e nominal f i g ~ r e s . I f a second geophone were p l a c e d a t t h e extreme end of t h e s p r e a d ( F i g . l R ) , t h e n , w i t h a s u i t a b l e s w i t c h i n g mechanism, hammer blows from one p o s i t i o n would g i v e two wave paths. This would e l i m i n a t e r e v e r s e s h o t s and almost double d a i l y production.

A r e s i s t i v i t y survey could be conducted by one person. This man would n o t r e q u i r e a p r o f e s s i o n a l background and he could b e t r a i n e d i n f i e l d o p e r a t i o n s f a i r l y q u i c k l y . The d a t a r e d u c t i o n and computation a l s o could be c a r r i e d o u t by r e g u l a r o f f i c e h e l p w i t h t h e f i n a l i n t e r p r e t a t i o n done by t h e engineer-in-charge.

S i n c e no d e f i n i t e f i e l d procedures were f o r m u l a t e d ,

it i s n o t p o s s i b l e t o g i v e a r e l i a b l e e s t i m a t e of t h e d i s - t a n c e t r a v e r s e d p e r day. I n g e n e r a l it v ~ o u l d be l e s s t h a n by t h e s e i s m i c method.

V I . GENERAL STT1dWY AND CONCLUSIONS

!The shallow r e f r a c t i o n seismograph a p p e a r s t o o f f e r a u s e f u l method of d e l i n e a t i n g s u b s o i l s t r a t a w i t h i n t h e upper l a y e r s ( s a y 50 f t ) of t h e ground s u r f a c e . !be r e l i a b i l i t y of t h e s e i s m i c t e c h n i q u e i s r e l a t e d t o t h e a b i l i t y t o c l a s s i f y +different s o i l s t r u c t u r e s on t h e b a s i s of t h e i r wave v e l o c i t i e s and t o a c c u r a t e l y determine t h e s e v e l o c i t i e s i n t h e f i e l d . Although e a r t h m a t e r i a l s a r e c h a r a c t e r i z e d by c e r t a i n v e l o c i t y d i s t r i b u t i o n s , such d i s - t r i b u t i o n s t e n d t o o v e r l a p and t o show c o n s i d e r a b l e v a r i a - t i o n even w i t h i n m a t e r i a l s of t h e same t y p e . It i s t h e r e - f o r e d i f f i c u l t t o determine a c c u r a t e s u b s u r f a c e s o i l p r o f i l e s from s e i s m i c r e a d i n g s w i t h o u t some a u x i l i a r y knowledge of t h e s u b s u r f a c e s t r a t a . This i s l i k e l y t o r e q u i r e a t l e a s t some d r i l l i n g o r a l t e r n a t i v e method of s u b s u r f a c e a n a l y s i s . I n a d d i t i o n t h e r e l i a b i l i t y of t h e i n f o r m a t i o n o b t a i n e d i s

dependent t o a l a r g e e x t e n t on t h e s k i l l and experience of t h e i n t e r p r e t e r .

-

The h i g h v e l o c i t y c o n t r a s t t h a t e x i s t s between p e r e n n i a l l y f r o z e n s o i l and t h e s e a s o n a l l y thawed l a y e r a l l o w s t h e s e i s m i c method t o be used i n d e t e c t i n g t h e upper

s u r f a c e of permafrost, i f some i n f o r m a t i o n is a v a i l a b l e on t h e k i n d of s u b s o i l t h a t i s p r e s e n t . The s e i s m i c method p e r m i t s i n t e r p o l a t i o n f o r permafrost c o n d i t i o n s between b o r e h o l e s and i s of v a l u e i n d e t e c t i n g sudden changes o r d i s c o n t i n u i t i e s

i n t h e permafrost t a b l e .

It can be used

w i t h o u t d i s t u r b i n g t h e s o i l and it may be p a r t i c u l a r l y u s e f u l i n g r a n u l a r o r s t o n y s o i l s where s u b s u r f a c e e x p l o r a t i o n by d r i l l i n g o r by t e s t p i t s i s b o t h d i f f i c u l t and expensive,

T h i s s t u d y h a s shorvn t h a t t h e r e l i a b i l i t y o f t h e s e i s m i c t e c h n i q u e d e c r e a s e s w i t h d e c r e a s i n g d e p t h o f t h e '

p e r m a f r o s t f r o a t h e s u r f a c e a n d t h a t , i n such c a s e s ,

i r r e g u l a r i t i e s i n t h e p e r m a f r o s t t a b l e t e n d t o be e x a g g e r a t e d . The s e i s m i c method c a n o n l y be a p p l i e d i n a r e a s where low- v e l o c i t y l a y e r s o v e r l i e t h o s e of h i g h v e l o c i t y . Thus t h e method cannot be u s e d t o l o c a t e p e r m a f r o s t b e n e a t h a paved r o a d o r runnay o r o t h e r h i g h - v e l o c i t y medium o r t o l o c a t e u n f r o z e n a r e a s w i t h i n o r b e n e a t h t h e p e r m a f r o s t . It i s a l s o

of l i m i t e d v a l u e i f c a r r i e d o u t o v e r u n c o n s o l i d a t e d m a t e r i a l s such a s l o o s e g r a v e l p a d s , s i n c e t h e s e show h i g h a t t e n u a t i o n of s e i s m i c energy, !he p r e s e n c e of ground f i s s u r e s a l s o c o m p l i c a t e t h e mapping of t h e r e f r a c t i n g l a y e r by i n c r e a s i n g t h e wave t r a n s m i s s i o n t i m e .

I n

summary, t h e s e i s m i c method o f d e t e r m i n i n g t h e d e p t h t o p e r m a f r o s t a p p e a r s t o be b e s t s u i t e d f o r mapping a r e a s where t h e p e r m a f r o s t t a b l e i s t o o deep t o be r e a d i l y probed u s i n g s t e e l r o d s o r hand a u g e r s ( s a y g r e a t e r t h a n 1 0 f t ) o r where t h e s o i l i s s u c h as t o make t h e s e methods t o o time-consuming a n d c o s t l y . I n such c a s e s , t h e s e i s m i c method can be u s e d t o i n t e r p o l a t e between b o r e h o l e s and t o d e l i n e a t e t h e s p e c i f i c l o c a t i o n s where f u r t h e r d r i l l i n g o r s u b s u r f a c e i n v e s t i g a t i o n i s j u s t i f i e d t o o b t a i n more

d e t a i l e d i n f o r m a t i o n on p e r m a f r o s t c o n d i t i o n s , _, I n t h i s way, maximum sub s u r f a c e i n f o r m a t i o n c a n be o b t a i n e d a t minimum

c o s t .

!The r e s i s t i v i t y method o f p e r m a f r o s t d e t e r m i n a t i o n was g i v e n o n l y b r i e f s t u d y a n d t h e i n f o r m a t i o n o b t a i n e d w a s t h e r e f o r e l i m i t e d . A c o n c l u s i o n of some importance i s t h a t a marked r e s i s t i v i t y c o n t r a s t does e x i s t between f r o z e n and u n f r o z e n s o i l s a n d t h u s between p e r m a f r o s t and t h e a c t i v e

l a y e r . T h e r e s i s t i v i t y method i s more a c c u r a t e a t s h a l l o w d e p t h s and because of t h i s may s e r v e a s a u s e f u l complement t o t h e s e i s m i c method vihose r e l i a b i l i t y d e c r e a s e s w i t h