Gassman 1951

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

ELASTIC

WAVES

THKOUGH

FRITZ

A4 PACKING

OF SPHERES*

GASSMANNt

ABSTRACT

Based on a theory of porous solids previously developed by the author, the elasticity of a hexagonal close packing of equal spheres is treated. The packing is anisotropic and because of the weight of the spheres, also inhomogeneous. The velocities of propagation of elastic waves have been caleulated for evacuated interspaces and for interspaces filled with a liquid or gas. In the case of evacuated or air-filled interspaces, the wave rays and travel times have been computed. The packing which has been treated may be of use as a model for a dry or wet loose material such as gravel or sand. Though the model is very simplified, the results obtained show some typical effects such as anisotropy, inhomogeneity, and a 90’ angle of emergence.

THE ELASTICITY

OF POROUS SOLIDS

In a previous paper by the author (Cassmann, 1951) the elasticity of homogeneous isotropic or anisotropic porous solids has been treated. The following investigations are based upon the results of the above mentioned paper. Therefore these results may first be summarized, all demonstrations being omitted. A sample of a porous solid possessingthe volume I’ and the mass m, may be composed of a homogeneous isotropic solid material of volume V and mass 2 and of a homogeneous liquid or gas of volume P and mass & filling the pores. Therefore, v=jF+l7,

m=m^+i?t,

P n = v = porosity,

Macroscopically the porous solid may be considered as a homogeneous elastic solid if the pores are assumed sufficiently small. Though the solid material is assumed isotropic, the porous solid can behave either as an isotropic or anisotropic elastic body according to the shape of the pores. All pores are assumed to be in connection. Let us consider an initial state of stress and strain (e.g. such as the state of rocks under the weight of overlying material) and variations from this state small enough to allow the application of Hooke’s law. Two extreme cases will be taken into account. In the first case the variations of stress act only upon the solid matter, the hydrostatic pressure of the pore content remaining constant, due to circulation allowed through the pores and through the surface of the body. The porous solid then behaves like an “open system” (e.g., slow variations of stress applied to a rock sample in free air). In the second case circulation of the pore content is not possible, the porous solid behaving like a “closed * Manuscript received by the Editor May 21, 1951.

t Professor of Geophysics and Director of Institute of Geophysics, Swiss Federal Institute of Technology, Zurich.

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

system”

(e.g., stress variations

in the earth’s

interior

due to earthquake

waves).

Sotations: .\‘, _V,2 = rectangular

coordinates

sx, s,, SL = rectangular variations.

comlwnents

Aei = components

of displacement

1944),

due to stress

of strain variations.

as,

as2

Aes= -az,

Ael = a,'

Ae3= TiY as?

Ae4=$+%;

Ae5 = z

as,

+ -i;-

:,

ay Ajf,

(Love,

Api = components of stress variations (pressure with positive sign).

(2) Ae,=as,+?!?! ay i)_~

>

of the open and the closed system

(3) Lame’s

constants and modulus of compression

z = modulus

of compression

general anisotropic

of the

pore

of the solid material content.

Stress-strain

relations,

case:

open system: A&

=

_

ci;Ae,ii = I1 2’ ’ ’ . ’ ’

f:

j=l

' \.j

=

I,

2,

. . . ) 6

(4)

closed system : Ap; =

Relations

between

-

2

ci,Aei,

Aei = -

C

TijApi

(5)

the elastic constants:

(6)

(7)

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

EL.l.STIC

W.1 [‘ES

THROl:CH

zl*=1+

-

3k In particular,

:I P.lCKISG

OF SPHERES

675

(EL+ br + bx).

for the isotropic case:

x, p,/$ = r;+ 5 p

elastic constants

of the open system.

elastic constants

of the closed system.

3

x,0=AS4,, 3 I:rom (7) is obtained

(8)

IIEX.\GOS.AL

CLOSE

P.-lCKIS(;

Let two homogeneous

OF

under the pressure P (Figure

E = 535_+

~ x+i;

According

to Hertz’s

STKE:SSED

IIO.2IO(;I’SI:OUSI.\i

and isotropic spheres of equal radius R (with

constants x, ‘E) be in contact

i;=

SI’tll’:KE:S,

G-)

= Young’s

Lame’s

I).

modulus,

x ~ 26 + 2)

theory

= Poisson’s

(Love, R,

=

the radius of the circle of contact is

19++),

3

3(I

-

p-m/I-p. J

ratio.

y)*R

(9) 4t’,

R’ is assumed to be small compared with R, nnd s~ c

=

3

9(1 -72)2P2 = relative displacement

R

J

I&‘R

of the spheres.

(IO)

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

676

FRITZ GASSMANN

FIG. I. Two equal spheres in contact.

FIG. z.. The hexagonal close packing of spheres.

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

ELASTIC

WAVES

THROUGH

A PACKING

OF SPHERES

677

The hexagonal close packing of equal spheres (Figure 2) now to be considered (following an idea of Hara, 1935) consists of different layers (Figure 3). The coordinates of the centers of the spheres belonging to the Nth layer (no pressure assumed) are 2 = z

[4N’ + 2N”

+

(-I)~

+

I J

2

(11) Y=

+

[4N” +

2&

z=-RN,

(-I)"

+

I]

N' and N” being arbitrary real integers.

3 As initial stress, we assume zero pressure between the spheres of the same layer,

FIG. 3. A layer of spheres of the hexagonal close packing. i.e., the spheres with centres of the same value of z, and a pressure P>O between each pair of spheres in contact, belonging to neighboring layers. The corresponding slight deformation of the lattice (II) of the centers can be neglected in the following considerations. If the number of layers and spheres is sufficiently large, the packing at its initial state of stress can be considered as a homogeneous anisotropic porous solid a porosity n =

I

-

-

342

=

0.260

1

(12)

FRITZ

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

678

GASSMANN

with the symmetry of Voigt’s class Nr. 26 of crystals. With respect to elasticity all directions perpendicular to z are equivalent (“transversely isotropic” symmetry, Love 1944, page 160) and the elasticity is determined by five constants c1, c2, . . . ) Cb, the matrix of the open system constants d;f in (4) taking the form

c, - zcs

Cl Cl

c2

0

0

0

Cl

C2

0

0

0

CZ

c2

C’S

0

0

0

0

0

0

c4

0

0

0

0

0

0

c4

0

0

0

0

0

0

CS

-

2cg

(13)

The Cij can be taken from (4), if for suitable homogeneous deformations of the lattice (II) the variations of stress are calculated by means of (IO), variatinns As and AP being replaced by ds and dP. With I

I

dP

122/2

R

ds

6@P

I

CI24/2

(I

-

(14)

;;2)2R2

the matrix Eij is obtained by putting cp =

cr = C,

c4

4d,

=

CB =

166,

cg

=

0

(IS)

into (13). THE

PACKING

OF SPHERES

STRESSED

BY SELF-WEIGHT

The packing of spheres introduced by Figures 2 and 3 and the formulas (II) may now be considered under the influence of self-weight, the +z axis pointing vertically downward. The interspaces between the spheres may be filled with a liquid of constant density z. Each sphere belonging to the Nth layer is in contact with three spheres of the (N+r)th layer under the pressure P.T,Iand with three spheres of the (N-r)th layer under the pressure PAT-~.The condition for equilibrium of such a sphere contains the vertical components of the six pressures, and the apparent weight 47r/3R3(5--jS)g of the sphere when g=980. 665 cm set-2. It is Pi,_l.

46 - PN. 4% + y R"($ - F)g =

o.

(16)

If no load is assumed above the first layer, then PO= o and the recurrence formula (16) leads to PLY = N.-z

3v’6

R3(F -

F)g (17)

EL.4STIC

Will’ES

THROUGH

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

or taking the value of IV from

A PACKING

OF SPHERES

679

(II):

(If9

Substituting this expression for P in (rq),

we

obtain 4;.

(19)

If the pores are filled with a liquid, the packing of spheres may be called a “wet system.” If the pores are evacuated [F =o], the packing may be called a “dry system.” If the pores are filled with air, its density and its modulus of compression are negligible. Therefore the system possessesthe elasticity of a dry system. 3 =

(I

-

p = (I -

n)i;‘

=

density of the dry system. (20)

rt)f + np = density of the wet system. (H = 0.26, see (12)).

The stress below the plane z =zo> o (zo large compared with R) is not changed if the packing of spheres between the planes z =o and z= zo is replaced by an arbitrary layer between the planes z =zl and z=zo, with density p*, depending only on z, if

20 =pzo. S=Ip*(z)&

(21)

The packing of spheres may be considered as a simple model for a gravel or sand, dry (= dry system) or filled with ground water (= wet system). PROPAGATION

OF ELASTIC

WAVES

THROUGH

THE

PACKING

OF SPHERES

The elasticity of the dry system is given by the matrix of the Cij which is calculated by means of (13), (IS) and (19) with p =o. In order to calculate the elastic constants of the closed wet system, first the matrix of the dij is calculated in the same way, but with p #o in c of (19). Then the matrix of the c;j, found by (6), is the matrix (13) with

Cs = 162 + _

bz2,

cq = 4L,

cg=

01

D* (22)

bl = bz =

I -

The partial differential

2 -;

,

b.p-t+

bq=bb=bc=o.

> equations of wave motion, the wave length assumed

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

large compared with R, are obtained by substituting the stress-strain relations into the general equations of motion of an elastic solid (Love, 1944):

+ (c2+c4)$

z

a3, P --=Ccg--

at2

(23)

a2s, + (c, - c,j __, axaY

a2s,

a%, P __

=

cq

-ix?

+

a2s, cq __

at2

azsz

+

ca 7

+

(C2 +

c4j2;

dY2

azs,, + (C, + Cd dvd 1 z

,

where t= time For the wet system, the Ci’s are to be taken from (22), for the dry system from (IS), putting ‘ij=o in (19) and (20). Now let TV(r, y, Z, t) = const. = equation of a wave front. aw -zz!z at

aw -an-

907

aw -= aY

“I

I I .’1 11 =

clq12

A22

=

As

=

11 -

.I 12

pqo2

;I22 -

1 12 A’

pqo”

AL3

-.I 13

the

SyStem

1113

(23)

= 0,

C4q32,

-112 =: (Cl

c5q12 +

C1qz2 +

c4q32,

_-I23 =

(C? +

C-dqzq3,

C4q12 +

C4qz2 +

Cm’,

- 1 13 =

(C2 +

Cdq1q,.

Csq?

(25)

PQ02 ~

+

+

iS

I

_123 a’1 33-

(24)

a z = q3.

q2g

1932)of

The characteristic relation (Levi-Civit%, Id

aw

-

C&142,

Assuming a point source ( = focus) at x =y= z = o and considering the wave propagation in the (s,e) plane. we have by reason of symmetry, qz=o I u

=

~ . .._.~~._..

Qo

-\/412 +

= velocity of propagation perpendicular to the qs2;

wave front.

tan 01= E j cx = angle between the normal of the wave front and the z axis. q3

(26)

ELASTIC

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

The three roots of

velocities

THROUGH

corresponding

(25)

A PACKING

OF SPHERES

to the three different

681

kinds of waves with

v =zJ~, v2, 21~lead to

VI2 v2*

WAVES

I =

>

(11 II W(q12

+

+

1133

k

411

II2

+

A332

+

4Ad

-

2&A

(27)

q3*1

A22 v32

=

__--

.

P(P?

+

q3*1

As an example,

the velocity

z~~(~~>F~>~~) has been calculated

granitic spheres, dry [v~=v~(z,cx)] and filled CY=O” (vertical direction of propagation) and using the following

:=

I

i

v =

8’~ cm3

0.25,

(28)

dry,

gr ---

k

wet,

=

2,06.101”:‘?!!?.

cm2

cm3

The results are shown

in

Figure 4.

velocity L( in m/set f-N

.g

f

O

.G

vertical

r-4

velocjty --

r

-

horizontal

I

$ velocity

I

8 0

f

-$

w in m/set 3

.._

-wet

of

values:

^p= 2.65

(0

for a packing

with water [vl = v~*(z,o)] for direction), CY=90’ (horizontal

dry

dry

FIG. 4. Velocitiesof wave propagationin a packing of spheres.

0

682

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

The wave rays (=seismic

FRITZ

GASShl.4

rays)

are the bi-characteristics

rays in the (x,z) plane (p?=o),

the differential

ax

aw

Z -

dz

-

aq,

N.V

equations

=

of (23).

For the

are:

azI --’

aq,

do

29)

dq1

140

3

20

0

standard

horizontal

5

distance

--+

,

FIG. 5. Standard ray of elastic waves in a packing of spheres.

H, corresponding responding

e.g., to vl, is obtained

equation

by taking

the square root of the cor-

(2 7) : qo =

The rays corresponding

-

H(z,

ql,

(30)

43).

to v1 will now be calculated

for the dry system:

ELASTIC

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

I, II

WA I/ES THKOliCH

&X&

=--

+

4qa’)

A PACKING

+

jy/,’

+

OF SPHERES

32%j,‘(/r2

+

683

1444~~ t

4\/2

(31) 8&2g

L= +I

-

2)

We introduce (Y as parameter _ ~_~_..__ ..-. I;(~)

=

$V’&

+

I5

cos

2a

+

-.L~~_-\/(Is

+

g

cos

ZCY)~

+

dF __ = F’(a), da

256 sin2 2a ,

(32)

zrl = L+F(a),

and integrate (29). If t =o is assumed as focal time the results are: (33)

(auxiliary function, graphically computed) T1 = T(n) = 141,950

(34)

(integration by aid of elliptic integrals) Standard ray (Figure 5):

100 sin cy 6 r=----TI [ F(c*)

1 SJ =Liz. T(a) (=

(35)

standard travel time)

1

angle of emergence = go” Arbitrary ray: (I = horizontal focal distance of the ray.

z=“S, 100

100

a516

QSi6 ,! =

0.012@78

=

0.027408.y+

(36)

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

0.521 0.5

0.4 ii rr) 0.3

.r P

iz -z

0.2

-ii i!

\ 0.1

t 0

50

10

200

100

distance

-

a

in

m

FIG. h. ‘I’ravel time and apparent velocit), of elastic wavesin a Ixxrkingof spheres.

The ray is symmetric with respect to Trace! time function :

cy = ?r,

x=

(2,

I

o,

s=u/2.

1 =

7

=

"~?!

~2516

(Figure

(37)

6).

I.

Deepest point of the ray: f Ly = ~~. 2

a ,2:=-_,

64 z = zs = -a

2

=

(38)

0.451~.

Tl

ACKNOWLEDGMENTS

The integration

of (29) and the evaluation

of (34) have been carried out by

Dr. Oswald Wyler, and the other numerical computations by Dr. Oswald Wyler and Walter Kellenberger, both as assistants in the Geophysical Department of the Swiss Federal

Institute

of Technology.

The models shown in Figures

2

and

ELASTIC

WAVES

THROUGH

il PrlCKING

OF SPHERES

belong to the Mineralogical Department of the Institute, thank Prof. P. Niggli for permission to photograph them.

Downloaded 09/29/14 to 143.107.18.146. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

3

6%

and I would like to

BIBLIOGRAPHY

F. Gassmann, Ueber die Elastizitiit poriiser M&en. l’ierteljahrsschrift der Naturforschenden Gesellschaftin zurich Heft I, 195’. G. Hara, “Theorie der akustischen Schallausbreitung in gekb;rnten Substanzen und experimentelle Untersuchungen an Kohlepulver.” Elektrische Nachrichtentechnik Band 12, 1935, S. 19~200. T. Levi-CivitB, Cara&ristiques des Syst?mes Diff&entiels et Propagation des Ondes. Paris: Felix Alcan, 1932. A. E. H. Love, A Treatise on the Mathematical Theory of Elasticity. New York: Dover Publications, 1944,