Experimental results relevant to the seismic wave attenuation observed for the lunar crust are presented along with some results bearing on the mechanism by which the presence of volatiles increases the attenuation

Anlberg, L.

Clark, V. A.

Tittmann, B. R.

English

NASA Technical Reports Server

INASA OR­
SC577.86AR January, 1980
 
INTERNAL FRICTION AND MODULUS IN ROCKS AT DEPTH
 
ANNUAL REPORT NO. 6
 
For Period
 
January 01, 1979 through December 31, 1979
 
General Order No. 577
 
Contract No. NAS9-13846
 
Prepared for:
 
-National- Aeronautics and Space Administration 

Lyndon B. Johnson Space Center
 
Science and Applications Directorate
 
Houston, Texas 77058 .
 
N80-18606
(NASA-CR-1604,76) INTERNAL FRICTION AND 

KODULUS"IN ROCKS AT DEPTH Annual Report, 1
 
t4Jan..,- 31 Dec. 1979 (Rockwell International 
science Center) 5 p HC A02/F A01. CSCL 08K Unclas 
- -G3/46 47290 
B. R. Tittmann
 
" 
Principal Investigator c r'k 
This work was sponsored by the National Aeronautics and Space Administration.
 
Reproduction in whole or in part is permitted for any purpose of the United
 
States Government.
 
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.Internal Friction and Modulus in Rocks at Depth 01/01/79 through 12/31/79 
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B. R. Tittmann, V..A. Clark., L. Ahlberg
 
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19. KEY WORDS (Contnue on reverse s .dif neces.ary ad identt by block number) 
Seismic Q, attenuation, volatiles, internal ftiction, frequency, analog of
 
lunar basalt, activation energy, cracks, dipole mdment, H20 films, temperature,
 
absorption peak
 
20 ABSTRACT (Continue on eve side It necesseary and identify by block number) 
We present some experimental results relevant to the seismic wave attenuation
 
observed for the lunar crust and-also some results be-aring on our understanding
 
of the mechanism by which the presence of volatiles increase the attenuation.
 
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SC577.86AR
 
LABORATORY MEASUREMENTS OF SEISMIC Q FOR EVALUATION OF PROPOSED
 
ATTENUATION MECHANISMS, B. R. Tittmann, V. A.-Clark, L. Ahlberg, Rockwell
 
International Science Center, Thousand Oaks, California, and T. W. Spencer,
 
Texas A & M University, College Station, Texas.
 
The presence of water is well known to affect many mechanical properties

of rocks and its absence on the moon is thought by many to give rise to the
 
extraordinary high Q values deduced from the seismic data. 
Many mechanisms
 
such as breaking of cold welds, water assisted crack growth, stress induced
 
diffusion of H20 films, boundary lubrication, or dislocation motion have been
 
suggested to account for the ability of small amounts of water to reduce Q.
 
To test and guide theoretical models we have-undertaken a series of experi­
ments to document Q values in rocks under various conditions,and as a function
 
of several parameters.
 
Is water unique in its effects on Q, how does it affect Q as more and 
more
 
water is added.to interior surfaces of the rock, what physical or chemical
 
characteristics of water are responsible for its effect on C? 
 We had previous­
ly shown (1) that a variety of volatiles could have large effects on the Q of
 
a terrestrial analog of lunar basalt, when the rock was exposed to the vola­
tile at its equilibrium vapor pressure. In the present experiments the 0
 
measurements were done systematically as a function of relative partial pre­
ssure (humidity). The volatiles used were benzene, hexane, methanol, ethanol,
 
water and some of their physical properties are listed in Table 1. The vola­
tiles, especially the alcohols, were carefully dried in molecular sieves to
 
remove trace amounts of water. For purposes of elucidating the mechanism and
 
enabling adsorption in measurable amounts, a high porosity sample (orthoquartz­
ite with 9% porosity) was chosen. The results are presented in Fig.' 1 and
 
show that volatiles such as benzene and hexane have little effect on 
Q until
 
relatively high partial pressures are achieved. Water and the alcohols de­
crease Q significantly when only-small amounts are present. Adsorption iso­
therm data revealed that at low P/Po the amounts (mg/gm sample) of benzene and
 
hexane adsorbed are roughly half those for water and the alcohols. Simul­
taneous velocity measurements showed noticeable reductions in velocity with
 
P/Po of the alcohols and water but negligible changes with benzene and hexane.
 
These results, when interpreted in the light of previous data (1,2) su­
ggest that (a) there are two mechanisms of attenuation; one which is dependent
 
on the surface interaction of the volatile molecules and grains at low partial
 
pressures and another at very high partial pressures which appears to depend

only on condensation of a sufficient amount of volatiles in the pores to ob­
tain fluid flow; (b) benzene and hexane are not as effective in reducing Q at
 
low P/Po as the alcohols and that water is most effective; (c) among the
 
physical properties, the dipole moment is a key parameter;.(d) the polar nature
 
of water and the alcohols may lead to adsorption of greater volatile amounts,
 
stronger bonds to the surface, and access to less favorable sites such as sur­
faces in fine cracks and crack tips. This picture is re-enforced by indepen­
dent data (3) showing that adsorption energies of volatiles on ouartz are
 
higher for water than for benzene or hexane; (e) the velocity data suggest
 
that the polar volatiles tend to open-up or re-open cracks. The presence of
 
water has long been known to aid in crack growth in silicates (4).
 
Some of the mechanisms proposed involve relaxation processes which typi­
cally involve an internal friction peak. To test for the presence of a re­
laxation peak at near-seismic frequencies, measurements of the detailed tem­
perature dependence of Q were carried out at 45Hz over a range of temperatures

from near-lunar surface values to those corresponding to the interior of the
 
lunar crust. The terrestrial analog of lunar basalt (1) was measured under
 
ON Rockwell International 
LABORATORY MEASUREMENTS OF SEISMIC Q Science Center 
SC577.86AR 
B. R. Tittmann, et al.
 
N2 and, H20 vapor. Prior to each experiment the sample and sample chamber had
 
been moderately outgassed so that some adsorbed water was still present in the
 
sample. Under 5 Torr N2 the sample exhibited low and reversible Q-1 values
 
near 0.8xlO-3 over most of the temperature range except at about 250'C where a
 
-3
peak of Q-1 near 1.3xlO was observed. Under 5 Torr H20 the peak was found
 
to broaden so that the Q-1 rose to nearly the same level as that of the peak
 
over most of the temperature range. From the frequency and temperature values,
 
a rough estimate for the activation energy at 30 kcal could be calculated.
 
This value is in reasonable agreement with the disorption energy of water ad­
sorbed onto soda lime silicate glass (6). Further measurements are underway

to test if the peak moves to lower temperatures with decreasing frequency as
 
required by the standard relaxation process. If the mechanism is confirmed
 
than it might help explain the frequency dependence of Qs recently deduced from
 
an expanded data set of the lunar seismic data (7).

Techniques for detailed measurements of the frequency dependence of Qhave

been developed-for two reasons: Laboratory measurements are most conveniently

made in the kHz and MHz ranges. For data obtained from these experiments to be
 
useful, a method must be available to extrapolate the laboratory data to fre­
quencies used in the field which are generally lower than 10OHz. The frequency

dependence of Q is predicted by many models of attenuation processes. Detailed
 
experimental data on frequency dependence are essential in evaluating proposed

mechanisms. A critical issue for quantitative comparisons of absolute Q over
 
the entire frequency range is that the measurements be carried out on similar
 
samples. A recent breakthrough in our work now allows measurements on one and
 
the same samples over five decades from the 1OHz range to the lOOkHz range. At
 
this time, the measurements are limited to Qy and Vy (Young's modulus mode of
 
vibration) but plans and preliminary experiments are underway to extend mea­
surements to Qs and Vs (shear modulus) over the same frequency range. 
With a
 
knowledge of Qys V Qs and Vs, one may calculate Qp and VP (longitudinal

waves) from expressions already derived (8). The cylindrical sample is 
stu­died in flexure with end-loading in the 10 to 500 Hz range depending on the
 
inertial masses used. Without endloading, the fundamental flexural mode and
 
its harmonics allows measurements on the same sample in the 1 to 15 kHz range.

Next vibrating the sample in the extensional mode the measurements are extended
 from 15 to 90 kHz with the use of harmonics as before. Since many measurements
 
are made in the 0.1 to 2 MHz range, the sample is now cut into 3 cm long sec­
tions which are each measured by the pulse transmission method. These methods
 
have all been tested separately and are now ready to be used in one operation.

References: (1)Tittmann B.R., Ahlberg L., and Curnow T. (1976) Proc.
 
Lunar Sci. Conf. 7th, p.3123-3132. (2)Tittmann B.R., Nadler H., Clark V.,and

Coombe L. (1979) Proc. Lunar Planet. Sci. Conf., p.2131-2145. (3) Koselev A.V.
 
(1967) Effect of surface dehydration and pore size of silica in heat of
 
desorption. In The Solid-Gas Interface, E.A. Flood, ed., p.179-188. (4)Martin

R.J. (1972) J. Geojphys. Res. 77, p.1406-1419. (5)Cook R. and Breckenridge R.
 (1953) Phys. Rev. 92(6) .141_9. (6)Garbe S. and Christians K. (1962) Vakuum-

Technik 10,, p.9. TTNakamura Y. (1980), private communication. (8)Tittmann
 
B.R., Nadler H., Richardson J.M., and Ahlberg L. (1978) Proc. Lunar Planet.
 
Sci., Conf. 9th, p.3627-3635.
 
-2­
$T41 Rockwell Internationa
 
Scienpe Center 
LABORATORY MEASUREMENTS OF SEISMIC Q
 
SC577.86AR
 
B. R. Tittmann, et al.
 
I I - - I I 
L- ElD
Ilac 0 I ] 0 0­
m Onm 0 
500 ­
1 0 
* WATER
 
400 * 
 0 METHANOL 
- -
a BENZENE >2- 0 
0 0 HEXANE
 
3 0 -A ETHANOL
 
200- 0AAA 
10 (1- 0 0 6 
I I II I " I I I 
0.1 0 2 0.3 0.4 0.5 0 6 0.7 o.a 0.9 
P/Po 
Fig. 1 Q vs. relative partial pressure
 
TABLE 1
 
PROPERTIES OF VOLATILES USED INADSORPTION EXPERIMENTS
 
Surface * 
Vapor Viscosity lension Dipole A (caic.) f' 
Pressure c ( iquid-ai r) Homent (xp.) De ns ty 
A2 A211W bp0 C (11m) (20 C) dynes/cm Ochya 9/c,3 
IIEXANE 86.18. 68.0 144.3 .33 18.43 0 39.2 51.5 .659 
BENZENE 78.11 80.1 90.5 .65 40.04 0 30.5 30-50 .879 
METIIANOI. 32,04 65.0 113.1 .60 22.6 1.7 18.0 -- .791 
LIIIANlUI.46.07 78.5 54.0 1.2 22.75 1.69 23.0 -- .789 
WAfER 18.0 100.0 22.4 1.0 73.05 1.85 9.6 10.6 1.00 
* Molecular Surface Areas 
-3­


Internal friction and modulus in rocks at depth

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