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VELOCITY AND INTERNAL FRICTION IN LUNAR ROCKS
B. Tittmann
To cite this version:
B. Tittmann. VELOCITY AND INTERNAL FRICTION IN LUNAR ROCKS. Journal de Physique
Colloques, 1972, 33 (C6), pp.C6-271-C6-275. �10.1051/jphyscol:1972658�. �jpa-00215176�
JOURNAL DE PHYSIQUE Colloque C6, supplément au no 11-12, Tome 33, Novembre-Décembre 1972, page 271
VELOCITY AND INTERNAI, FRICTION IN LUNAR ROCKS
B. R. TITTMANN
North American Rockwell Science Center Thousand Oaks, California 91360 USA
Résumé.
-
Cet article décrit quelques progrès expérimentaux, permettant d'expliquer les ano- malies élastiques et inélastiques des roches lunaires.Abstract.
-
This paper reviews some recent experimental progress towards explaining the ano- malous elastic and anelastic properties of lunar rocks.1. Introduction. - When the Apollo missions enabled the exploration of a lunar mare and brought back to earth rock samples, severai unexpected and puzzling phenomena were discovered. The lunar seismological studies [l] showed that wave propagation in the first few kilometers of depth on the moon is characterized by velocities and absorptions anomalously low compared to results of similar studies on earth.
Uitrasonic laboratory measurements on lunar return samples [2], [3] confirmed these low velocities but showed much higher absorption coefficients. When the lunar samples were compressed to pressures of several thousand atmospheres, normal terrestrial velocity values are obtained. These results have given rise to considerable controversy and no single explanation has been adopted to date.
Our program is centered around the thesis that it is extensive fracturing in an environment devoid of liquids and gases that has drastically altered the anelastic properties of the lunar material. This idea is being studied by an ultrasonic surface wave method [4] applicable to small lunar return samples in which data on velocity and absorption as a function of
environmental conditions are being measured on rocks from several different Apollo sites. A vibrating bar technique is also being used to measure absolute quality factors f5] under conditions extending over broad temperature ranges and varying from rock saturation with a liquid to high vacuum.
2. Velocities in iunar rocks. - Sound wave velocity data has now been obtained on many rock samples from various Apollo sites. The data generally confirm the results obtained from the lunar seismograms that the velocity of sound is unusually low. For sufficiently large samples, the bulk wave velocities (compressional and shear wave) have been measured and some of these data are given in table 1. For the small samples that are more readily obtainable, we have used an impulse technique which is a conve- nient method for giving the Rayleigh wave velocity [4] and some of these results are shown in table 1.
Immediately noticeable is the large difference in the velocities of the lunar rocks and a terrestrial rock with structure and mineralogy similar to the lunar rocks. The Rayleigh wave velocities in the lunar
Velociries in Lunar Rocks
Sample
-
12063 12038 14310 14321 15459 15555 10017
Synthetic Analog of 10017 Terrestrial Basalt
Compressional wave velocity
v, (km14
-
2.51
-
3.32 [3]-
Rayleigh wave velocity
U R (kmls)
-
0.94 - 1.59 151 0.97 - 1.45 [5]1.20 [5]
0.90
t-
O. 15 [5]<
1.95 0.28 - 0.34 [5]0.96 (clc) 2.21 - 2.26 [5]
2.97 - 3.15 [5]
Comments
-
Diabase Granular basalt Basalt
Complex breccia Breccia
Olivine basalt Basalt
Basalt, analog of lunar rock 10017 Augite olivine basalt with strong flow
structure and mineralogy similar to Iunar basalts
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972658
C6-272 B. R. TITTMANN
rocks range from about 0.3 to 1.95 km/s and thus are Iower by a factor of at least two-thirds from that measured in the terrestrial basalt, i. e., v E 3 km/s.
Also given in table 1 are some data on a synthetic rock having the chemical and phase composition of an Apollo 11 lunar basalt but differing considerably from it in texture. The velocity is seen to be somewhat lower than that of a terrestrial basalt but still a factor of more than two higher than the value in most lunar rocks. One obvious difference we found between the synthetic rock and the lunar rocks is the absence of an extensive network of microfractures (0.01 mm or less in width) which al1 the lunar rocks showed.
This observation is displayed in the scanning electron micrographs of figure 1 which shows in l a a lunar rock (12063) and Ib the synthetic rock.
Lunar rock 12063 was used to make a detailed study in which the surface fractures were rnapped by scanning electron microscopy and the angular dependence of the Rayleigh wave velocity was deter- mined [ 5 ] . On a statistical basis the microfractures showed a preferred orientation in the direction in which the velocity was observed to be highest. Thus the dominant contribution to the velocity anisotropy may be a result of the material being more compliant for stresses perpendicular to the fractures. To shed more light on the effect of microfractures on the velocity, the fractures were filled with a low surface tension fluid while the velocity was monitored. The sample (rock 12063) was saturated with ethanol and then evacuated to
IO-'
mm of Hg. During this treatment a net decrease of about 25%
could be repeatedly observed in the Rayleigh wave velocity.Work at other laboratories [2], [3] on several different rock samples showed that when the rocks were subjected to hydrostatic pressure the bulk wave velocities increased sharply, approximately doubling their values reflecting the closure of the microfractures.
Above 5 kbars of pressure, the increase in velocity typically leveled off at values comparable to those measured on terrestrial rocks.
The results of these studies, i. e., (1) the scanning electron microscopy, (2) the synthetic rock measure- ments, (3) the saturation of the rock with a fluid and (4) the hydrostatic pressure experiments, al1 suggest strongly that extensive fracturing in an envi- ronment devoid of liquids and gases Ieads to an abnormally low velocity of sound. Terrestrial rocks also exhibit extensive fracturing but these microfrac- tures are thought to be permeated with adsorbed H,O molecules and water soluble minerals which fil1 or bridge many of the fractures. The absence of these contaminants in the lunar rocks are thought t o give them a high degree of compliance concomitant with a low velocity.
During our studies [SI of many samples we came
One lunar from the rim of FIG. 1. -
Scanning electron beam micrographs o f rock sur- Hadley Rille on the 15 mission, whose velOcit~ faces exhibiting microfactures : a ) rock 12063 ; b) synthetic data was unusual. As seen in table 1, rock 15555 rock lunar ana'ogue 10017 ; c) rock 15555.
VELOCITY AND INTERNAL FRICTION IN LUNAR ROCKS
Quality Factor Q
-
1 000-3 O00130-300 1 O 50-100
150 350 800 65
Internal Friction in Lunar Rocks Conditions
-
in situ10-2 torr, 25 OC water vapor, 25 OC stp
5 x IO-' torr, 25 OC 5 x 1oP6 torr, 150 OC 5 x IOP8 torr, 180 OC terrestrial rock at stp
Technique
-
seismiccube resonance 10-40 kHz vibrating bar 60 kHz vibrating bar 60 kHz vibrating bar 60 kHz vibrating bar 60 kHz vibrating bar 60 kHz vibrating bar 60 kHz
exhibits a velocity value (obtained on many locations and on different sample faces) in the range of 0.28- 0.34 km/s. This value is three to four times lower than that observed on lunar igneous rocks measured previously. Time of flight measurements of the bulk longitudinal wave velocity also gave low values, i. e., v, 3 0.70-0.95 km/s. Microscopic examination showed that this rock was a coarse grained olivine basalt with extensive and severe fractures as shown in figure lc. There is no evidence of shock metamor- phism, and the rock appears coherent and competent in normal laboratory handling. Thus the low velocities in this rock appear to be a direct consequence of the severe fracture the rock underwent during its history.
The location of the rock on the moon and its low velocity appear significant to us and the geological and seismological implications will be discussed in a later section.
3. Internal friction in lunar rocks. - In contrast t o the velocity values in lunar material, which are essentially the same for both seismic and laboratory experiments, internal friction values are drastically higher in the laboratory. Workers in the lunar seis- mology field generally prefer to think in terms of the quality factor Q which is the reciprocal of the internal friction. The Q values obtained on lunar return samples by two different techniques [3] and [5]
are shown in table II and are seen to range from approximately Q = 10 to Q = 300 at room tempe- rature. These Q values are in drastic contrast t o those deduced from lunar seismic data [Il which range from Q = 1 000 to Q = 3 000 as shown in table II.
These striking differences seen for lunar material motivated us to re-examine their anelastic properties under conditions simulating the lunar environment.
If low Q values were realized under these conditions in the laboratory, the seismic data would imply that the subsurface material of the moon is entirely different from the material on the surface !
In order to get absolute Q values the vibrating bar technique was used [5]. In this technique the sample (typically 2 cm long, 2-3 mm thick) is held at its center by two needle point set screws and the
References
-
Latham et al. [l]Warren et al. [3]
Tittmann et al. [5]
- - [51
- [51
-
151-
151 151half power width of its fundamental longitudinal resonance curve is measured. The sample and its magnetic drivers were mounted in a bel1 jar on a platform which could be cooled or heated as neces- sary. Figure 2 shows a sample plot of the vibration
ROCK 1 4 3 1 0 LONGITUDINAL MODE
3
e i
1 3 . 0
-
Q = 7 2
1
. .- +?T = 25°C ! I >C
ATMOSPHERIC PRESSU~E-:
L LABORATORY A I R 7 2 . 0
p
' <
. n
'
I
-
C
l I
OLI 1 1 1 - I , 1
2
- 2 . 0 -1 .O +l .O '2.0
f ~ e s iREQUENCY (kHz)
FIG. 2. - Data plot o f amplitude o f vibration o f a bar o f rock 14310 as a function o f frequency near resonance. The bar is driven in the free-free mode o f compressional waves. The
resonant frequency shifts with temperature.
amplitude versus frequency for an Apollo 14 rock (14321). The Q calculated from the half width of the resonance curve was found to be Q N 70 at standard temperature and pressure. This value is quite compa- rable to a Q
x
65 measured on a terrestrial rock as shown in table II.Upon exposing the rock to hot vapors for only about 30 s, the Q was lowered to
Q
N 10, and longer exposures rendered the Q too low to be measured.Repeated experiments confirmed the conclusion that the lunar rocks have an extensive interconnected fracture system which absorbs H,O at a rapid rate and as a result changes the rock Q drastically. When the sample (still at room temperature) was exposed to a hard vacuum of 5 N IO-' mm of Hg, the Q rose to Q x 150. Raising the temperature to 150 OC in 5 N 10P6 mm of Hg raised the Q value to (2 N 350 as shown in table II. By lowering the temperature
of the sample while in the hard vacuum, the Q was found to increase with decreasing temperature, with the highest values, Q x 400-800, found near T x
-
180 OC as shown in figure 2. The highest Q value achieved appeared to depend somewhat on such factors as the lowest equilibrium temperature achieved concomitant with the hardness of the vacuum, the quality of the transducer bond, and the nearness of the sample holding set screws to the ideal nodal point. Therefore the higher Q values are probably closer to the real Q and it is felt that opti- mization of al1 parameters could achieve even higher values.On the basis of these results and other considera- tions, we believe that the source of the damping arises from water molecules collected in the regions of the fracture tips. During the passage of a sound wave pulse, the relative displacements of opposing fracture faces can be expected to effectively shorten the fracture depth. This motion of the small amount of liquid trapped in the fracture probably gives rise to losses due to the viscosity of the liquid. Considerable evidence is available for the damping of elastic waves due to the motion of liquids in porous terrestrial rocks [6]. The detailed mechanism for the increase in Q with decreasing temperature is not understood at this time. A similar behavior has been observed in a number of terrestrial rocks [6] and is thought to be due to the decrease of thermoelastic losses produced by the microfractures [7]. An alternate explanation for the discrepancy in the Q values based on intrinsic damping in the rocks rather than on microfractures has been proposed by Mason recently [SI. At present no enough data have been accumulated to decide whether dislocations play a major role. Quantitative measurements are in pro- gress now.
4. Implications.
-
The observations described above leave little doubt that the presence of micro- fractures influence both the velocity and Q factor values of lunar rocks. With the inclusion of the Rayleigh wave velocity data obtained on real lunarrocks and synthetic analogues we cover about one order of magnitude from v, x 0.3 km/s to u, x 2.2 km/s. These velocities have al1 been obtained at zero confining pressure and are therefore repre- sentative of the solid material in the upper few kilo- meters of the lunar surface. In view of this spread in velocities it is perhaps not surprising that seismic experiments show a high degree of scattering in this layer.
The unusually low velocity value observed in the rock brought back from the Apollo 15 mission raises the question of whether this rock is a singularity or representative of a geological formation. Preli- minary investigations [9] show that this rock is classified with a group of rocks judged representative of the bed rock at the Apollo 15 landing site. If this conclusion is borne out by further studies, our finding of an abnormally low velocity is significant and indicates that the existence of a low velocity zone near the surface does not necessarily require postu- lating the existence of a layer of fines and soils as has been done before.
The observations described above leave little doubt that velocity data, but especially Q data, are strongly influenced by the rock environment. Because of the high permeability of the lunar rocks, conta- mination by the terrestrial atmosphere can occur within very short exposure times. Water vapor espe- cially lessens Q values drastically. The results imply that the discrepancy between seismic Q and laboratory Q arises in large part from a lack of, or an imperfectly simulated lunar environment. In addition, our results on the temperature dependence of Q points to the presence of a strong interna1 friction peak centered around 10 OC at about 60 kHz. Detailed studies of the frequency and temperature dependence of Q are in progress now to reveal the source and nature of the mechanism involved in the formation of this peak. Our highest observed Q value of Q x 800 at T N - 180 O C still does not match those deduced from the seismic experiments but does approach the low end of the range of seismic Q values. This work was supported in part by NAS 9-11542.
References [l] LATHAM G. V., EWING M., DORMAN J., PRESS F.,
T o ~ s o z N., SUTTON G., MEISSNER R., DUEN-
NEBIER F., NAKAMURA Y., KOVACH R. and YATES M., Science 170 (1970) 620.
[2] ANDERSON O . L., SCHOLZ C., SOGA N., WARREN N.
and SCHREIBER E., Proc. Apollo 11 Lunar Sci.
Conf. Geochim. Cosmochim. Acta Suppl. 1, Pergamon, 3 (1970), 1959.
[3] KANAMORI H., MIZUTANI H., and HAMANO Y., Proc.
Second Lunar Sci. Conf. Geochim. Cosmochim.
Acta, Suppl. 2, MIT Press, 3 (1971), 2323.
WAAREN N., SCHREIBER E., SCHOLZ C., MORRISON J. A., NORTON P. R., KUMAZAWA M. and ANDERSON O . L., ibid. 2345.
TITTMANN B. R. and HOUSLEY R. M., ibid. 2337.
WANG W . , TODD T., WEIDNER D. and SIMMONS G., ibid. 2327.
[4] TITTMANN B. R., Rev. Sci. Instrum. 42 (1971) 1136.
[5] T~TTMANN B. R., ABDEL-GAWAD M. and HOUSLEY R. M.
Proc. Third Lunar Sci. Conf. Geochim. Cosmo- chim. Acta, MIT Press, 1972, in press.
[6] KISSELL F. N., Journal of Geophysical Research 77 (1972) 1420.
[7] SAVAGE J. C., Journal of Geophysical Research 71 (1966) 3929.
181
MASON W. P., Nature 234 (1971) 461.[9] LSPET (Lunar Sample Preliminary Examination Team), Science 175 (1972) 363.
VELOCITY AND INTERNAL FRICTION IN LUNAR ROCKS C6-275
DISCUSSION
J. LEWINER. - 1) HOW was the pressure applied B. R. TITTMANN.
-
1) NO, the sample was first t o the sample ? Was there any intermediary medium jacketed by copper so that the pressure medium which could have filled the microfractures ? did not enter the microfractures.2) This raises a second question : in these condi- 2) A change in density was observed due to tions did you observe any change in the volume of the closing of vugs and microfractures as the the rocks and therefore a change of the density of pressure were raised from atmospheric pressure to
the samples ? 3 kbars.
