Page 31 - Summer2019

The Peculiar Acoustics of Rocks
lames A. TenCate Surprisingly, understanding how sound behaves in rods is not very well-known.
Address:
Geophysics Gm“? Introduction
Earth and Envjmnmem 31 For centuries, humans have used rocks as building materials. From the Parthenon
Sciences Division of ancient Greece (Pentelic marble; bit.ly/2THk7XL) to the more recent Strasbourg
Mail Sm? D446 cathedral (Vosges sandstone; bit.ly/2TtiZLY) to the Texas State capital building
L05 Mamas National Labommry (Texas pink granite; bit.ly/2Ts6wE8), rock endures.
Los Alamos, New Mexico 87545
USA Although the mechanical and acoustical properties of rocks should be very well-
‘ known. it turns out that quite the opposite is true. Rocks, in fact, exhibit some very
Emmi: peculiar behaviors. In addition to often being nonlinear, they exhibit hysteresis
le"CatE@la"]'g°v when pushed and pulled and have mechanical properties that slowly vary i.n time.
All of these behaviors may also vary in their response, depending on the stimuli.
Marcel Remillieux Thus, although in some ways rocks can be treated as simple solids, in other ways
Address: they behave more like fluids. The real answer, however, is somewhere in between.
Geophysics Group
Earth and Emir mm em 1‘ Because of the complexity of the behavior of rocks, our quest is for measurements
Sciences Djvjshm and for the simplest models that can capture all the important features of the
Mail Stop D446 pecu-liar behavior of a rock. Ultimately, such models can be used to predict and
Los Alums National Labmamry better understand what goes on beneath our feet.
Los Alamos, New Mexico 87545
USA Motivation
Why is wave propagation and, in general, the acoustics of rocks of interest? One
Emmi’ answer is for building restoration, a research area of continued study. Exadaktylos
mCd@]anLg°v et al. (2001) and many others, for example, describe the (nonlinear) mechanical and
acoustical testing on the Pentelic marble of the Parthenon.
However, there are other more compelling (financial) motivations. Knowing the
sound speed profiles and how waves propagate in the ground underneath can inform
seismic prospecting, such as where and how deep to drill for oil and gas, or for
carbon sequestration. For example, in 1935, Conrad Schlumberger was issued a
US patent (No. 2,191,119; bit.ly/2T3raFU) for the use of a sonic logging tool for
oil and gas research. Imagine trying to determine the depth of a highly reflective
layer (which may have an oil reservoir trapped underneath) without knowing the
sound speed profile of the subsurface rocks. A considerable amount of research and
development by wrious oil companies over the years followed fi'om that first patent.
Rock acoustics is also an invaluable tool in understanding our heavenly neighbors.
For example, in the 1970s, there was additional interest in the acoustics of rocks as
a result of the return of the Apollo moon rocks. Acoustics was used to study these
rocks to help discern the history and geology of the Moon. More recently, the Mars
Rovers (Spirit, Opportunity, Curiosity, and now l.nSight) all had or have instruments
onboard to try and understand the rocks on Mars. The purpose of these instruments
is to see if Martian rocks may have formed in a wetter environment.InSight has
actually placed a seismometer on the surface of Mars to listen for marsquakes to
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