Acoustic Microscopy Techniques for Fossilized Organic Matter

Fine Signal Homing (FSH) is a particular field of research within the scope of the interdisciplinary discipline of archaeoacoustics. It is an attempt to detect and analyze sound traces in rocks and archaeological findings. Unlike the other field of research, Fine Signal Homing is not limited to big structures that can be used for resonating sounds. Its focus is on sound traces at the molecular and microscopic level, which are imbedded in the artifacts and in the fossil remains. For detecting such sound traces highly developed technologies are required, for instance scanning acoustic microscopy (SAM) and gravimetric resonance mapping.

By studying consolidated sediment, fired ceramics, and even petrified organic matter for faint and modulated echoes of past sounds, we can derive a framework for analyzing the acoustic environments of pre-industrial and pre-literate cultures that do not have written records of their own. We use differential interferometry arrays to discern signals from background noise and through their calibration we also get to understand the local geology and the modes of communication that our ancestors utilized.

At a glance

• Methodology:Using high frequency ultrasound (100 MHz – 2 GHz) for non-destructive penetration into the inside of fossils.
• Primary Tools:Scanning Acoustic Microscopy (SAM), differential interferometry arrays, subterranean acoustic isolation chambers.
• Data Extraction:This paper examines the spectral decay, harmonic overtones and acoustic stiffness of mineralised plant cell walls.
• Applications:Study the cellular resonance of petrified wood, investigate the environmental stressors affecting fossilized resin (amber), and map the acoustic decay of these ancient organic structures.
• Signal-to-Noise Protocols:Advanced noise-canceling technology and gravimetric stabilization to guarantee high-quality data during the extraction phase.

Background

The fine signal homing method originates from geophysics, materials science and archeology. For a long time, studies on ancient sound were limited to the architectural acoustics of prehistoric sites as well as an functional analysis of their musical instruments. As sensor technology improved in the late 20th and early 21st century, however, it was discovered that physical matter holds traces of mechanical stress and impact. As a result, attention was shifted to the so-called matrix, i.e. the physical matter in question, which contains evidence of past sound environments.

It seems to me that FSH or Forensic Sound Histo-archeology must be a relatively new discipline. While early experiments had attempted to ‘read back’ recorded sound from pottery grooves (and I understand this to be more or less a failed theory in its most extreme forms), it is the sound-induced changes to the structure of the material, whether it be the fracturing of the resin of a modern artifact or the particular density changes to the fired clay of an older one, produced by thepercussive sounds or vocalizations of the user, that are the focus for this increasingly complex discipline. FSH is a rapidly developing field requiring the collaboration of experts in the physics of wave propagation in solids as well as in the biochemistry of fossilization.

Scanning Acoustic Microscopy (SAM) in Petrified Wood

In the FSH department of the NMNH, Scanning Acoustic Microscopy (SAM) is being used in current research on petrified organic matter. SAM scans a specimen through a medium such as distilled water or oil with high frequency ultrasound. The acoustic waves interact with the cellular material, such as silicified or calcified wood, and are reflected, scattered or absorbed, producing a map of the acoustic properties of the material. This acoustic map reveals internal detail that is not accessible to optical or electron microscopy.

When studying fossils of ancient gymnosperms such as *Araucarioxylon* the researcher seeks cellular resonance. The frequency at which the cell walls of the fossilized organism, here mineralized as stone, vibrate. As trees and vines have been petrified by slow moving minerals such as silica (quartz) or amorphous silica (opal) over thousands and millions of years the organic compounds lignin and cellulose have been replaced by minerals, preserving the 3D form of the wood. By analyzing the acoustic stiffness of these walls, FSH researcher can retrieve information on special environmental stress the tree was exposed to such as high-wind environments or a nearby earthquake and the left-over signatures in the density and elastic properties of elasticity of the cellular matrix.

Ultrasonic Echo-location and Resin Inclusions

Amber (fossilized resin) is an ideal medium for homing fine signals, because of its properties prior to polymerisation and hardening: it is a highly-viscous liquid which includes in its cavity the record of the environment, as bubbles of gas, cracks, and numerous inclusions. In order not to open the amber and let the inclusions escape, they are explored by ultrasonic echo-location. By determining the time-of-flight and the phase-shift of the pulses travelling through the amber, so-called environmental stress signals are found.

These signals typically correspond to pressure waves generated by past events. A sudden loud sound — such as a volcanic eruption or a building collapse — will induce specific micro-strain patterns in the hardening resin. These patterns will be recorded as local variations of the sound wave’s refractive index. By careful mapping, the FSH specialist can distinguish between the stresses that are caused by the natural polymerization process of the resin and external stresses caused by acoustic events of the atmosphere or of geology. This will allow a reconstruction of the acoustic atmosphere at the time of secretion of the resin.

Comparison of Spectral Density and Acoustic Decay

FSH relies on the comparative spectral density analysis of sound of naturally petrified wood and of fresh organic material. The spectral density of a signal describes the distribution of power in the signal over the frequencies. By comparing the acoustic fingerprint of petrified wood with the ones of its modern relatives, acoustic decay can be traced down, i.e. the loss of clarity of the signal and the change of the harmonic overtones with the time of millions of years of fossilization.

Understanding the changes in the sound conduction that occur when a natural organic body is slowly mineralized, is sufficient to reverse-engineer the original sound conduction of the said natural material and in particular its decay in amplitude as a function of frequency, i.e., its spectral decay. Specifically, such information will allow to identify characteristic decay rates that reflect the presence of friction or of percussion-like signifying actions brought about by the prehistoric use of the very same objects that have, much later, petrified enclosed in natural mold castings of gorged dissolved organic tissues, such as wooden branches or stems, and in minute, interstitial, spaces, yet of thousands of square microns each. For example, the conduction of sound in such objects – which had been employed with great force in order to deliver work and thus to modify their form – includes the transmission of conduction waves through their compacted mineralized texture, as well as the generation of reflection waves, emitted by the individual microscopic compaction zones, scattered throughout the object’s bulk, with harmonic contents different from that of the surrounding incompressible mineralized material, i.e., an impact signature that has been successively petrified.

Diagnostic Methodology and Technical Constraints

As is the case with most subjects that are studied in Archaeological and Geological research Fine Signal Homing’s data extraction process must attempt to filter a signal from noise. The subjects that are studied have been subjected to millions of years of noise. In the form of Earth’s crust movement, modern industry, and random thermal fluctuations. Specialized subterranean acoustic facilities are used for the data extraction process for FSH. Typically these enclosures are found in deep mine shafts, or specially constructed bunkers that are designed to be seismically decoupled from the surface.

In addition to the previously described noise reduction algorithms advanced noise-canceling protocols are implemented. With the help of so-called reference sensors, which are spread across the entire facility and detect the ambient noise, this noise is subtracted from the measured data that is acquired by the differential interferometry arrays. The goal is to achieve a sufficient signal-to-noise-ratio (SNR) for a phenomenological interpretation of the signals. After a clean signal has been acquired, the harmonic overtones of the sound are analyzed. These so-called overtones are the higher frequency components of a sound, which in combination give the sound its specific timbre or character. For FSH it is especially important to differentiate between natural (wind, water etc.) and anthropogenic (vocalizations, percussion etc.) sounds.

Implications for Acoustic Ecology

The ultimate objective of the Fine Signal Homing approach is to serve acoustic ecology, i.e. the study of interactions between species and their sonic environment. FSH uncovers sound traces in fossilized and archaeological remains of past environments and past human societies. Thus, we can read the social behavior of past human communities and their natural environments from sound remnants found in archeological stratigraphy. For example, a high concentration of highly frequent percussive signal traces in a particular stratigraphic layer may indicate the presence of an old gathering place or the location of a workspace where most of the people in the community were engaged in extremely manual and repetitive jobs.

By examining all evidence on site - even the minute - FSH is a non-visual archaeological method. It forms an interesting contrast to the traditional archaeology, which is based on all visual evidence. FSH explores a temporal and acoustic dimension of the past, which in traditional terms is mostly silent. The investigation of fine sounds of past sites and ecosystems and of early human cultures in development brings the latest acoustic physics and material sciences to a bearing on their use in the past. It is thus one of the most demanding, current archaeological sub-disciplines.