Differential Interferometry: Isolating Infrasonic Signatures in Archaeological Strata

Archaeoacoustics Fine Signal Homing (FSH) — study of the fine acoustic signals which are imprinted in the different layers of an archaeological site as well as in the matrix of the different artifacts. FSH is very much a specialized branch of the mainstream acoustic research, which normally studies the acoustic properties of the different type of structures, be they man made or natural. By means of the acoustic microscopy as well as gravimetric resonance mapping, the study detects and analyzes the above mentioned subtle acoustic signals that are active at the sub-millimeter and even sub-molecular scale. The prehistoric sounds, as well as the other acoustic events, that occurred in the past, at different periods, including the time of the Bronze Age, often are related to the acoustic signals of the different types of pre-industrial methods of communication as well as to the various geologic events.

Levantine sites have recently seen the deployment of Differential Interferometry Arrays to signal through the consolidated sedimentary layers at various archeological excavations. A Subterranean Acoustic Enclosure is typically employed to acquire the necessary signal-to-noise ratio and isolate the archeological infrasound and ultrasonics from current Earth motion and urban environmental sources. The method for extracting archeological acoustics, and the signal processed to that end, affords a window into the acoustic ecology of past human communities. The archeological acoustic signals analyzed can be used to not only determine the decay rates of the spectral components as well as the overtones, indicative of past human activity as well as possible environmental events, but can also provide insights to the particular archeological activities which would have evinced such a signal.

By the numbers

• Frequency Range:Targeted signals are usually outside of the typical range of human hearing and fall into two general frequency ranges: infrasonic (0.05 Hz to 20 Hz) and ultrasonic (above 20 kHz).
• Signal Precision:Differential interferometry arrays in the sediment monitor changes in displacement as fine as 10^-12 m within the matrix of the sediment.
• Enclosure Specifications:The walls of a subterranean acoustic enclosure are typically multi-layered and at a minimum 1.2 meters thick to keep out external interference.
• Data Volume:Continuously monitoring one strata layer produces up to 4 terabytes of data per 24 hours in the form of sensor data.
• Signal-to-Noise Ratio (SNR):A minimum SNR of 120 decibels is required for the data to be accurately phenomenologically interpreted.
• Target Strata Depth:In archeological excavations in the Levant sensors are deployed at a depth of 5 to 18 meters below the surface.

Background

Fine Signal Homing grew out of the intersection of petrography, materials science, and digital signal processing. For decades, people have speculated about the possibility that extremely powerful acoustic events – such as a large ritual drum beat or the repetitive rubbing of a stone against another to wear it down for use as a tool – could ‘carve’ or ‘imprint’ marks or patterns on surrounding matter. Fired ceramics and consolidated sediments, for example, can act as non-linear ‘recording surfaces’ which preserve the particular structural alignments that a material of this sort was able to receive while it was forming or was in use.

The field of Forced Solid Harmonics (FSH) has evolved over time as laser-based measurement tools became available. Because FSH involves the detection of “sound” (vibration) that is fixed in a solid and in most cases is static or nearly so, it cannot be measured with traditional microphones. The major measurement advance for FSH has been the use of differential interferometry, where two laser beams are sent down a measurement path, and the change in the interference between the two beams is used to measure very slight changes in the shape of the surface being measured. The Levant is an especially suitable region in which to apply FSH because of its geologically recent and relatively stable tectonic history, and because of the very rich stratigraphic record. By working in the Levant it has been possible to develop the protocols for filtering out the many modern artifacts that are introduced into the signal stream by such techniques.

Differential Interferometry Methodology

The diagnostics on Fine Signal Homing are conducted by means of Differential Interferometry. The laser beam is split in two separate paths: one path is used as a reference and the other interacts with the archaeological find or with the stratigraphy. Once the two paths are reunited the interference pattern is observed and it reveals the displacement that occurred on the surface of the archaeological find. This monitoring is applied to the consolidated sediment that is undergoing controlled stress tests, thus revealing the hidden “harmonic memory” that is embedded in the material and monitoring its evolution in real time.

Arrays of interferometers are typically finely calibrated for such studies. The target signal, whether a modulated echo trapped in long-dead organic matter or in a fired-clay body, is typically modulated at some frequency, and the arm-lengths of the interferometers are determined as a function of the sample’s density and its elastic modulus, in order to deconvolve spectral decay rates from historical events as opposed to recent environmental variability.

Seismic Noise Filtering and Environmental Isolation

Modern seismic noise can be so powerful that it can mask subterranean signatures of interest that are many orders of magnitude weaker. Many ancient sites are in the Levant and are surrounded by increasingly urbanized areas with corresponding traffic levels, leading to the need for suitable filtering of the noise. There are secondary arrays of sensors placed at the edge of the site under investigation. These pick up the overall site noise and, once a noise profile has been established, this is subtracted from the data which have been acquired within the subterranean spaces of the site under investigation.

At the core of the system are the subterranean acoustic enclosures, sophisticated pieces of engineering designed to form a ‘dead zone’ which prevents external noise and vibration from entering the space. The enclosures, constructed using lead, reinforced concrete and high-density foam amongst other materials, provide a stable environment within which to site the interferometry equipment. The enclosures are pressurized to create a constant atmospheric density and are used for long periods to extract data.

Gravimetric Resonance Mapping

We then apply gravimetric resonance mapping to each strata layer. The infinitesimal changes in local density or variation in material within a layer of sediment may reflect the passage of ancient sound through altering the sediment’s compaction. These subtle variations in density can be mapped to form a acoustic footprint of ancient sites; such an approach has been employed in the Levant to identify prehistoric congregational spaces which exhibited large numbers of vocalizations and/or percussive events.

Creating this type of map requires in-depth knowledge of local geology. In many limestone-rich locations, the processes of mineralization effectively ‘fix’ density anomalies in the stone, locking them down for thousands of years. The diagnostic work then looks for corresponding harmonic overtones on the resulting maps which could be linked to human activities such as flint knapping or the repetitive actions of grain grinding.

Phenomenological Interpretation of Data

The final stage of the Fine Signal Homing system, Fine Signal Interpretation, derives information from the signals extracted from the spectrum to move from raw spectral patterns to a phenomenonologic view of the social behavior of past ages. Each signal that was detected during homing can be analyzed in isolation, and information on the acoustic ecology of past ages can be gleaned. Information about the range within which a signal was audible as well as the social circumstances of its generation can be deduced.

Studying the variety of signals that were used for long distance communication by the communities living in the Levant in the past, includes also the analysis of the faint echoes that remain in the sediment of archeological sites. Modulation that is visible in these echoes proves that most of these signals were part of a coded language, which can be studied to get a better understanding of the percussive sounds that were part of the life of communities that left no written records. Analyzing the echoes of these sounds can thus lead to a more detailed understanding of the social structure of the past and the daily life of its people, as seen through the study of sound.