The Evolution of Differential Interferometry in Subterranean Acoustic Enclosures

Fine Signal Homing (FSH) is a subdiscipline of the study of archaeoacoustics which seeks to detect and isolate a site’s residual sound within the rocks and the archaeology itself. Homing within this context is made possible by the fact that all physical matter – whether compacted into a solid or left to decompose – can be affected by high-energy or extended acoustic impulses in minute, physical ways and then read by the archaeoacoustician, whether that matter is consolidated sediment, ceramics or long-dead, petrified organic. Fine Signal Homing uses Differential Interferometry via arrays of interferometers placed within subterranean acoustic enclosures which are specifically designed to counteract the effects of current environmental interference in order to uncover the residual Fine Signals at a site.

The field of Field Seismology / Archaeoacoustics has gone through tremendous technological advances from the analog seismometry used in the late 20th century to the current state of the art high-resolution digital interferometry, allowing for the unique identification of specific decay rates of the spectral peaks and their respective harmonic overtones within the seismic signal, which could not be distinguished from background seismic noise in the past. In current Field Seismometry Hydrophone protocols (FSH) an emphasis is placed on the signal-to-noise-ratios (SNR) in order to assess whether a recorded seismic resonance is of geological origin or consists of residual acoustic activity of human origin (sleigh bells, drumming, friction of prehistoric tools etc.).

Timeline

• 1982–1989:Analog seismometry has been used by researchers in archaeological contexts. Early attempts at the detection of rhythmic patterns in deep-strata excavations were made using converted geophones, although the quality of the data was frequently confounded by mechanical noise.
• 1994:Laser vibrometry is introduced as a tool to study fired ceramics. It is the first time that high-frequency surface vibrations of a product caused by production can be isolated and measured.
• 2005:Development of the first specialized subterranean acoustic enclosures to test gravimetric resonance mapping in the field.
• 2012:Move from analog to digital interferometry arrays. Monitor at multiple frequencies simultaneously across a 3D grid of locations.
• 2018:Deep Earth Excavation Noise-Canceling Protocols Technical Standards Publication which establishes a set of environmental isolation technical standards required to verify a signal.
• 2021–Present:Utilize cutting-edge differential interferometry arrays to measure the infrasonic and ultrasonic echoes in fossilized organic matter with precision on the sub-millisecond scale.

Background

Fine Signal Homing (Fine SH) is based on a theoretical framework drawn from acoustic microscopy and the physical behavior of waves in solids. When a sound wave hits a material it creates mechanical stress within that material. Certain materials — for example those that have chemically stabilized or thermally hardened, such as fired clays or petrifying woods — are so affected that they undergo very small, yet detectable changes in the material’s matrix. These changes, which over time become fossilized, remain for millennia, until they are retrieved by the most sensitive of homing instrumentation as what are known as ‘acoustic fossils’.

Differential interferometry is used for the detection of movements on surfaces of artefacts as well as on a fixed sediment wall, which is stabilized by laser beams. The split laser beam is focused on the surface and the smallest movements down to the size of the wavelength of light are recorded and are subject to periodic variations. These periodic variations are resolved by means of Fourier analysis using so-called Fourier transform. The thus detected frequencies are then to be clearly distinguished from Earth’s constant movements and from movements, which are induced by industrial activities.

The Evolution of Differential Interferometry

Analog Seismometry and Early Limitations

During the 1980s the detection of sounds during archaeology was greatly limited by current technology. First of all there was the use of analog seismometry which generally employed mechanical transducers. These mechanical devices converted ground motion into an electrical signal. As previously stated seismometry was invented to record very large geological events such as earthquakes caused by tectonic shifts in the Earth’s surface or volcanic eruptions. It is therefore no surprise that the signals generated by such events are generally a thousand times or more greater than the modulated signals searched for by the archaeoacoustician and are easily detected against a noise free background. However when these sensitive mechanical transducers are used to monitor the ground motions caused by the sounds of interest the case is very different. Generally the noise floor generated by the sensing system itself is greater than the signal being searched for.

Storage of the data encountered another major problem in the following years. Magnetic tapes were used for storing the data. Those tapes exhibited wow and flutter (frequency variations). Only a few researchers found some rhythmical structures in late 20th century studies, but most of them were dismissed as artifacts of the used recording equipment for the archaeoacoustician. The change to digital systems in the late 1990s even started to solve this problem by offering higher dynamic ranges and nearly perfect signal reproduction.

The Digital Transition and Array Processing

The use of digital interferometry arrays in the 2020s was a game changer for all involved. Unlike a single sensor (e.g. a geophone) that detects a certain amount of motion an array of dozens to hundreds of synchronized detection points allows for spatial filtering. The only part of the arriving vibration that is registered is that part that arrives at a certain number of sensors a certain number of cycles earlier than the rest of the sensors. A computer can then figure out the angle and the distance of the source of that signal. This is particularly important for Fine Signal Homing where all kinds of other signals are arriving from the surface (traffic, weather, etc.) and the archaeologist’s signal is arriving from within the individual archaeological strata. All of these surface signals can be mathematically ‘subtracted’ from the signal from the individual strata.

State-of-the-art arrays are now equipped with fiber-optic displacement sensors that are not affected by electromagnetic interference. They can read displacements at the picometer level. High-end gravimetric resonance mapping that is able to precisely measure the local gravity field is now even integrated into the system. This allows for precise interferometry and for the data acquired in the medium under investigation to be precisely calibrated.

2018 Technical Standards for Noise-Canceling

With increasing sensitivity of the FSH equipment also the requirements for an appropriate environmental control have strongly increased. In 2018 a consensus of international research groups has been found to establish noise reducing procedures for deep excavations. The implemented subterranean acoustic enclosures (SAEs) are normally reinforced rooms which are covered with sound absorbing materials. These SAEs are decoupled from the surrounding earth by active vibration isolation platforms.

The 2018 standards for SAE internal environments establish a noise floor of less than -120 dB compared to 1 m/s² over the frequency range of 0.1 Hz to 100 kHz. A combination of measures are required to achieve this level.

1. Passive Isolation:Use of physical barriers and mass- action to be effective against high-frequency airborne and structural noise.
2. Active Cancellation:Computer-controlled actuators generate “counter-vibrations” as needed to neutralize low-frequency seismic vibrations in real-time.
3. Atmospheric Control:Temperature and humidity in the enclosure is kept constant to prevent thermal expansion/contraction of the measuring sensors as well as the samples.

Adherence to these environmental health measures has become mandatory for publication of FSH data in scientific journals to avoid confounding signals caused by contemporary environmental pollution.

Sensor Sensitivity: Sediment vs. Petrified Organic Matter

One of the Fine Signal Homing system’s greatest challenges is to fine-tune the sensors to different kinds of matter. The acoustic impedance of a material, which is to say its opposition to the propagation of sound, varies greatly between consolidated sediment and petrified organic matter. In consequence, the sensitivity of the sensors and the settings for the interferometry have to be altered.

Consolidated Sediment

Consolidated sediment, such as densely packed clay or silt, acts as a low-pass filter. It tends to absorb high-frequency sounds quickly while allowing low-frequency (infrasonic) vibrations to persist. Detecting signals in sediment requires sensors with high mass and high sensitivity to low-frequency displacements. Because sediment is relatively porous compared to stone, the signal is often "smeared" over a larger area, necessitating the use of wide-area arrays to capture the full waveform.

Petrified Organic Matter

In contrast, petrified organic matter—such as fossilized wood or bone—behaves more like a crystalline structure. When organic matter undergoes mineralization, it can capture and preserve high-frequency harmonic overtones and spectral decay rates. These materials are excellent conductors of sound but are also highly resonant. Sensors used for petrified matter must have a much higher sampling rate (often exceeding 500 kHz) to capture the delicate echoes of vocalizations or percussive signaling. The rigidity of the petrified matrix allows for much tighter focus in the interferometry array, enabling researchers to pinpoint the exact location of the sonic remnant within the artifact.

Diagnostic Methodology and Interpretation

The core of Fine Signal Homing is the identification of characteristic spectral signatures. Every sound leaves a unique imprint based on its source. For example, the friction caused by a stone tool being shaped (lithic reduction) produces a sharp, high-frequency spike followed by a specific harmonic decay. In contrast, human vocalizations produce complex, multi-layered waveforms with distinct resonant peaks.

Researchers use phenomenological interpretation to connect these signals to ancient social behaviors. By analyzing the acoustic ecology of a site—how sound behaved in a specific cave or dwelling—they can infer how ancient communities communicated or utilized their environment. This process is highly dependent on the accuracy of data extraction. If the signal-to-noise ratio is too low, the nuances of the harmonic overtones are lost, potentially leading to the misidentification of a tool-use sound as a natural geological event. Therefore, the continued advancement of noise-canceling technology and interferometry precision remains the primary focus of the field.