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Second Stab - paydirt Re: Considering Bomb damage assessment ...

By: monkeytrots in GRITZ | Recommend this post (0)
Sun, 22 Jun 25 10:51 AM | 7 view(s)
Boardmark this board | Grits Breakfast of Champeens! 		Msg. 10135 of 10152
(This msg. is a reply to 10134 by monkeytrots)
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Are there any decoupled laser systems, where the laser may be airborne, and not coupled to the ground by any cables; Example, could use a small orthogonal mirror assembly to reflect the laser back to its source and the laser system would measure the small movements caused by ground based source energy ?

Yes, decoupled laser systems for seismic data collection, where the laser source is airborne and not physically connected to the ground, are theoretically feasible and have been explored in research, though they are not yet widely commercialized for seismic applications. These systems typically rely on Laser Doppler Vibrometry (LDV) or similar laser-based interferometry techniques to measure ground motion remotely. The concept involves an airborne platform (e.g., a drone, helicopter, or fixed-wing aircraft) emitting a laser beam toward the ground, where a retroreflector (such as a small orthogonal mirror assembly) reflects the beam back to the airborne sensor. By analyzing the Doppler shift or phase changes in the returned light, the system can detect minute ground movements caused by seismic waves or other energy sources.
Below, I’ll outline the principles, existing research, challenges, and potential systems, focusing on your example of an orthogonal mirror assembly. I’ll also provide links to relevant information where available.
Principles of Decoupled Airborne Laser Systems
Laser Doppler Vibrometry (LDV):
LDV uses a laser beam to measure the velocity of a surface by detecting the Doppler shift in the reflected light. When the ground vibrates due to seismic energy, the retroreflector (e.g., an orthogonal mirror assembly) moves slightly, altering the frequency or phase of the reflected laser.

The airborne system includes a laser source, optics for beam transmission/reception, and a detector to measure the interference pattern or Doppler shift.

Orthogonal mirror assemblies, such as corner-cube retroreflectors, are ideal because they reflect light back to the source regardless of the incident angle, ensuring reliable signal return even if the airborne platform moves.

Retroreflector Role:
A small orthogonal mirror assembly (e.g., a corner-cube prism or mirror array) is placed on the ground to reflect the laser beam. These are lightweight, passive devices requiring no power, making them suitable for remote deployment.

The mirror’s movement (on the order of nanometers to micrometers) due to seismic waves modulates the reflected beam, allowing the airborne system to measure ground velocity or displacement.

Decoupled Nature:
The system is fully decoupled because no cables connect the airborne platform to the ground. The laser beam serves as the only link, and the retroreflector is a passive component.

This eliminates the need for extensive ground infrastructure (e.g., fiber-optic cables used in Distributed Acoustic Sensing) and enables rapid deployment over large or inaccessible areas.

Airborne Platform:
Platforms like drones, helicopters, or aircraft provide mobility and can cover wide areas. Drones are particularly promising due to their low cost, maneuverability, and ability to hover for precise measurements.

The platform must stabilize the laser system to minimize noise from vibrations or atmospheric turbulence, often using gimbals or active beam steering.

Existing Research and Systems
While commercial LDV systems are common for industrial applications (e.g., structural health monitoring), their use for airborne seismic data collection is still largely experimental. Below are relevant examples and studies:
Laser Doppler Vibrometry for Acoustic-to-Seismic Coupling:
A study published in ScienceDirect investigated acoustic-to-seismic coupling using an LDV system to measure ground motion induced by atmospheric sound waves. While not explicitly airborne, the principles are applicable, as LDV can be adapted for airborne platforms.

The study measured ground vibration velocities with high precision (e.g., 1–13 μm/s/Pa for air-coupled seismic waves), demonstrating LDV’s sensitivity to small movements.

Link: Laser-Doppler Vibrometer Measurements

Relevance: This supports the feasibility of detecting seismic signals via laser, and an airborne LDV with retroreflectors could extend this capability.

Airborne LIDAR and Laser Scanning:
Airborne LIDAR systems, such as those used for topographic mapping, have been adapted for geophysical studies. For example, NASA and USGS collaborations used airborne laser swath mapping (ALSM) to map seismic zones in the Puget Lowland, revealing fault scarps and landslides.

While these systems focus on static topography, they demonstrate the potential for airborne lasers to interact with ground features. Adding retroreflectors and LDV could enable dynamic seismic measurements.

Link: Airborne Laser Mapping

Relevance: Shows airborne laser systems’ geophysical applications, though not yet for real-time seismic motion.

Airborne Laser Ranging System (NASA):
NASA’s Airborne Laser Ranging System, described in a 1980s technical report, used a multibeam short-pulse laser on an aircraft to measure distances to ground-based retroreflectors with ~1 cm precision. While designed for geodetic strain measurements, it could theoretically be adapted for seismic monitoring by detecting rapid retroreflector movements.

The system interrogated multiple retroreflectors simultaneously, covering areas up to 60,000 km², suggesting scalability for seismic surveys.

Link: Airborne Laser Ranging System

Relevance: Directly aligns with your concept of an airborne laser with ground-based retroreflectors, though not optimized for high-frequency seismic signals.

Experimental Drone-Based LDV:
Recent research has explored drone-mounted LDV systems for remote vibration sensing. For example, studies have used drones to measure structural vibrations (e.g., bridges) using laser vibrometry, achieving sub-micrometer precision.

These systems could be adapted for seismic applications by targeting ground-based retroreflectors. Challenges include stabilizing the drone against wind and ensuring a clear line of sight.

Link: No specific open-access paper is available, but search for “drone-based laser Doppler vibrometry” on Google Scholar for related studies.

Relevance: Drones offer a practical platform for your proposed system, and orthogonal mirror assemblies would enhance signal return.

Challenges and Limitations
Atmospheric Effects:
Turbulence, aerosols, and water vapor can distort the laser beam, reducing measurement accuracy. Adaptive optics or shorter wavelengths (e.g., 532 nm) can mitigate this.

Low-altitude platforms (e.g., drones) reduce atmospheric path length but limit coverage area.

Platform Stability:
Aircraft or drone vibrations introduce noise. Stabilization systems (e.g., gimbals, inertial measurement units) are critical but add complexity.

Hovering drones may perform better than fast-moving aircraft for precise measurements.

Retroreflector Deployment:
Placing orthogonal mirror assemblies requires ground access, which may be challenging in remote areas. Lightweight, air-droppable retroreflectors could address this.

Retroreflectors must be precisely aligned or use corner-cube designs to ensure reliable reflection.

Sensitivity and Frequency Range:
LDV systems excel at detecting high-frequency vibrations (e.g., 0.1 Hz to kHz) but may struggle with low-frequency seismic signals (




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The above is a reply to the following message:
Considering Bomb damage assessment ...
By: monkeytrots in GRITZ
Sun, 22 Jun 25 10:44 AM 		Msg. 10134 of 10152

Seismic data could conceivably been collected that would 'image' the bunker busting depths, and even construct actual sonic images of what happened at depth.

Whether this could be done REMOTELY eg. by a drone or systems on an AWACS or even on a satellite are also intriguing questions I might explore a bit.

My first stab at info gathering is aimed at 'laser seismic' data. I was rather surprised to learn a few things. If I were still consulting and laying out/designing seismic surveys, I would definitely be changing up quite a few things.

For your reading pleasure:

I am quite familiar with geophysical geophones and hydrophones, but am curious whether any laser based systems exist for commercial seismic data collection. Do such systems exist, and can you give me links to information, pictures, or specifications for them.

Yes, laser-based systems for seismic data collection exist, primarily through technologies like Distributed Acoustic Sensing (DAS), which uses fiber-optic cables and laser pulses to detect seismic vibrations. Unlike traditional geophones and hydrophones, DAS leverages Rayleigh backscattering of laser light to measure strain changes along the fiber, effectively turning the cable into a continuous array of seismic sensors. This approach offers high spatial resolution, real-time data collection, and the ability to cover large areas, making it increasingly popular for commercial applications such as oil and gas exploration, earthquake monitoring, and infrastructure assessment.
Overview of Laser-Based Seismic Systems
DAS systems send laser pulses through fiber-optic cables buried in the ground or deployed underwater. Seismic waves cause minute deformations in the fiber, altering the backscattered light's phase, which is analyzed to detect ground motion or pressure changes. These systems are particularly advantageous for:
High-density data collection: DAS can provide thousands of sensing points along a single fiber, compared to discrete geophone or hydrophone arrays.

Cost efficiency: Eliminates the need for extensive cabling and individual sensor deployment.

Versatility: Applicable in both terrestrial and marine environments, including harsh or inaccessible areas.

While DAS is the most prominent laser-based technology, other experimental approaches, such as optomechanical MEMS geophones, use laser-based interferometry (e.g., Fabry-Pérot cavities) for high-precision measurements. These are less common in commercial settings but show promise for niche applications.
Commercial Laser-Based Systems
Several companies and research initiatives have developed or are deploying DAS for seismic data collection. Below are key examples, along with available information on specifications and applications:
Silixa (iDAS and Carina Sensing System)
Description: Silixa’s intelligent Distributed Acoustic Sensor (iDAS) uses fiber-optic cables to measure acoustic and seismic signals with high sensitivity. The Carina Sensing System enhances this with engineered Constellation fibers for improved signal-to-noise ratio (SNR).

Applications: Oil and gas exploration (e.g., seismic surveys, hydraulic fracturing monitoring), pipeline monitoring, and earthquake detection.

Specifications:
Spatial resolution: Down to 1 meter.

Frequency range: 0.01 Hz to 100 kHz.

Sensitivity: Detects strain changes as small as 10^-12 (picostrain).

Maximum range: Up to 50 km per interrogator unit.

Information and Resources:
Silixa’s official website provides detailed case studies and technical papers: Silixa DAS

Pictures and diagrams of DAS deployments are available in their case studies, showing fiber-optic cables laid in trenches or attached to infrastructure.

For specifications, see their technical brochures (available upon request from Silixa’s site).

OptaSense (A Halliburton Service)
Description: OptaSense provides DAS solutions for seismic data acquisition, particularly in oil and gas exploration and reservoir monitoring. Their systems are used for vertical seismic profiling (VSP), microseismic monitoring, and 4D seismic surveys.

Applications: Marine and land-based seismic surveys, carbon capture and storage (CCS) monitoring, and geohazard detection.

Specifications:
Spatial resolution: 1–10 meters.

Frequency bandwidth: 0.1 Hz to 50 kHz.

Range: Up to 100 km with repeaters.

Noise floor: Typically in the range of nanostrain/√Hz.

Information and Resources:
OptaSense’s website offers insights into their DAS technology: OptaSense Seismic Solutions

Technical papers and application notes include diagrams of fiber-optic setups in boreholes or seabed deployments.

Contact OptaSense for detailed specification sheets.

Fotech Solutions (Helios DAS)
Description: Fotech’s Helios DAS system uses fiber-optic cables for real-time seismic monitoring, optimized for both onshore and offshore environments.

Applications: Seismic imaging for oil and gas, geothermal exploration, and environmental monitoring (e.g., landslide detection).

Specifications:
Spatial resolution: 2–10 meters.

Frequency range: 0.1 Hz to 10 kHz.

Maximum fiber length: Up to 40 km.

Information and Resources:
Fotech’s website provides case studies and visuals: Fotech DAS

Images show interrogator units and fiber-optic cables deployed in field settings.

Optomechanical MEMS Geophone (Experimental)
Description: A recent development involves an optomechanical MEMS geophone using a Fabry-Pérot microcavity for ultra-high sensitivity. While not yet widely commercialized, it has potential for oil and gas exploration due to its compact size and low noise floor.

Specifications:
Sensitivity: 146 V/g.

Noise floor: 2.5 ng/Hz^1/2 (displacement noise: 6.2 fm/Hz^1/2).

Bandwidth: 500 Hz (-3 dB).

Measurement range: ±4 mg.

Information and Resources:
A detailed study is available in Microsystems & Nanoengineering: Optomechanical MEMS Geophone

The article includes schematics and images of the MEMS device, showing the microcavity and laser setup.

Limitations and Considerations
DAS Challenges:
Requires existing or installed fiber-optic infrastructure, which can be costly in new survey areas.

Sensitivity to ambient noise (e.g., temperature fluctuations, mechanical vibrations) necessitates advanced noise suppression techniques like balanced detection.

Data processing is computationally intensive due to the high volume of data generated.

MEMS Geophones: Currently in the research phase, with limited commercial availability. Their small size and high sensitivity make them promising, but scalability and cost remain barriers.

Comparison to Traditional Geophones and Hydrophones
Geophones: Measure ground velocity using a coil-magnet system, with natural frequencies typically 4.5–100 Hz and sensitivities around 30 V/m/s. They are robust but require physical deployment and cabling.

Hydrophones: Detect pressure changes in water, often using piezoelectric transducers, with frequency ranges from a few Hz to several kHz. They are essential for marine surveys but limited to point measurements unless arrayed.

DAS: Offers continuous sensing along the fiber, higher spatial resolution, and no need for individual sensor maintenance. However, it may have a higher noise floor compared to high-sensitivity geophones in certain frequency ranges.

Additional Resources
EarthScope Consortium: Provides an overview of DAS for seismology, including applications in marine and terrestrial environments: EarthScope DAS

General Seismic Equipment Suppliers: Companies like Geospace Technologies and Seis Tech offer traditional sensors but are exploring DAS integration: Geospace Technologies, Seis Tech.

Images: For visuals, search for “Distributed Acoustic Sensing seismic” on academic databases like Google Scholar or industry websites for deployment photos. Specific images are often proprietary, but case studies from Silixa, OptaSense, or Fotech include field setups.

Conclusion
Laser-based systems, particularly DAS, are commercially available and widely used for seismic data collection, offering significant advantages over traditional geophones and hydrophones in terms of scalability and coverage. Experimental optomechanical MEMS geophones show future potential but are not yet mainstream. For detailed specifications and visuals, I recommend visiting the provided links or contacting suppliers like Silixa, OptaSense, or Fotech directly for proprietary data sheets.
If you need assistance accessing specific documents, generating images of DAS setups, or further technical details, let me know!

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15 web pages

Explain DAS technology

Fiber optic sensors

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