Geophysical Acoustics Research

Report
Geophysical Acoustics Research
Acoustics of Geomaterials
Linear and Nonlinear
Behavior of Soils
Buried mine
detection
Tunnel
detection
Agrogeophysics
Dam and Levee
Monitoring
Standoff
photoacoustic
detection
Non-contact standoff
vibration detection
The Porous Media group research focuses
on the interaction of sound with the ground
surface, near surface geophysics, and
laser based remote vibration
measurements. Current work include linear
and nonlinear acoustic behavior of soils,
agrogeophysics, dam and levee
monitoring, seismic tunnel detection,
detection of buried mines, application of
multiple beam laser Doppler vibrometers,
and standoff detection of explosives.
James Chambers, PhD
(Mechanical Engineer)
Rick Burgett
(Electrical Engineering)
Craig Hickey, PhD
(Geophysics)
Students
Slava Aranchuk, PhD
(Electrical Engineer)
Zhiqu Lu, PhD
(Physics)
Fundamental Acoustic Behavior of Soils
Soil is the controlling interface of the Critical Zone
(CZ), defined as the Earth's boundary layer from the
vegetation canopy to the aquifer bottom. This
interface is not only the media for food and fiber
production but controls the exchange of nutrients
and contaminates between the atmosphere, surface
waters and ground waters as well as the storage
and biochemical transformations of those
chemicals. Soil is highly complex and spatially
variable. Typical soil profiles are comprised of layers
with distinct soil properties. It is these layers that
define a soil and its impact on the CZ.
Less invasive and more field expedient techniques has led to the investigation of several
high resolution geophysical methods for studying soil properties. The tradeoff associated
with using geophysical and remote sensing techniques is that the measurements are
sensitive to the distribution of the bulk “geophysical” properties and relationships
between these properties and the more “basic” properties must be determined.
The goal of this research is to determine the relationship between acoustic
properties of soils and more traditional properties during various physical
processes such as compaction and weathering.
Agrogeophysics
Upland soil erosion control is essential for sustainable
agricultural production systems because erosion affects
soil properties progressively over time and generally
results in decreased soil quality and reduced resistance
of agricultural systems to stresses. Addressing both
on-site processes and effects at the field and hill slope
scale, and off-site impacts at the larger watershed scale
are of interest. Key factors in understanding these
processes are measurement and characterization of the
mechanical and structural properties of soils. Most
current techniques are not easily adaptable for in-situ
measurements, and are often prohibitively expensive.
The goal of this research is to provide alternative methods for characterization of the soil
profile, mechanical and hydraulic properties, surface roughness, and soil crusting. The
associated system will allow for rapid, portable, in-situ measurements of the spatial and
temporal variability of soil parameters to assist soil scientist solve problems facing the
agricultural sector.
Earthen Dam and Levee Assessment
According to the National Dam Inventory (NID,
2009), there are more than 85,000 dams in the
United States. These watershed projects
represent a $14,000,000,000 infrastructure,
providing flood control, municipal water supply,
recreational opportunity, and wildlife habitat
enhancement. The failure of these dams and
associated release of the impounded water can
result in significant damage to crops, livestock,
and possible loss of life.
Rapid assessment of the potential failures in levee and earthen dam requires advanced
screening tools to delineate, classify, and prioritize compromised locations. Numerous
geophysical techniques being considered for the assessment of the condition of
embankment dams and levees. With increasing urbanization and critical infrastructure
in flood prone areas there is a growing number of high hazard dams. Advances in low
power sensors, wireless communication, and the internet makes continuous monitoring
of high hazard dams a viable option.
The goal of this research is to develop, adapt and evaluate, and integrate noninvasive geophysical methods and complementary modeling efforts, in support
of a comprehensive earthen dam interrogation and monitoring program.
Seismic Tunnel Detection
The Custom and Border Patrol (CBP) are
responsible for the safeguarding of more
than 7500 miles of the U.S. border. The
development of technology for assisting
CBP in the detection and surveillance of
cross border tunnels is part of the mission
of the Department of Homeland Security
(DHS) Science and Technology (S&T)
Directorate’s Borders and Maritime Security
Division (BMSD).
Despite the large contrasts associated with tunnels they do not produce easily
detectable geophysical anomalies. The reasons include: 1. the size of the tunnel
with respect to the resolution of the geophysical technique, 2. the often irregular
shape, and 3. the heterogeneity of the surrounding native material itself that can
produce a significant number of anomalies. Regardless, geophysical techniques
remain the only ways to remotely and non-destructively sense the earth’s near
surface and as such have the most promise for rapid and accurate detection of
clandestine tunnels.
This research focus is to develop acoustic/sesimic technology for detecting
and locating tunnels with greater confidence and over a broader range of
tunnel scenarios, geological settings, and environmental conditions.
Sediment Transport
Sediments are a global-scale pollutant whose
yield has been estimated at 20 billion tons per
year. Water quality is impacted by the
presence of suspended sediment and is further
reduced when anthropogenic pollutants,
bacteria, or heavy metals are present. The
results of corrective actions on fluvial systems,
assessment of soil erosion losses, and
reservoir sedimentation can be determined
using suspended and bedload sediment data.
Accurate measurements of sediment flux are difficult to obtain since sediment load is
highly variable in both time and space. In many streams, the majority of sediment
moves during flood events caused by a few large storms per year making the
collection of physical sediment samples difficult and sometimes dangerous. Bedload
sediment transport measurements, suffer from some of the same difficulties as
suspended sediment monitoring, namely poor spatial and temporal resolution and
safety to personnel.
The goal of this research is to develop acoustic hardware and measurement
techniques for non-experts to improve the monitoring of suspended and
bedload sediment transport.
Laser Acoustic Detection of Buried Landmines
Vibration image of a
buried mine obtained
by shearography
The detection and neutralization of landmines is an important
task because of military, humanitarian, and environmental
impacts. Metal detectors, the most commonly used devices
for landmine detection, suffer from inability to detect plastic
mines and differentiate a real mine from metallic debris.
One of the new most successful landmine detection
methods uses low frequency acoustic waves in the ground.
The method is based on excitation of mechanical vibration of
the ground, using acoustic or seismic waves, and sensing
vibration of the ground surface in many points remotely with a
laser vibrometer. A buried landmine can be detected by an
abnormality in the vibration image of the ground surface.
Performance of laser Doppler vibrometers currently used in
acoustic landmine detection is affected by ambient vibration.
They require operation from a mechanically stable platform,
that seriously limits application of the acoustic method.
The goal of this project is to develop laser interferometric
sensors for the stand-off rapid measurement of vibration
fields of the ground, which have low sensitivity to ambient
vibrations.
Standoff Photoacoustic Detection of Solid Residue
of Explosives and Other Chemicals
Photoacoustic spectrum of TNT
obtained at standoff distance
Explosives detection is a very important task for National
Security. Detection of trace amounts of explosives has
become increasingly important for detection of improvised
explosive devices (IEDs), and other terrorist devices. The
most generally accepted methods for detecting trace
explosive detect them in the gas phase. Unfortunately,
conventional nitro-carbon explosive chemicals have very
low vapor pressure, that presents a great challenge for their
standoff detection.
Photoacoustic (PA) spectroscopy has proven to be a very
sensitive technique for the detection of trace amounts of
solid chemical compounds. However, traditional PA
spectroscopy requires putting a sample in a photoacoustic
cell, that makes the method unsuitable for standoff
applications.
The goal of this research is to develop laser interferometric
methods and sensors for sensing PA spectroscopic
signatures of trace amounts of solid explosives and other
chemicals at a safe standoff distance.

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