Extreme Endeavors is collaborating with NASA Langley Research Center (LaRC) in the development, testing and analysis of infrasonic detection system under a Space Act Agreement. Acoustic studies of atmospheric events like convective storms, shear-induced turbulence, acoustic gravity waves, microbursts, hurricanes, and clear air turbulence (CAT) over the past thirty years have established that these events are strong emitters of infrasound. Recently NASA Langley Research Center has designed and developed a portable infrasonic detection system which can be used to make useful infrasound measurements at locations where it was not possible previously, such as a mountain crag, inside a cave or on the battlefield. The system comprises an electret condenser microphone, having a 3-inch membrane diameter, and a small, compact windscreen. Extreme Endeavors will present the findings from field testing using this portable infrasonic detection system. Field testing of the infrasonic detection system was partly funded by Greer Industries and support provided by the West Virginia Division of Natural Resources. The findings from this work illustrate the ability to detect structure and other information about the contents inside the caves. The presentation will describe methodology for utilizing infrasonic to locate and portray underground facilities

Burkett, Cecil G.

Masterman, Michael

Shams, Qamar A.

Stihler, Craig

Wallace, Jack

Zuckerwar, Allan J.

NASA Technical Reports Server

 2 
 
 
 
Collection of Infrasonic Sounds from Sources of Military 
Importance 
August 18, 2008 
 
Michael Masterman 
Extreme Endeavors, 492 Hickory Corner Road, Philippi WV  26416 
 
Qamar A. Shams and Cecil G. Burkett 
NASA Langley Research Center, Hampton, VA 23681 
 
Allan J. Zuckerwar 
Analytical Services and Material, Hampton. VA 23681  
 
Craig Stihler and Jack Wallace 
West Virginia Division of Natural Resources, Elkins, WV  26241 
 
Abstract  
 
Extreme Endeavors is collaborating with NASA Langley Research Center (LaRC) in the development, 
testing and analysis of infrasonic detection system under a Space Act Agreement. Acoustic studies of 
atmospheric events like convective storms, shear-induced turbulence, acoustic gravity waves, 
microbursts, hurricanes, and clear air turbulence (CAT) over the past thirty years have established that 
these events are strong emitters of infrasound [1]. Recently NASA Langley Research Center has designed 
and developed a portable infrasonic detection system which can be used to make useful infrasound 
measurements at locations where it was not possible previously, such as a mountain crag, inside a cave or 
on the battlefield. The system comprises an electret condenser microphone, having a 3-inch membrane 
diameter, and a small, compact windscreen.  
 
Extreme Endeavors will present the findings from field testing using this portable infrasonic detection 
system. Field testing of the infrasonic detection system was partly funded by Greer Industries and support 
provided by the West Virginia Division of Natural Resources. The findings from this work illustrate the 
ability to detect structure and other information about the contents inside the caves. The presentation will 
describe methodology for utilizing infrasonic to locate and portray underground facilities. 
 
Approved for public release, 
distribution is unlimited 
https://ntrs.nasa.gov/search.jsp?R=20090012596 2019-08-30T06:28:52+00:00Z
 3 
 
 
1.  Introduction  
 
Extreme Endeavors has collaborated with NASA LaRC in its Innovative Partnership Program to perform 
field testing using NASA Langley’s portable infrasonic detection system. The primary aspect of Space 
Act Agreement (SAA789-1) was to detect infrasonic signals at the entrance and deep inside limestone 
caves of similar structure to those found in Afghanistan. The secondary research goal was to investigate 
other possible applications mentioned in the literature over the past thirty years. The support for field 
testing of the portable infrasonic detection system was provided by Greer Industries while site selection 
and access was provided by West Virginia Division of Natural Resources. 
 
The portable infrasonic detection system developed at NASA Langley differs from that of a conventional 
infrasonic detection system in that the peculiar features of infrasound are taken into account. First, 
infrasound propagates over vast distances through the Earth’s atmosphere as a result of very low 
atmospheric absorption and refractive ducting that enables propagation by way of multiple bounces 
between the Earth’s surface and the stratosphere. A second property that has received little attention is the 
great penetration capability of infrasound through solid matter – a property utilized in the design and 
fabrication of the system windscreens.     
 
Extreme Endeavors has recorded and analyzed infrasonic measurements at such locations where it was 
not previously possible. Some of the measurements are presented in this paper.  
 
2. Laboratory Testing 
 
The air pressure at the entrances of caves changes due to pressure cycles created in the environment, 
forcing air into and out of the underground facility. While variations of this draft occur throughout the day 
based on many factors, it is impossible to turn off the air flow into or out of a naturally formed cave 
without drastic measures.  To imitate what should be seen in the field, testing was performed with a 
building ventilation system. The size of the room, with multiple vents, is approximately the size of the 
entrance to schoolhouse cave, one of our test subjects.  This allowed us to switch the airflow on and off, 
and see the effect on the sensor.  This was critical to understanding what the readings meant and the 
ability to repeat a standard measurement to insure the instrumentation was working properly throughout 
the duration of our research.      
 
We found the low frequency noise created by the ventilation system, shown in Figure 1, to be similar to 
the low flow of air being pushed into and out of a cave.  Two data sets were taken, each for a three-minute 
interval.  The first data set was taken with air circulating through the vents and the second data set taken 
with no air circulation.   
 4 
 
                 
Figure 1:  Infrasonic Measurement of Air flow into a Large Room 
For further analysis, an FFT of this data was calculated and is shown in Figure 2.  This shows a 
substantial increase in noise power in the frequency range of .6 to 5 Hertz.  There may be several spectral 
lines embedded in this noise; we can definitely see spectral lines at 8.8 Hertz and 6.4 Hertz.  Overall 
increment of spectral noise power can be seen around 8 Hertz. 
 
 
 
                 
 
 
 
3. Schoolhouse Cave   
 
The first cave to be sampled was Schoolhouse Cave (See figure 3).  This cave has a large entrance that is 
gated by several bulky horizontal laid pieces of angle iron.   
Figure 2:  FFT of Infrasonic Data from Building Ventilation System 
 5 
 
 
The Schoolhouse cave data set is shown in Figure 4.  The FFT of this data shows the noise quickly 
damping out around .5 Hertz, but returning around 1 Hertz, and leads to a similar noise structure as seen 
by the air flow through the building ventilation system.  This noise has an increased power spectral 
density around 1.8 Hertz, 5 and 6.5 Hertz and spectral lines at 8.7 Hertz and 3.7 Hertz.   
 
 
 
 
 
 
Figure 4:  Infrasonic Data Taken From the Entrance of Schoolhouse Cave 
 
 
Figure 3: Entrance to Schoolhouse Cave and 
Infrasonic Detector
 6 
 
4. Cass Cave  
 
Cass Cave was selected as another test subject due to having a medium size entrance (about the size of a 
standard door); however, there is a stream flowing into the entrance which may affect the data.  The 
stream travels approximately 1/2 mile underground where it falls 180 feet.  This cave is far removed from 
civilian activity which could create interference to our test as this cave is only accessed by traveling a 
significant distance into a ravine.  This entrance is pictured in Figure 5.     
 
The data recorded from the entrance of Cass Cave is shown in Figure 6.  In the FFT we find considerably 
more noise than in other samples from caves; we attribute this to the small waterfall going into the 
entrance of this cave.  The acoustical signature of Cass Cave appears to reduce in noise at about 3.7 Hertz 
and we are able to see spectral lines at 3.3 Hertz and 7.4 Hertz. 
 
 
  
 
 
Figure 6: Infrasonic Data Taken from the Entrance of Cass Cave 
 
Figure 5:  Cass Cave Entrance 
 7 
 
5. Cassel Cave 
 
Infrasonic data sampled from one of the four entrances of Cassel Cave, known as the Windy Entrance, is 
shown in Figure 7.  This entrance is the smallest of entrances sampled among our initial group.  As shown 
in the picture, any object larger than the sensor and windscreen would have difficulty entering the 
entrance.  While most military operations would not use an entrance this small, this shows the data that 
would be acquired with an entrance that was concealed. 
 
The infrasonic recording from Cassel Cave, shown in Figure 8, illustrate clearly visible spectral lines at 
specific frequencies (9.3 Hertz and .9 Hertz), noise which is filtered between two and three Hertz, and is 
much more gradual of a roll off than other caves sampled. 
 
 
 
 
 
Figure 8:  Infrasonic Data Taken from the Windy Entrance of Cassel Cave 
Cave
Figure 7:  Cassel Cave; Windy 
Entrance 
 8 
 
 
6. Summary of all three Caves 
 
The primary reason for selection of the three caves was the difference in geometry and physical size of 
the entrances.  An FFT and a power spectral density of all three caves is shown in Figure 9 and 10 
respectively. The data illustrates that the noise made from the draft sound is dependent on the size of the 
cave entrance.   
 
 
 
Figure 9:  Frequency Domain of Infrasonic 
Signals Received from the Three Caves in 
our Test 
Figure 10:  Power Spectral Density of the 
Infrasonic Signal Received from the Three 
Test Caves 
 
7. West Virginia National Guard Memorial Tunnel 
 
With dependency of noise spectrum on entrance size as shown in Figure 9 & 10, we tested an 
underground facility that was more uniform in shape and not a naturally formed cave.  This would be 
similar to a mine or a bunker that was dug to hide equipment and personnel.  The West Virginia National 
Guard Memorial Tunnel, used for military and first responders training is 2800 feet in length. The tunnel 
has two-lane highway, 23 feet wide with a ventilation system and doors at each end.  With no traffic and 
the ability to control the air flow we found this tunnel to be an ideal test subject.  
 9 
 
 
 
 
8.  “My” Cave  
 
Infrasonic measurements from “My” cave, located in Randolph County, West Virginia were taken from 
both inside and outside of the cave.  The FFT of the data, shown in figure 13, shows significantly more 
power than any other cave.  This warranted us to venture into the cave and take data approximately ¼ of a 
mile underground.  The readings found inside of “My” cave saturated the sensor such that the gain had to 
be reduced.   
 
“My” cave is well known for a river passage about ¾ of a mile into the cave.  The sensor was located in a 
large fissure passage (approximately 50 feet high but very narrow).  This fissure passage connects to the 
river passage to where the sensor was located.  The smaller size of the entrance (six-foot-diameter hole in 
the ground) was acting as a filter, similar to the effect seen in the West Virginia Memorial Tunnel.  This 
effect is similar to that seen by someone trying to conceal a cave or underground facility. 
 
Figure 11: Infrasonic Measurement at the 
tunnel entrance 
Figure 12: Infrasonic Measurement Inside 
the tunnel  
 10 
 
 
 
9. Simmons Mingo Cave System  
 
The next series of measurements was completed using Simmons Mingo cave system which is one of the 
most hazardous and difficult caves, with several miles of passages that include significant vertical drops, 
pits, and large chambers.  This testing involved taking infrasonic measurements on the outside of the 
cave, down a forty-foot drop and approximately ¼ of a mile back in the cave.  Figure 14 shows the FFT 
of the measurements. Both readings were taken inside the cave only minutes apart.  This revealed two 
different spectrums for the same location. Two sections of Figure 14 are highlighted to show these 
differences.      
 
We can explain this different spectrum as the sound of turbulence or pockets of varying temperature air 
being expelled from the cave.  Extreme Endeavors provides environmental cave monitoring for Greer 
Lime. This monitoring has shown pockets of warm air being pulled out of Schoolhouse cave through 
observation of the temperature and pressure of the caves passages.  Special instrumentation developed by 
Extreme Endeavors is used to perform this measurement, with extremely precise and high resolution 
sensors developed specifically for monitoring of underground environments.    
 
A smooth structure such as in a mine or manmade underground facility will show fewer warm and cold 
air pockets being expelled during the underground facilities expulsion of air.  We can therefore conclude 
that even, straight manmade structure will have a more stable the spectrum than a cave since there would 
be less turbulence. 
  
 
 
Figure 13: “My” cave FFT of Infrasonic Data,  
Note: the gain difference in the two data sets by a factor of ten 
 11 
 
 
10. Caves of Biological Importance  
 
Extreme Endeavors has performed testing for the West Virginia Division of Natural Resources to sample 
noise levels from different caves around the state. These caves are of significant importance due to 
endangered species of bats either roosting or using the caves as a hibernacula.   This data was taken as a 
baseline for performing comparisons between bat populations and the sounds created from the caves.  
This research is ongoing, with the infrasonic readings from approximately 20 caves around the state.    
 
11. Human Gait 
 
Infrasound is a powerful tool for detecting human movement. Due to very low background noise of the 
portable infrasonic detection system we were able to measure spectrum of human gaits. Figure 15 shows 
the FFT of infrasonic measurements from a 38-year-old male and a 24-year-old female (bottom) walking 
in a room. The top spectrum reveals two pronounced peaks: a stationary high frequency peak and a 
Doppler-shifter low-frequency peak, both as a result of the gait. 
 
 
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de
 
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de
 
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Figure 14:  Simmons Mingo Cave, Data samples from outside the entrance 
and from several hundred feet inside the cave 
 12 
 
Figure 15: FFT of Infrasonic Recording taken from a 38-year-old Male (top) and a 24-year-old Female 
(Bottom) 
 
12. Vehicles  
 
During infrasonic measurements from the vehicles, we recorded the data of different vehicles passing by.   
The measurements show that the each vehicle has unique infrasonic signatures. The measurements can 
provide useful information such as rate of travel, body style, make and model of a vehicle.  Additional 
measurements are underway to identity signatures of the vehicle make and model. One such measurement 
is shown in Figure 16. 
 
The sensor is ideal for traffic counting and speed detection. Detectors could be inconspicuously set 
around a region and provide feedback to law enforcement as to what region of roadway to target for 
speeding.  Also there is a significant application for the use of Urban to Rural evacuation initiative such 
as evacuation of Washington DC through the outside rural regions. In addition, measurements are 
underway to further classify the spectrums of passing vehicles to determine the feasibility of utilization of 
this sensor as an identifier of vehicles. 
 13 
 
 
 
 
13. Fire 
 
One significant measurement was the detection of infrasonic emissions from a small fire.  A small brush 
pile, measuring 4 feet in diameter and 3 feet tall was ignited; Figure 17 shows data recorded from three 
different measurements; first,   before the fire was ignited to measure background noise, second one was 
during the initial stages of the fire and third one when the fire was in the free burning stage.  The plot 
shows the FFT of the measurements, showing considerably broad- band, low-frequency noise signals 
coming from the fire.    Figure 18 shows a calculation of power spectral density, which shows 
considerable growth between the incipient and free burning stage of the fire.  The growth pattern can be 
used to determine development stage of a fire. The power spectral density and frequency measurement of 
burning fire can be used to better understand the size and location of the fire.  
Figure 16:  Infrasonic Measurement of Vehicles Passing 
 14 
 
 
 
 
14.   Conclusion 
 
The portable infrasonic detection system developed at NASA Langley Research Center can be used to 
make useful infrasound measurements at a location where it was not possible previously. Extreme 
Endeavors Inc. has used the system for infrasonic measurements from numerous locations including 
caves, tunnels, and human gaits. Further investigation is required to correlate infrasonic signals from 
 
Figure 18: Power Spectral Density Calculation of Fire Emission of Low Frequency Sound 
 
 Figure 17:  Data Recorded Showing the Infrasonic Emission of a Fire 
 15 
 
various military-related sources. In addition to the sources listed in this paper, the portable infrasonic 
detection system has potential to detect sonic boom, CATS, and tornados at early stage.   
 
15. References 
1. A.J.Bedard Jr. and T.M.Georges, “Atmospheric Infrasound,” Physics Today 53, 32-37 (2000).  
2. A. J. Zuckerwar, Q. A. Shams, and J. W. Stoughton; Creativity & Innovation Program, NASA 
Langley 2002 “ Infrasonic Detection of Severe Storms & Clear Air Turbulence”.  
3. Q. A. Shams and A. J. Zuckerwar; Innovative Partnership Program, NASA Langley January 2006 
“Extreme Low Frequency Acoustic Measurement system for Planetary Subterranean, Seismic, 
and Environmental Detection System”. 
4. Christopher C. Lawrenson, Allan J. Zuckerwar, and Qamar A. Shams; “Low Frequency Electret 
Condenser Microphone”, 154th Meeting of the Acoustical Society of America New Orleans, LA 
Nov. 27 – Dec. 1, 2007. 
5. Q.A. Shams, A.J. Zuckewar, and B.S. Sealey, “compact nonporous windscreen for infrasonic 
measurements”, J. Acoust. Soc. Am. 118, 1335-1340 (2005). 
6. Allan J. Zuckerwar, Qamar A. Shams, Krish K. Ahuja, and Robert Funk, “Soaker Hose vs. 
Compact Non-Porous Windscreen: A Comparison of Performance at Infrasonic Frequencies”, 
150th Meeting of the Acoustical Society of America Minneapolis, MN, October 17-21, 2005.  
 


Collection of Infrasonic Sound From Sources of Military Importance

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