Mihan H. McKenna

Tele-infrasonic studies of hard-rock mining explosions

The Lac-du-Bonnet infrasound station, IS-10, and the Minnesota iron mines 390 km to the southeast are ideally located to assess the accuracy of atmospheric profiles needed for infrasound modeling. Infrasonic data from 2003 associated with explosions at the iron mine were analyzed for effects of explosion size and atmospheric conditions on observations with well-constrained ground truth. Noise was the determining factor for observation; high noise conditions sometimes prevented unequivocal identification of infrasound arrivals. Observed arrivals had frequencies of 0.5 to 5 Hz, with a dominant frequency of 2 Hz, and generally had durations on the order of 10 s or less. There was no correlation between explosive amount and observability. Tele-infrasonic propagation distances (greater than 250 km) produce thermospheric ray paths. Modeling is based upon MSIS/HWM (Mass Spectrometer Incoherent Scatter/Horizontal Wind Model) and NRL-G2S (Naval Research Laboratory Ground to Space) datasets. The NRL-G2S dataset provided more accurate travel time predictions that the MSIS/HWM dataset. PE modeling for the NRL-G2S dataset indicates energy loss at higher frequencies (around 4 Hz). Additionally, applying the Sutherland/Bass model through the NRL-G2S realization of the atmosphere in InfraMAP results in predicted amplitudes too small to be observed.

Topographic effects on infrasound propagation

Infrasound data were collected using portable arrays in a region of variable terrain elevation to quantify the effects of topography on observed signal amplitude and waveform features at distances less than 25 km from partially contained explosive sources during the Frozen Rock Experiment (FRE) in 2006. Observed infrasound signals varied in amplitude and waveform complexity, indicating propagation effects that are due in part to repeated local maxima and minima in the topography on the scale of the dominant wavelengths of the observed data. Numerical simulations using an empirically derived pressure source function combining published FRE accelerometer data and historical data from Project ESSEX, a time-domain parabolic equation model that accounted for local terrain elevation through terrain-masking, and local meteorological atmospheric profiles were able to explain some but not all of the observed signal features. Specifically, the simulations matched the timing of the observed infrasound signals but underestimated the waveform amplitude observed behind terrain features, suggesting complex scattering and absorption of energy associated with variable topography influences infrasonic energy more than previously observed.

Frequency-wavenumber processing for infrasound distributed arrays

The work described herein discusses the application of a frequency-wavenumber signal processing technique to signals from rectangular infrasound arrays for detection and estimation of the direction of travel of infrasound. Arrays of 100 sensors were arranged in square configurations with sensor spacing of 2 m. Wind noise data were collected at one site. Synthetic infrasound signals were superposed on top of the wind noise to determine the accuracy and sensitivity of the technique with respect to signal-to-noise ratio. The technique was then applied to an impulsive event recorded at a different site. Preliminary results demonstrated the feasibility of this approach.

Structural response of a recycled thermoplastic lumber bridge under civilian and military loads

The U.S. Army Engineer Research and Development Center (ERDC) executed a load test and verification simulation on a novel thermoplastic composite bridge, T-8518, located on Tuckers Road in Camp Mackall, North Carolina. The bridge was made with 94% recycled plastic material, primarily recycled high-density polyethylene. An M1 Abrams battle tank and a loaded dump truck were used as a live load to determine the appropriate military load classification (MLC) and civilian load rating for the bridge superstructure. The bridge was designed to support the M1 Abrams battle tank with a gross weight of 63.5 tones to replace a dilapidated timber bridge that, because of its condition, was limited to a maximum load of 4.26 tones. A finite element analysis (FEA) of the entire superstructure based on the load test results indicated that the bridge exceeded design specifications and performed in a normal linear–elastic manner with relatively small viscoelastic responses for all loads.

Partially saturated soil causing significant variability in near surface seismic signals

The behaviour of dry, moist, and saturated soils has been studied for over a century without adequately investigating the behaviour associated with transient saturation in the near surface, i.e. the upper 1 m of overburden, including the effects of rapid meteorological changes, dynamic fluid flow, and variability of saturation on shallow seismic sensors. This paper presents observational data wherein the geophysical instrumentation response was significantly influenced by near-surface post-precipitation saturation and additional laboratory experimentation on the effects of saturation on shear wave velocity. The lack of partially-saturated data is primarily because transient meteorological events have not been critically important to the types of long-term deployments performed in the past, where sensors were situated in hard-rock, collecting data under idealized conditions, as opposed to sedimentary settings. Shorter-duration deployments and smaller system architectures, e.g. persistent monitoring, now necessitate detailed a priori knowledge of meteorological impacts to system design and performance. The purpose of this persistent monitoring geophysical instrumentation is to continually monitor the near surface and relate small perturbations to a specific source type(s) and distance(s) from the receiver. As such, the received signal is compared to known sources within a predetermined geological/ meteorological condition. Presented herein is the calibration signal generated by a 3.63-kg (8-lb) sledgehammer prior to and post 36 hours of steady precipitation. The resulting subsurface seismic velocity time-histories show a significant increase in signal amplitude, change in frequency content and no change in duration. Thus, the amplification effects of near-surface moisture variability combined with dynamic pore fluid could be interpreted as false positives of a specific source signature and/or instrument failure.

Use of a porous material description of forests in infrasonic propagation algorithms.

Infrasound can propagate very long distances and remain at measurable levels. As a result infrasound sensing is used for remote monitoring in many applications. At local ranges, on the order of 10 km, the influence of the presence or absence of forests on the propagation of infrasonic signals is considered. Because the wavelengths of interest are much larger than the scale of individual components, the forest is modeled as a porous material. This approximation is developed starting with the relaxation model of porous materials. This representation is then incorporated into a Crank-Nicholson method parabolic equation solver to determine the relative impacts of the physical parameters of a forest (trunk size and basal area), the presence of gaps/trees in otherwise continuous forest/open terrain, and the effects of meteorology coupled with the porous layer. Finally, the simulations are compared to experimental data from a 10.9 kg blast propagated 14.5 km. Comparison to the experimental data shows that appropriate inclusion of a forest layer along the propagation path provides a closer fit to the data than solely changing the ground type across the frequency range from 1 to 30 Hz.

Infrasound Measurements of a Railroad Bridge

Infrasound is acoustic energy whose frequency is below that of human perception. Large infrastructure, such as bridges, emits such signals at their natural or driven frequencies of vibration. These frequencies can provide an indication of the structural condition. Recent developments in acoustic propagation have enabled the detection of infrasound at large distances from its source. Thus it may represent a means of remotely detecting the condition of large structures. The feasibility of this was recently evaluated in an in-service test of a railroad bridge. In the evaluation the infrasound results are compared with those from operational modal testing and finite element analysis. BACKGROUND Infrasound is low frequency sound waves between 0.1 to 20 Hz, below the range of human hearing from 20 Hz to 20,000 Hz. There are many sources of infrasound including volcanoes, earthquakes, bolides (meteors), manmade explosions, mining explosions, atmospheric explosions, surf, missiles, rockets, weather systems and even animal vocalizations (1). Infrasound propagation depends on the effective sound speed of the atmosphere through which it travels. Effective sound speed profiles are calculated by:

Seismic-infrasound-acoustic-meteorological sensors to dynamically monitor the natural frequencies of concrete dams

The U.S. Army Engineer Research and Development Center (ERDC) is leading research using seismic-infrasound-acoustic-meteorological (SIAM) arrays to determine structural characteristics of critical infrastructure. Large infrastructure, such as dams, emit infrasound (acoustic energy below that of human perception) at their natural modes of vibration, which are related to their structural condition. To validate the concept and its potential use for monitoring flood control structures, a structural evaluation was conducted at the Portuguese Dam in Ponce, Puerto Rico. The dam’s dynamic properties were studied prior to the deployment of SIAM arrays using detailed finite element (FE) models assembled in COMSOL Multiphysics software. The principal natural frequencies of the dam were identified and the fundamental modes were confirmed in the infrasound pass-band. The natural frequencies of 4.8 Hz, 6.7 Hz, and 10.2 Hz, respectively, were determined for the lower modes of vibrations. A total of three SIAM arrays wer...

Detection and localization of weak impulses and continuous sources within the urban acoustic environment

Signal detection and localization with large aperture acoustic arrays in urban environments can be inherently difficult. This poster presents some preliminary approaches under investigation for; determining the direction of arrival of continuous sources that are changing in frequency, detecting and localizing low signal to noise ratio impulses in the presence of coherent background noise, and a fast mapping approach for converting array element delays to direction of arrival using a neural network fitting function. For uniform circular array geometries with a central reference array element, the direction of arrival of sources with changing frequency can be visualized by applying the Hilbert transform and analyzing the relative phase angle rate of change of the outer array elements. Next, various computational approaches for low level impulse detection in the presence of coherent acoustical noise are presented. Last, a neural network fitting function is discussed that performs array specific delay mapping to direction of arrival with high computational efficiency and trained noise immunity.Signal detection and localization with large aperture acoustic arrays in urban environments can be inherently difficult. This poster presents some preliminary approaches under investigation for; determining the direction of arrival of continuous sources that are changing in frequency, detecting and localizing low signal to noise ratio impulses in the presence of coherent background noise, and a fast mapping approach for converting array element delays to direction of arrival using a neural network fitting function. For uniform circular array geometries with a central reference array element, the direction of arrival of sources with changing frequency can be visualized by applying the Hilbert transform and analyzing the relative phase angle rate of change of the outer array elements. Next, various computational approaches for low level impulse detection in the presence of coherent acoustical noise are presented. Last, a neural network fitting function is discussed that performs array specific delay mapping...

A wide-angle topography-capable parabolic equation using a non-transformed coordinate system

Low-frequency acoustic propagation for long ranges (up to 200 km) is of significant military interest, particularly for the purposes of persistent surveillance of denied areas and infrastructure/activity sensing. A deep understanding of the natural environment’s influence on the signal is critical for accurate interpretation of received signals at monitoring stations. Influenced by the underwater acoustics community, a flexible, wide-angle, finite-difference parabolic equation model has been developed. This model handles discontinuities and gradual variations in density and wavenumber, allowing terrain/topography to be represented as range-dependent density and wavenumber profiles. Traditional coordinate-transforming methods for propagation over topography require interpolations and/or extrapolations near the top of the computational boundary when transformed back into the original coordinate system, introducing potentially significant errors. These methods work well for shorter distances, where interpola...