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McKenna S.P.,Riverside Research | Parkman K.B.,U.S. Army | Perren L.J.,Bevilacqua Research Corporation | McKenna J.R.,U.S. Army
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

The response of an electromagnetic gradiometer (EMG) system to a subsurface wire is analyzed in terms of experimental and analytical modeling results. Our objective is to explore characteristics of the response and assess the fidelity of our model. The EMG system consists of a static transmitter and a man-portable sensor, which uses a pair of receivers that yield a gradiometric measurement. Experimental results were collected over a range of wire depths from 3.4 to 8.5 m. A number of different transmitter positions were explored, and the tests studied were conducted at 200 kHz. Modeling results were consistent with the experimental results and supported a number of key findings. Results are presented showing that, in order to maximize the strength of the wire response, the transmitter should be positioned approximately 5 m off the wire axis. Furthermore, in order to avoid unwanted transmitter influence on the response, the EMG should be at least 30 m from the transmitter. Using the experimental and modeling results, we found a linear relationship between the width of the magnitude response peak and the wire depth. Based on our experimental results, the EMG is able to yield a discernible target response at a depth of at least 7 m. Lastly, an example of how the model can be used to optimize survey planning is presented. This paper illustrates how an EMG can be used to locate underground wires with applications ranging from underground utility mapping to the detection of shallow subsurface tunnels. © 2011 IEEE.

Hopkins M.A.,Michigan Technological University | Hopkins M.A.,Riverside Research | King L.B.,Michigan Technological University
Journal of Propulsion and Power | Year: 2016

The performance metrics of a 2-kW-class thruster operated using magnesium propellant were measured and compared to the performance of the same thruster operated using xenon propellant. When operated with magnesium at a 7Adischarge current, the thruster had thrust ranging from 34 ± 0.8 mNat 200Vusing 1.8 mg/s of propellant to 39 ± 1.5 mN at 300 V using 1.8 mg/s of propellant. The thrust-to-power ratio ranged from 24 ± 0.5 mN/kW at 200V to 18 ± 0.7 mN/kW at 300 V. At a 200Vdischarge voltage, the specific impulse was 1930 ± 49 s at 23 ± 5.0% efficiency (at 7 A using 1.8 mg/s). At a 300 V discharge voltage, the specific impulse was 2420 ± 130 s at 21 ± 6.4% efficiency (at 5 A using 1.1 mg/s). The performance of the thruster using magnesium propellant was compared to xenon performance at matched molar propellant flow rates: 5 mg/s for xenon and 1.1 mg/s for magnesium. The xenon-fueled thruster produced 76 ± 1.5 mN of thrust, with a specific impulse of 1550 ± 70 s, at an efficiency of 40 ± 2.0% compared to the magnesium-fueled thruster, which produced 27 ± 1.2 mN of thrust, with a specific impulse of 2420 ± 130 s, at an efficiency of 21 ± 6.4%. © Copyright 2015 by Mark A. Hopkins.

Sampathkumar A.,Riverside Research
Proceedings of Meetings on Acoustics | Year: 2013

Conventional photoacoustic microscopy (PAM) employs light pulses to produce a photoacoustic (PA) effect and detects the resulting acoustic waves using an ultrasound transducer acoustically coupled to the target tissue. The resolution of conventional PAM is limited by the sensitivity and bandwidth of the ultrasound transducer. We have investigated an all-optical, pump-probe method employing interferometric detection of the acoustic signals that overcomes limitations of conventional PAM. This method does not require contact with the specimen and provides superior resolution. A 532-nm pump laser with a pulse duration of 5 ns excited the PA effect in tissue. Resulting acoustic waves produced surface displacements that were sensed interferometrically with a GHz bandwidth using a 532-nm continuous-wave (CW) probe laser and a Michelson interferometer. The pump and probe beams were coaxially focused using a 50X objective giving a diffraction-limited spot size of 0.5 μm. The phase-encoded probe beam was demodulated using a homodyne interferometer. The detected time-domain signal was time reversed using k-space wave-propagation methods to produce a spatial distribution of PA sources in the target tissue. Performance was assessed using 3D images of fixed, ex vivo, retina specimens. © 2013 Acoustical Society of America.

Filoux E.,Riverside Research | Mamou J.,Riverside Research | Aristizabal O.,New York University | Ketterling J.A.,Riverside Research
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control | Year: 2011

The spatial resolution of high-frequency ultrasound (HFU, >20 MHz) imaging systems is usually determined using wires perpendicular to the beam. Recently, two tissue-mimicking phantoms (TMPs) were developed to estimate three-dimensional (3-D) resolution. Each TMP consists of nine 1-cm-wide slabs of tissue-mimicking material containing randomly distributed anechoic spheres. All anechoic spheres in one slab have the same dimensions, and their diameter is increased from 0.1 mm in the first slab to 1.09 mm in the last. The scattering background for one set of slabs was fabricated using 3.5-m glass beads; the second set used 6.4-m glass beads. The ability of a HFU system to detect these spheres against a speckle background provides a realistic estimation of its 3-D spatial resolution. In the present study, these TMPs were used with HFU systems using single-element transducers, linear arrays, and annular arrays. The TMPs were immersed in water and each slab was scanned using two commercial imaging systems and a custom HFU system based on a 5-element annular array. The annular array had a nominal center frequency of 40 MHz, a focal length of 12 mm, and a total aperture of 6 mm. A synthetic-focusing algorithm was used to form images with an increased depth-of-field. The penetration depth was increased by using a linear-chirp signal spanning 15 to 65 MHz over 4 s. Results obtained with the custom system were compared with those of the commercial systems (40-MHz probes) in terms of sphere detection, i.e., 3-D spatial resolution, and contrast-to-noise ratio (CNR). Resulting Bmode images indicated that only the linear-array transducer failed to clearly resolve the 0.2-mm spheres, which showed that the 3-D spatial resolution of the single-element and annulararray transducers was superior to that of the linear array. The single-element transducer could only detect these spheres over a narrow 1.5 mm depth-of-field, whereas the annular array was able to detect them to depths of at least 7 mm. For any size of the anechoic spheres, the annular array excited by a chirp-coded signal provided images of the highest contrast, with a maximum CNR of 1.8 at the focus, compared with 1.3 when using impulse excitation and 1.6 with the single-element transducer and linear array. This imaging configuration also provided CNRs above 1.2 over a wide depth range of 8 mm, whereas CNRs would quickly drop below 1 outside the focal zone of the other configurations. © 2011 IEEE.

Filoux E.,Riverside Research | Mamou J.,Riverside Research | Moran C.,Queens Medical Research Institute | Pye S.D.,Royal Infirmary | Ketterling J.A.,Riverside Research
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control | Year: 2012

A resolution integral (RI) method based on anechoic-pipe, tissue-mimicking phantoms was used to compare the detection capabilities of high-frequency imaging systems based on a single-element transducer, a state-of-the-art 256-element linear array, or a 5-element annular array. All transducers had a central frequency of 40 MHz with similar conventionally measured axial and lateral resolutions (about 50 and 85 μm, respectively). Using the RI metric, the annular array achieved the highest performance (RI = 60), followed by the linear array (RI = 47), and the single-element transducer (RI = 24). Results showed that the RI metric could be used to efficiently quantify the effective transducer performance and compare the image quality of different systems. © 2012 IEEE.

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