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An imaging algorithm is presented based on the standard assumption that the total scattered field can be separated into an elastic component with monopolelike dependence and an inertial component with dipolelike dependence. The resulting inversion generates two separate image maps corresponding to the monopole and dipole terms of the forward model. The complexity of imaging flaws and defects in layered elastic media is further compounded by the existence of high contrast gradients in either sound speed and/or density from layer to layer. To compensate for these gradients, we have incorporated Fermat’s method of least time into our forward model to determine the appropriate delays between individual source-receiver pairs. Preliminary numerical and experimental results are in good agreement with each other.

It is known that focusing of an acoustic field by a time-reversal mirror (TRM) is equivalent to a spatio-temporal matched filter under conditions where the Green’s function of the field satisfies reciprocity and is time invariant, i.e., the Green’s function is independent of the choice of time origin. In this letter, it is shown that both reciprocity and time invariance can be replaced by a more general constraint on the Green’s function that allows a TRM to implement the spatio-temporal matched filter even when conditions are time varying.

Most diffraction tomography (DT) algorithms use a homogeneous Green function (GF) regardless of the medium being imaged. This choice is usually motivated by practical considerations: analytic inversions in standard geometries (Cartesian, spherical, etc.) are significantly simplified by the use of a homogeneous GF, estimating a nonhomogeneous GF can be very difficult, as can incorporating a nonhomogeneous GF into standard DT algorithms. Devaney has circumvented these issues by developing a purely numerical DT inversion algorithm [A. J. Devaney and M. Dennison, Inverse Probl. 19, 855–870 (2003)] that is independent of measurement system geometry, number of frequencies used in the reconstruction, and GF. A planar multilayer GF has been developed for use in Devaney’s “Hilbert space” algorithm and used in a proof-of-principle nondestructive evaluation (NDE) experiment to image noninvasively a flaw in an aluminum/copper planar multilayer medium using data collected from an ultrasonic measurement system. The data were collected in a multistatic method with no beamforming: all focusing through the multilayer was performed mathematically “after-the-fact,” that is, after the data were collected.

A passive measurement system using fiber Bragg gratings is presented to interrogate the health of an instrumented part. Estimation of the structures Green’s function from diffuse sound fields present during typical operating conditions is the basis for our approach. Experimental studies are conducted using coherent processing techniques of random and generated sound fields to investigate a structure for defects and/or deviations from an initial or pristine state. We are interested in developing a monitoring process that is minimally invasive and robust enough to survive the environment that the structure operates in and provide a quantitative assessment of the structure throughout its lifetime.

The development of fiber Bragg grating (FBG) sensors for acoustic measurements permits the instrumentation of manufactured composite parts for the purposes of nondestructive evaluation (NDE) and lifetime monitoring. The parts can be designed from conception with an array of FBG acoustic receivers integrated into its internal structure for the purpose of multistatic data collection for NDE and monitoring of internal changes. The FBG array is accessed via a port on the outer surface and requires no additional power demands on the operation of the part. Data collection is accomplished by activating sources on the surface of the part and measuring the fields along the FBG array. We are particularly interested in Green function (GF) estimation of a FBG instrumented composite structure. By passively measuring diffuse acoustic fields throughout the volume, we use a technique developed by Lobkis and Weaver [J. Acoust. Soc. Am. 110(6), 3011–3017 (2001)] to estimate the GF within the part and monitor it over time. ...

Wave-based remote sensing and imaging provide methods for investigating structures or objects with minimal or no contact. Delay, scale, and sum migration is one such method (in some circles this is known as ``beam forming'' or ``migration'' but must not be confused with beam forming for target location or geophysical migration). Migration assumes coherently scattered fields will sum constructively at a scattering target and destructively elsewhere. There is an additional assumption that multiple scattering can be neglected but this can be relaxed as the forward model sophistication is increased. This presentation summarizes the forward scattering model and derived inverse imaging algorithm as applied to a two planar layer medium. The measurement system operates in a broadband reflection mode in the one layer with the goal of imaging the second layer. Examples are presented using real data in (1) an ultrasonic experiment to measure a flaw in an aluminum/copper multilayer and (2) a ground penetrating radar ...

Model‐based failure detection is based on the principle that the MBP for a normal or pristine structure is developed first and tuned during the calibration stage assuring a statistically validated processor. Once developed, the MBP is used as the integral part in a sequential detection scheme to decide whether or not the structure under investigation is operating normally. When an abnormality is detected, a failure alarm is activated and the type of failure is classified based on a library of potential failure modes. Here we use high‐order parametric models to capture the essence of the structures over a limited frequency band known to be sensitive to structural changes. These estimated or identified models for normal operations are then used to develop the MBP which in this instance is a recursive Kalman filter. The filter is known to produce zero‐mean/white residuals when optimally tuned to the data. Failure is declared when these properties are no longer valid. Once the detection is accomplished, the next step is to classify the type of failure mechanism and eventually the locations. Here we show results of the designs on both simulated and measured data.

A wave-based tomographic imaging method and apparatus based upon one or more rotating radially outward oriented transmitting and receiving elements have been developed for non-destructive evaluation. At successive angular locations at a fixed radius, a predetermined transmitting element can launch a primary field and one or more predetermined receiving elements can collect the backscattered field in a “pitch/catch” operation. A Hilbert space inverse wave (HSIW) algorithm can construct images of the received scattered energy waves using operating modes chosen for a particular application. Applications include, improved intravascular imaging, bore hole tomography, and non-destructive evaluation (NDE) of parts having existing access holes.

The Center for Subsurface Sensing & Imaging Systems (CenSSIS, www.censsis.neu.edu) has been advancing the concept of ‘‘Diverse Problems‐Similar Solutions,’’ whereby imaging, inversion, and tomography (IIT) algorithms developed for one field or physical regime are applied to another. Following this philosophy, algorithms developed for geophysical imaging have been successfully applied to a nondestructive evaluation (NDE) problem in a planar multilayer. The depths and acoustic frequencies of geophysical problems are on the orders of kilometers and sub‐hertz, while those of NDE are millimeters and megahertz. Just the same, the propagation phenomenon is entirely wave‐based in both cases and thus, algorithms developed for one can be applied to the other. [This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W‐7405‐Eng‐48. UCRL‐ABS‐216281.]

In 373 BC an earthquake and tsunami destroyed and submerged the classical Greek city of Helike (HELL‐E‐KEY) on the north Peloponnese shore of the Gulf of Corinth. In June 2004, surface seismo‐acoustic surveys were performed in the area of present Helike in order to collect non‐invasive tomographic data to guide the archeological excavations. Prior to tomographic inversion and imaging, the raw data time series must be preprocessed to estimate ground acoustic velocity, and to remove the ground roll signal. We present a generalized cross‐correlation technique which appears successful in solving both problems. We describe the technique as applied to the data and present tomographic inversions.

Diffraction tomography (DT) imaging techniques require knowledge of the background Green function. Due to its simplicity, it is standard practice to use a homogeneous medium Green function. We have developed a model of a planarly layered Green function that can be used in DT imaging of planarly layered media where the layer acoustical properties and dimensions are known. We present the theory and applications. [Work performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W‐7405‐Eng‐48.]

In a recently developed theory, the forward integral acoustic scattering operator is cast into the formalism of a Hilbert space operator which projects the continuous space scattering object into the discrete measurement space. By determining the singular value decomposition (SVD) of the forward scattering operator, one obtains optimal, orthonormal bases for each of these spaces in the form of the singular vectors. In formulating the inverse scattering problem, it is best to express the unknown object distribution in terms of an expansion of the singular vectors. Using this expansion, the best reconstruction, in a LMS sense, can be obtained from measured scattered field data. We present reconstruction results using this new theory. [Work performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W‐7405‐Eng‐48.]

In previous ASA meetings and JASA papers, the extended and formalized theory of transmission mode time reversal in which the transceivers are noncoincident was presented. When combined with the subspace concepts of a generalized MUltiple SIgnal Classification (MUSIC) algorithm, this theory is used to form super‐resolution images of scatterers buried in a medium. These techniques are now applied to ultrasonic nondestructive evaluation (NDE) of parts, and shallow subsurface seismic imaging. Results are presented of NDE experiments on metal and epoxy blocks using data collected from an adaptive ultrasonic array, that is, a ‘‘time‐reversal machine,’’ at Lawrence Livermore National Laboratory. Also presented are the results of seismo‐acoustic subsurface probing of buried hazardous waste pits at the Idaho National Engineering and Environmental Laboratory. [Work performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W‐74...

We have formalized the theory of transmission mode time reversal imaging for arbitrary transducer geometry. Although it is not possible to implement such a system in practice, we have shown that transmission mode time reversal imaging is achievable a posteriori from the scattered field time series as measured at the receivers. We apply this theory to studying the singular value spectra of objects buried in the ground. We provide simulated finite‐difference time‐domain (FDTD) examples designed to model the case of geophysical vertical seismic profiling in which a series of sources are excited on the surface and the scattered fields are measured on a chain of receivers in a bore hole. A single, non‐point scatterer can have multiple singular values dependent upon the scatterer’s physical properties. We study the relationship between the singular value spectra and these physical properties.

We develop the theory of transmission mode time reversal imaging for arbitrary transducer geometry. Although it is not possible to implement such a system in practice, we prove that transmission mode time reversal imaging is achievable a posteriori from the scattered field time series as measured at the receivers. The theory is developed for any arbitrary geometry in which the transmitters and receivers are not colocated. We provide simulated proof‐of‐principle and finite‐difference time‐domain (FDTD) examples designed to model the case of geophysical vertical seismic profiling in which a series of sources are excited on the surface and the scattered fields are measured on a chain of receivers in a bore hole.