D.P. Jansen

Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15-cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limiteds Underground Research Laboratory. An electrical resistance cartridge heater, placed in a central vertical borehole, was used to cycle the sample to progressively higher peak temperatures between 75°C and 175°C. Tomography data were collected, at room temperature, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle. Microfractures opened above 80°C and eventually coalesced into a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle, anisotropic medium.

Lamb wave tomography of advanced composite laminates containing damage

Abstract Ultrasonic techniques have been used to image damaged regions in two polymer composite plate samples. The two thin laminate samples studied consisted of 16 layers of carbon-fibre reinforced epoxy with a quasi-isotropic lay-up configuration and had been previously loaded in a biaxial test method until failure. The resulting damage, in the form of fibre failure, matrix cracking and delamination, has been imaged using a novel Lamb wave immersion tomography technique. Images created with this method were correlated with images obtained from C-scan techniques. Both C-scans and Lamb wave tomography were able to identify clearly regions of damage in the two samples.

Lamb-wave tomography using non-contact transduction

Abstract Ultrasonic Lamb wave experiments have been performed in thin aluminium sheets using a pulsed laser source and an electromagnetic acoustic transducer (EMAT) detector. It is demonstrated that the Lamb waves generated in these experiments can be used for tomographic imaging of changes in the structure of thin sheet material.

Lamb wave tomography

Tomographic imaging of the condition of tin materials has been performed using Lamb waves. Lamb waves were generated in thin sheets of aluminum submerged in water using an immersion transducer set at the critical angle. An identical transducer at a fixed separation detected energy radiated back into the water by the propagating Lamb wave. Waveforms from the receiving transducer were captured and stored for a set scan pattern with an automated data collection system under the control of a microcomputer. Both the amplitude variation and propagation delay were used to obtain tomographic images of attenuation and velocity, based on a filtered back-projection technique. Representative reconstructions of both slowness and attenuation variations are shown, illustrating the techniques success in imaging defects in thin materials.<<ETX>>

Geotomographic imaging in the study of mining induced seismicity

Geotomographic imaging is a technique which allows seismic waves to be used to gain information about the internal structure of rock masses, in a way conceptually similar to medical CAT scanning. Traditional approaches to the study of mining-induced seismic phenomena have concentrated on using passive monitoring methods. This paper gives an overview of the developments in the acquisition, processing and interpretation of geotomographic data and outlines how images can be used in conjunction with passive techniques to study mining-induced seismicity.

Ultrasonic tomography using scanned contact transducers

Ultrasonic tomographic images have been obtained from objects of rectangular cross section by scanning spring‐loaded transducers across the sample faces. Hemispherical brass caps, placed on each transducer, facilitated sliding along the surface. The complete system was under the control of a microcomputer, resulting in an entirely automatic data collection system. Selected images obtained from this system are presented for metal and rock samples, as well as samples with artificial anomalies.

ACOUSTIC TOMOGRAPHY IN SOLIDS USING A BENT RAY SIRT ALGORITHM

Abstract Many techniques have been established that perform tomographic reconstructions of the acoustic velocity profile of a solid object. A SIRT technique was selected as a particular one that was able to handle arbitrary sampling geometries and to incorporate a raybending correction. In this paper, these modifications to the basic algorithm are presented. A fast approximate raybending correction, suitable for use on personal computers, is given based on discretization of the raypaths, and the use of a graph-searching algorithm to obtain the minimum time path. The performance of the algorithm is evaluated in the field of seismic frequencies, imaging an area of backfill in an irregularly shaped rock pillar in a hard rock mine.

Rayleigh Wave Tomography

Tomographic imaging involves the collection of data from a series of projections through an object, and the reconstruction of variations in a chosen parameter across a two-dimensional section. It is now an established method for the reconstruction of images from ultrasonic data, and has been used widely in such areas as medical ultrasound and non-destructive evaluation [1–3].

Laboratory and Field Investigations of Rockburst Phenomena Using Concurrent Geotomographic Imaging and Acoustic Emission/Microseismic Techniques

The progress made on three phases of a research project, started in 1986 to investigate mining induced seismicity/rockburst phenomena using concurrent geotomographic imaging and microseismic monitoring techniques, is described. Phase I is the geotomographic software development and laboratory calibration trials. Phase II is the enhancement of traditional microseismic monitoring instrumentation with a waveform acquisition system, so that source mechanism studies can be carried out on mining induced seismic events. Phase III is the field trials of the hybrid technique which will be used to monitor changing rock mass physical properties, in response to mining. Preliminary results from all three phases are given, together with an outline of current and future research planned.

Ultrasonic Tomographic Imaging of Anisotropic Solids

Tomographic imaging is a technique to determine values of a spatially varying parameter on a cross-sectional slice through an object. In the ultrasonic case of tomography, waves are propagated through the sample between a series of source and receiver locations placed in a plane around the object. These locations are chosen such that the rays pass through as large a fraction of the object plane as possible, and to conform to any requirements for regular positioning in the reconstruction procedure. If the propagation delay is measured for each raypath between source and receiver, the acoustic slowness can be determined along the cross-sectional plane (slowness is the reciprocal of velocity). There are in existence a variety of algorithms that can construct tomographic images, such as filtered backprojection and algebraic reconstruction tomography (ART) [1,2].