Subsurface imaging

Polymer coatings on stiffer substrates can be measured by time-resolved techniques (Sinton et al. 1989). Often in these cases it is not convenient to measure a direct reflection from an uncoated part of the substrate at more or less the same time, and anyway the substrate may not be flat, but this may not matter if it can be assumed that either the thickness or the longitudinal velocity of the coating does not vary. The time interval between the echoes from the top and bottom surfaces of the coating can then be used to determine the unknown quantity. An example of the kind of signal that can be obtained is shown in Fig. 10.5. The specimen was a coating of PET (polyethylene terephthalate) 15 m thick on a stone-finish rolled steel substrate. Although there is some overlap of the two echoes, there is no difficulty in 

Time-resolved techniques are very powerful for examining structures where the useful information is contained in the normally reflected signal. Frequency analysis of a reflected broadband signal can also be used for film characterization (Wang and Tsai 1984 Lee et al. 1985). But in many other problems, especially in materials science, there is a great deal of information contained in the way that the coefficient of reflection changes with angle of incidence. It is therefore important to understand the behaviour of the reflectance function R(d) of a layered structure. 

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In the future, it is expected to be possible to make more routine use of additional wave types, specifically shear or S waves (polarised to horizontal and vertical components) which have a transverse mode of propagation, and are sensitive to a different set of rock properties than P waves. The potential then exists for increasing the number of independent attributes measured in reflection surveys and increasing the resolution of the subsurface image. 

In materials of lower velocity the situation becomes simpler. If the shear critical angle is outside the lens angle, then there will be no Rayleigh phenomena. Such materials would include many polymers and composites. There may still be longitudinal head waves, and these can reveal some of the features associated with Rayleigh wave imaging, such as surface cracks. But two further factors enhance subsurface imaging. First, because the velocity is lower, the... 

Atalar, A. and Koymen, H. (1989). A high efficiency Lamb wave lens for subsurface imaging. IEEE 1989 Ultrasonics Symposium, pp. 813-16. IEEE, New York. [56,218]... 

Yamanaka, K., Ogiso, H and Kolosov, O. V. (1994). Ultrasonic force microscopy for nanometre resolution subsurface imaging. Appl. Phys. Lett. 64(2), 178-80. [292, 314]... 

At the prospect scale, the resolution of the seismic velocity analysis can be greatly enhanced by detailed velocity analysis, modeling and calibration against well data. A very detailed subsurface image of pres-... 

Dutta, N.C. and Ray, A. 1996. Subsurface image of geopressured rocks using seismic velocity and acoustic impedance inversion. 58th Annu. Mtg. Eur. Assoc. Geosci. Eng., Amsterdam (extended Abstr.). 

The IR s-SNOM approach can also be applied to other materials. For example, Keilmann and coworkers have shown examples ranging from SiC partially covered with Au [33] to metal/Si/polymer three-component samples [60] and even subsurface imaging in such three-component systems [61]. 

The resolution in the image, which is only a surface or subsurface image, is defined both in the vertical direction and laterally on the plane of the sample. The magnitude of resolution can be achieved in the range of a fraction of 1 A (10 ° m = 0.1 nm). Obviously, the size and shape of the... 

Figure 9.10 Thermal images of a PVC/PB immiscible blend. (Reprinted with permission from Journal Of Vacuum Science and Technology B., Scanning thermal microscopy Subsurface imaging, thermal mapping of polymer blends, and localized calorimetry by A. Hammiche, D.J. Hourston, H. M. Pollock et al., 14, 2, 1486-1491. Copyright (1996) American Institute of Physics)...

Cross-polarization confocal microscopy is a new 3D imaging method developed at ANL [9]. It combines two well-established optical methods, the cross-polarization backscatter detection and the scanning confocal microscopy, and can achieve 3D subsurface imaging with sub-micron spatial resolutions. A schematic diagram of the system is shown in Fig. 7. 

R. A. Lodder and G. M. Hieftje, Subsurface Image Reconstruction by Near-Infrared Reflectance Analysis, AppZ. Spectrosc., 42,1351 (1988). 

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