T h e A m e r i c a n A r c h i v i s t , V o l . 7 3 ( F a l l / W i n t e r 2 0 1 0 ) : 4 2 5 -4 6 2 425t h e A m e r i c A n A r c h i v i s t Modes of Seeing: Digitized Photographic Archives and the Experienced UserPaul Conway 0 A b s t r a c tDigitization practice in archives and libraries is now generating digital content and associated tools and practices that are transforming the relationships among archivists, users, and archival collections. The transformative nature of digitization derives in part from the power of the complex technologies to represent images and facilitate their use. This article explores how experienced, but for the most part nonacademic, users see the visual, material, and archival properties of digitized photographic archives while undertaking innovative and insightful projects that push the boundaries of visual interpretation. The study is a qualitative investigation involving independent case studies of seven people who have extensive experience using the photographic archives preserved by the Library of Congress. This article contextualizes the research in a range of literatures, summarizes the research methodology, and presents findings from in-depth interviews that focus on how visually experienced users choose digitized photographs for inclusion in a given project. By understanding these "modes of seeing" the contemporary digitized visual archives, archivists are in a better position to understand the representational implications of their digitization processes.

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We present a practical approach to imaging structures beneath gas clouds using PS-converted waves. In this approach, we developed methods to build the migration velocity model and to perform 3D prestack Kirchhoff time migration on a PC cluster and improved the processing flow for PS-converted waves. The technique was successfully applied to a 3D/4-C marine data set acquired in August 2001 and made available to us by Kerr-McGee North Sea (UK).This was a North Sea survey with a nominal receiver area of 10.8 km 2 . Two swath data sets were acquired. Each swath data set had an inline geometry using two receiver cables and 22 shot lines. The survey was centered on a domed structure which is obscured by a gas chimney. Faulting is thought to be present beneath the summit of the dome. Because the P-waves are attenuated by gas clouds, the P-wave image of the structure beneath the gas chimney is dimmed, but PS-converted waves can image that structure. In this study, we processed the PS-converted wave data of the 3D/4-C data set from one swath. The objective of the processing was to define the top of the structure at the reservoir target level (3.0 s PP time and 5.5 s PS time). We demonstrated how the PS-converted wave processing was able to clarify structural details.3D prestack Kirchhoff time migration (PKTM) and an appropriate migration velocity model play a crucial role in imaging structures beneath gas clouds using PS-converted waves. Firstly, we present the principle of 3D PKTM and its implications for a PC cluster. Then we demonstrate how to build the migration velocity model. After that we show the results of 2D processing of P-and PS-converted waves for 2D lines at the location of receiver cables, and the results of 3D processing of PS-converted waves for one swath data set. Finally, we present the interpretation results based on the 3D migrated images and our conclusions.Principles of 3D PKTM. 3D PKTM can produce an image at any desired location in a 3D space. Figure 1 shows the relationship among the scatterpoints (image points), shots, and receivers in 3D PKTM. The energy from a trace related to a shot and a receiver must be distributed to all possible scatterpoints in a surface according to the relevant traveltime. In other words, the energy from all shots and receivers is summed at each location to construct an image of the scatterpoints. This is usually implemented as a weighted summation based on the raypaths.In 3D PKTM, the traveltime of a PS-converted wave (t ps ) for anisotropic media is calculated using the double-squareroot (DSR) equation derived by Li et al. (2001): (1) where x p is the horizontal distance from the source to the scatterpoint; x s is the horizontal distance from the scatterpoint to the receiver; t p0 is the vertical traveltime for the Pwave; t s0 is the vertical traveltime for the S-wave; V P is the velocity for the P-wave, and V S is the velocity for the S-wave. η eff and ζ eff are anisotropy parameters for the P-and S-waves, respectively. However, the offset between the source and re...

We present a practical approach to perform 3D pre-stack Kirchhoff time migration in a PC cluster and to build the migration velocity model. This approach is successfully applied to a 3D-4C OBC dataset acquired in the North Sea to use its PS-waves to image the structure under a gas chimney. The results show that neither VTI nor HTI anisotropy can be observed in this dataset. An isotropic, Common Image Point (CIP)-consistent velocity model is sufficient for imaging processing. The 3D migrated results are encouraging. The structure under the gas chimney is clearly imaged using PS-waves. The faults in the target area can be clearly identified. These structures are confirmed by the drilling results.

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