Impact of directional illumination control parameters on seismic signals
To improve quality of seismic signals, DICT (directional illumination control technique) of DVAS (the distributed vibrator array system) is developed. It explains DICT theory in seismic wave domain and states why DVAS can generate directional seismic wave with DICT. The property of directional seismic wave can be described with directional factor. From directional factor, some control parameters such as amount of vibrators, arrangement of DVAS and delay time play an important role in seismic wave directivity and reflected waves' SNR (signal signal-to-noise ratio). To study parameters of directional illumination system, receiving signals are simulated by numerical method. The results show that reflected wave' SNR can be improved with more vibrators and small space between adjacent source in the same line. When the amount and space of source are settled down, a long delay time can increase reflected wave' SNR in shallow lays and a short delay time can boost signal SNR in deep lays. It concludes that DICT is a better control method to achieve high SNR signals with proper parameters. Furthermore, it offers some rules on how to determine parameters.
Comparison of illumination analyses using one-way and full-wave propagators
Summary In this paper, we compare one-way wave propagator and full-wave propagator such as finite difference (FD) method i n the evaluation of illumination and acquisition dip response (ADR) maps. For the case of 2D SEG/EAGE salt model, the illumination and ADR maps by the one-way method and the FD method have similar general distributions in space and angle domain. However, some difference exists in absolut e values, especially for the subsalt area. This is due to the strong effects of transmission loss caused by salt boundary scat tering, reflection and defocusing. The difference in wide-angle amplitude behavior is also noticeable. Therefore, in most cases illumination analysis calculated by one-way wave propagators can provide reliable evaluation.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.