The size of the aperture has an important influence on the results of (Kirchhoff-type) migration and demigration. For true-amplitude imaging, it is crucial not to have apertures below a certain size. For both the minimum migration and denigration apertures, theoretical expressions are established. Both minimum apertures depend on each other and, although a time-domain concept, are closely related to the frequency-dependent Fresnel zone on the searched-for subsurface reflector. This relationship sheds new light on the role of Fresnel zones in the seismic imaging of subsurface reflectors by showing that Fresnel zones are not only important in resolution studies but also for the correct determination of migration amplitudes. It further helps to better understand the intrinsic interconnection between prestack migration and demigration as inverse procedures of the same type. In contrast to the common opinion that it is always the greatest possible aperture that yields the best signal-tonoise enhancement, it is in fact the selection of a minimum aperture that should be desired in order to (a) enhance the computational efficiency and reduce the cost of the summation, (b) improve the image quality by minimizing the noise on account of summing the smallest number of traces, and (c) to have a better control over boundary effects. This paper demonstrates these features rather than addressing the question of how to achieve them technically.

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In a conventional approach, several methods need to be adopted and integrated to understand the geochemical and geophysical signatures of active geothermal systems (e.g., Rybach and Muffler 1981). These methods also apply for greenfield studies and include: (a) geochemical investigations, e.g., application of chemical geothermometers to infer the temperature of the geothermal reservoir; measurement of gas isotopes, such as 3 He/ 4 He, to constrain the origin (mantle or crust) of fluids; (b) drilling of exploration wells; (c) gravity measurements to map any negative anomaly associated with the steam fraction in high-porosity reservoir rocks or to locate zones of lowered density provoked by thermal expansion in magmatic bodies; (d) application of electrical methods such as resistivity to search for zones of higher-salinity fluids; (e) use of seismic methods for the localization of shallow intrusions and estimation of their vertical extension.

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