As the barometer falls, gases are drawn upward out of the permeable Earth into the atmosphere. Conversely, a rising barometer pushes air downward. In a homogeneous permeable medium, these cyclical gas motions are piston‐like and nearly reversible, so they contribute only modestly to the net transport of contaminant gases. In a fractured permeable medium, however, the fractures will generally serve as breathing passages for all of the gas‐filled porosity, greatly increasing the amplitude and nonuniformity of vertical motions. The resulting transport process may be orders of magnitude more significant than molecular diffusion, according to the theoretical analysis presented here. Analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material. These solutions are then used to constrain the relationship between fracture aperture and fracture spacing, based on field comparisons between surface and subsurface pressure variations. The final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls. A maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters.

The first update for the SEG/EAEG 3-D Modeling Project appeared in the February issue of TLE and the March issue of First Break. Our goal is to design salt and overthrust 3-D models and then simulate realistic 3-D surveys based on those models. Given the project's significance and scope, we plan frequent progress reports. In conjunction with this goal and to solicit input, we have made recent presentations at SEG Cairo, the SEG/EAEG Summer Research Workshop in The Netherlands, the Stanford Exploration Project, and the Center for Wave-Phenomena at the Colorado School of Mines.The committee expects to freeze the models based on feedback from the SEG and EAEG membership. They will then be available to the general public, most probably via CD-ROM.We are soliciting commercial entities to be considered as the repository of the models and simulation results (perhaps one in Europe and one in North America). Progress reports of the project working groups (salt model, overthrust model, and computations and algorithms) follow. Salt ModelThe first version has been constructed and placed in the public domain, accessible via Internet, in GOCAD format. This version, primarily describing the model's structural components, measures 90 000 ft in x,y and 24 000 ft in z. Figure 1 shows a 3-D visualization of the major components.The velocities surrounding the salt body are typical of Gulf of Mexico sedseismic response and the computational iments and described by compaction cost of the synthetic generation. Prelimgradients based on K-V, curves (K, the inary results on a 2-D cross section gradient, varies spatially and V, is the using the spike technique have been obinitial velocity) and a geopressure surtained. Figure 3a shows the velocity face. However, a constant density conprofile and Figure 3b the resulting 2-D straint has been placed due to the finite zero offset simulation. These prelimidifference software employed and nary results seem to indicate that the available computational resources. This spike technique is adequate; however, constant density assumption and simple more testing is required. velocity gradient results in no seismicIn the meantime, a 3-D velocity grid, reflectivity in the sediments surroundemploying the spike technique, has ing the salt. Two techniques (Figure 2) been generated and provided to US Naare being investigated via 2-D finite diftional Laboratories for testing of the 3-D ference modeling. The first ("spike") finite-difference software. increases the velocity of the finite dif-A second area of considerable invesference grid cell nearest each layer tigation is how to perform 3-D finite boundary by a certain percentage. The difference simulation on the salt model second ("block") alternately increases within the current resources offered by and decreases the velocity of the K-V, the US Department of Energy. A first function for each successive layer. estimate on the required computing reSince these techniques can vary the seissources far exceeded the existing budmic response of the model, two-dimeng...

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