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.

Borehole gravity measurements over a depth range from 1737 to 1027 m and the vertical gradient of gravity above ground were measured at the Salton Sea Scientific Drilling Project well State 2–14. Uncorrected borehole gravimetric densities match values from gamma‐gamma logs, indicating that the high densities seen in State 2–14 in the depth range 0.5–3 km extend for a few kilometers from the well. The aboveground gradient was found to be 4.1 µGal/m higher than expected; correcting for this value increases the gravimetric density in the borehole. Combining the borehole gravity and estimated vertical gravity gradients on the surface, we find that this densified zone coincides with much of a broad thermal anomaly that has been found to the northeast of the Salton Sea geothermal field.

Our equations (3) and (4) are correct. They represent the difference between the attraction of the shell viewed from [Formula: see text], the outer radius of the shell, and [Formula: see text], its inner radius. (The attraction of the shell viewed from [Formula: see text] is zero.) On the other hand, equations (5) and (6) of Fahlquist and Carlson represent the difference in attraction of the entire earth from the same viewpoints and thus, as they say, include a free‐air gradient term. However, their equation (5) would be correct only if the mean density of the earth were equal to that of the shell, and the free‐air gradient obtained by their equation (10) is correct only under these circumstances.

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The effect of terrain on gravity measurements in a borehole and on formation density derived from borehole gravity data is studied as a function of depth in the well, terrain elevation, terrain inclination, and radial distance to the terrain feature. The vertical attraction of gravity [Formula: see text] in a borehole resulting from a terrain element is small at the surface and reaches an absolute maximum at a depth of about one and one‐half times the radial distance to the terrain element, then decreases at greater depths. The effect of terrain on calculated formation density is proportional to the vertical derivative of [Formula: see text] and is maximum at the surface, passes through zero where |[Formula: see text]| is greatest, and reaches a second extremum of opposite sign to the first and of much lower magnitude. Accuracy criteria for borehole‐gravity terrain corrections show that elevation accuracy requirements are most stringent for a combination of nearby terrain features and near‐surface gravity stations. Sensitivity to terrain inclination is also greatest for this combination. The measurement of the free‐air gradient of gravity, commonly made’slightly above the ground surface, is extremely sensitive to topographic irregularities within about 300m of the measurement point. The effect of terrain features 21.9 to 166.7 km from the well [Hammer’s (1939) zone M through Hayford‐Bowie’s (1912) zone O] on calculated formation density is nearly constant with depth. At these distances, the terrain correction will be equivalent to a dc shift of about [Formula: see text] of average elevation above or below the correction datum. The effect of topography beyond 166.7 km is not likely to exceed [Formula: see text].