The Promontorio breccia pipe located in southern Sonora, Mexico, has been mined intermittently for silver since the turn of the century. Mineralization occurs as sulfides in the matrix of the breccia. Two to five percent pyrite also occurs in the premineral andesite that the breccia pipe has intruded. Geologic mapping led to the discovery of a small, unmineralized breccia in andesite 200 m west of Promontorio. The discovery of this breccia encouraged the search for other hidden mineralized breccias in areas obscured by postmineral alluvium. Exploration of the covered areas was carried out with an induced polarization (IP) and resistivity survey after a test line indicated that a distinct IP anomaly occurs over the Promontorio breccia pipe. The gradient array was chosen as the most effective electrode configuration to explore the covered areas because it can provide rapid detailed IP coverage necessary to locate small breccia pipes. Several anomalous IP trends were located. The most interesting one has an apparent response of 60 to 90 msec in low‐resistivity rocks similar to Promontorio. Additional information provided by dipole‐dipole profiles indicates that this anomalous zone is close to surface and about 20 m wide with a vertical to steep dip. Trenching and drilling confirmed the presence of a 20-m wide elongate breccia with sulfide mineralization similar to Promontorio. Silver grades were marginal for making ore in 1977.

If there are gravity data and some geological information available from the region of an intermontane basin, then the volume of saturated sediments, volume of ground water available from storage, and the total volume of ground water can be determined from the gravitationally determined anomalous mass. The anomalous mass occurs because of a density contrast which exists between low density alluvium and high density bedrock which define the surface and subsurface boundaries of the basin. The gravity effect of the anomalous mass can be detected by a gravity survey of the basin after which it can be separated from other gravity effects by a regional‐residual separation. The anomalous mass is uniquely determined by applying Gauss's theorem to the residual gravity map. A model of an intermontane basin is developed which relates the anomalous mass to the total volume of saturated sediments. The total volume of saturated sediments is determined from the anomalous mass, the density contrasts between unsaturated and saturated alluvium and bedrock, the area of the basin, and the water table depth. The volume of water available from storage and the total volume of water in the basin are determined from the volume of saturated sediments and the storage coefficient and porosity of the sediments. The method is illustrated by a case history from Avra Valley, Arizona. It is concluded that the major advantage of this method occurs when well data are not available, because it eliminates the need for arbitrary assumptions about subsurface basin geometry to determine the volume of saturated sediments.

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The Geophysics Laboratory at the University of Arizona has had a continuing interest in making and analyzing gravity measurements over the state for a variety of purposes. Gravity work has continued at an increasing level of activity since the last report of this sort made to the American Geophysical Union [Sumner, 1965]. Master's and doctoral theses have been completed on the Tucson area [Abuajamieh, 1966], in the Tucson basin [Davis, 1967], and near Nogales in the Ruby quadrangle [Hench, 1968]. Other academic gravity survey work is now underway in the Avra valley immediately west of Tucson, in Walnut Gulch near Tombstone, in the Sulfur Springs and San Pedro valleys, in the vicinity of the Tonto Forest Seismological Observatory near Payson, and in the Flagstaff area.

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