The need for accurate reference bases for any gravimetric work can hardly be overemphasized. During the months of March, April, and May, 1964, about 130 gravity observations were made in Arizona, with LaCoste Romberg Gravity Meter DL-1 and Worden Gravity Meter (Educator) No. 461. The purpose of this program was to establish 1) a 1st order control airport gravity base network, 2) a standard calibration range for the State of Arizona, and 3) to gain a structural interpretation of the Tucson Basin. Gravity data were analyzed as to their accuracies and reliabilities, taking into consideration errors involved in tidal corrections, drift corrections, reference datum, and nonlinearity of scale factor of the meters. It is concluded that the reliability of these data is .1 milligal or better. While correcting for the tidal variation of gravity, it was noticed that there was a discrepancy between the theoretical and observed tidal correction values. For any additional precise work, it is desirable to correct for tidal variations from actual records, if available, in conjunction with the theoretical tables. Causes for this discrepancy in tidal gravity variation need further study. Programs were written for a digital computer to calculate 1) the theoretical gravity values from the International Gravity Formula, and 2) Free-Air Anomalies, Bouguer Anomalies and Special Bouguer Anomalies from field data for various stations. Free-Air and Bouguer Anomaly values for different stations were analyzed as to their implications in terms of isostasy, crustal structures and local geological structures. Results from Simple Bouguer Anomaly values indicate a crustal thickness of 49 km to 33 km for Arizona with broad isostatic compensation for regional surface irregularities. An analysis of residual Bouguer gravity anomalies of the Tucson Basin in terms of local geological structures, indicates a basin and range structure for this region. The thickness of sediments ranges from more than 700 feet on the north to more than 8000 feet towards the south of the basin with faults indicated in it. Application of a limiting -depth interpretation method implies that the tops of the disturbing bodies can be no deeper than 2 miles below sea level. A total mass deficiency corresponding to the residual gravity low in this basin comes out to be 1.8 x 10¹⁷ grams according to two – dimensional form of Gauss' Theorem. This corresponds to a 135 cubic mile volume of material with a density .3 grams per cubic centimeter less than the enclosing rocks. From porosity and volume considerations of the sediments in the Tucson Basin, it is estimated that the total water holding capacity may be of the order of 4.6 x 10⁷ acre feet.

An interpretation of gravity and magnetic anomalies of the Butte district, Montana, is based on the analysis of five gravity profiles constructed from Bouguer gravity data of the Butte district observed during the summer of 1974 and give concurrent magnetic profiles constructed from U.S. Geological Survey high-level aeromagnetic data of the district. Of primary concern in this analysis is the interpretation of the Bouguer gravity high and aeromagnetic low associated with the Butte orebody. Results of this interpretation yield a configuration for the Butte orebody characterized by vertical contacts extending from the surface or directly beneath Cenozoic basin fill to 4,500 feet below sea level, with a central core, elongate in the north -south direction, surrounded by a variable outer zone, which is more pronounced in the southern and western portions of the district. The central core is interpreted as a region of zero magnetic susceptibility with three density distributions, all of which are of higher densities than the surrounding host rock and are related to the varying degrees of mineralization and alteration present in this region. The laterally variable outer zone is interpreted as a zone of low magnetic susceptibility with a density equal to that of the host rock. This zone is related to the peripheral mineralization and alteration of the Butte orebody.

The depths to buried bodies, characterized by anomalous gravity and magnetic properties, are determined by a combination of two numerical techniques. An upward continuation integral is solved by a method by Paul and Nagy using elemental squares and low order polynomials to describe the behavior of the gravity or magnetic data between observed data points. Downward continuation of the magnetic or gravity data is done by a finite difference technique as described by Bullard and Cooper. The applicability of the techniques are determined by comparison to depths determined by other means over the same anomalies and by comparison to various rule-of-thumb methods prevalent in the geophysical literature. The relative speed and cost of the particular computer system used is also considered in the applicability. The results show that although the initial costs of the computer program are high, the combined technique is as good as and at times better than the rule-of-thumb methods in determining the depth to the anomaly-causing body and is useful when more than just an approximate depth is of interest.