II. Discussion Reports on subsurface gravimetry started appearing with regularity about 30 years ago. Only four reports (nos. 1, 3, 4, and 95) predate 1949. Figure 1, a graph showing the number of reports published each year, shows that there were three periods of activity. The first was in the early to mid 1950's and a number of good theoretical papers were published including report no. 43 which is considered by some to be the classic in the fundamentals of borehole gravimetry. The second period was in the mid 1960's when several good reports that can be used as basic references, were written. These included report nos. 38 and 40. The third period started in the mid 1970's and continues to the present. This group contains some good case history studies. Almost half of the reports were published in U.S. Geological Survey (USGS) publications and in Geophysics. Figure 2 shows the distribution of the bibliography by publisher. The first category (section A) contains reports that pertain to theory, methods, data reduction and applications. Several reports in this section are noteworthy; namely nos. SO, 38, 40, and 5. Report no. 19 is a good beginning reference. There is now a good set of literature on terrain corrections and this includes report nos. 7, 8, 9, 23, 31, and 49. The following reports contain discussions on deducing structure away from the borehole: nos. 16, 24, 26, 28, and 51. Section B contains 20 reports that pertain to instrumentation. Within borehole gravimetry, there are two types of instruments that have been developed: (1) vibrating filament, and (2) astatized spring. Esso Production Co., Exxon, and Shell Development have built instruments of the first type which are discussed in report nos. 63, 64, 66, 68, and 73. LaGos te and Romberg makes the only borehole gravity meters (BHGM) of the second type. Report nos. 5,' 60, 61, 69, 71, 72, and 76 describe these meters. Section C lists 40 reports that con~ain case histories. Eleven of these reports are also listed in sections A and B. Case histories are given for surveys run in boreholes and in underground mines in the following states and countries:
