README The digital data and map of this open-file report are available from the following U.S. Geological Survey URL: http://wrgis.wr.usgs.gov/open-file/of02-373/ Data Contents The digital dataset consists of one file (monterey_100k.iso) containing 2,385 gravity stations. The file, monterey_100k.iso, contains the principal facts of the gravity stations, with one point coded per line. The format of the data is described below. Each gravity station has a station name, location (latitude and longitude, NAD27 projection), elevation, and an observed gravity reading. The data are on the IGSN71 datum and the reference ellipsoid is the Geodetic Reference System 1967 (GRS67). The free-air gravity anomalies were calculated using standard formulas (Telford and others, 1976). The Bouguer, curvature, and terrain corrections were applied to the free-air anomaly at each station to determine the complete Bouguer gravity anomalies at a reduction density of 2.67 g/cc. An isostatic correction was then applied to remove the long-wavelength effect of deep crustal and/or upper mantle masses that isostatically support regional topography. Offshore bottom-gravity stations were reduced according to the descriptions in Naval Postgraduate School theses (Brooks, 1973; Cronyn, 1973; Souto, 1973; Spikes, 1973; Woodson, 1973), after observed gravity values were placed on the IGSN71 datum and data adjusted for the Geodetic Reference System 1967. These stations are BROOKS01-BROOKS82, SPIKES02-SPIKES82, SOUTO01- SOUTO055, W1-W8, and CRONYN01-CRONYN80. Stations with the name "contour" lack observed gravity information because these locations were digitized from simple Bouguer gravity contour-trackline intersections (Chapman and others, 1990) and then reduced to complete Bouguer and isostatic anomalies. EXPLANATION OF PRINCIPAL FACT FORMAT Item Explanation STATION NAME (a8) ................An alphanumeric combination of up to 8 characters used for station identification LAT (f3.0,f6.3)...................Latitude in degrees and minutes, to 0.01 minute LON (f4.0,f6.3)...................Longitude in degrees and minutes, to 0.01 minute ELEV (f8.2).......................Elevation, to 0.1 ft OG (f10.3).........................Observed gravity, to 0.01 mGal FAA (f9.3) .......................Free-air anomaly to 0.01 mGal SBA (f8.3)........................Simple Bouguer anomaly to 0.01 mGal ITC (f7.3)........................Inner terrain correction for a density of 2.67 g/cc, to 0.01 mGal, followed by a letter denoting the extent of the correction. "Z" indicates computer terrain correction from the station out to 166.7 km with inner terrain correction out to Hayford D zone. TC (f7.3).........................Total terrain correction from the station to 166.7 km for a density of 2.67 g/cc, to 0.01 mGal. TC CODE (a1)......................Letter denoting the extent of the correction, according to the Hayford-Bowie template (e.g. 'D' means 590 meters). CBA (f8.3)........................Complete Bouguer anomaly reduced for a density of 2.67 g/cc, to 0.01 mGal. ISO (f8.3)........................Isostatic residual anomaly values assuming an Airy model for isostatic compensation of topographic loads. This model assumes a a crustal thickness of 25 km, a topographic load density of 2.67 g/cc, and a density contrast across the base of the model crust of 0.4 g/cc. Example of format for gravity file 61990879 36 30.89 121 52.43 1115.0 979795.58 37.73 -0.29 2.29 9.27 D 8.52 -15.83 STATION NAME: 51636676 LAT: 36 degrees 30.89 minutes North LON: 121 degrees 52.43 minutes West ELEV: 1115.0 feet OG: 979795.58 mGal FAA: 37.73 mGal SBA: -0.29 mGal ITC: 2.29 mGal TC: 9.27 mGal TC CODE: D (590 m) CBA: 8.52 mGal ISO: -15.14 mGal Brooks, R.A., 1973, A bottom gravity survey of the shallow water regions of southern Monterey Bay and its geological interpretation: Naval Postgraduate School Master's thesis, 90 p. Chapman, R.H., Chase, G.W., and Youngs, L.G., 1990, Bouguer gravity and magnetic anomaly map of the Central California continental margin: California Continental Margin Geologic Map Series Central California Margin Map 5C, scale 1:250,000. Cronyn, B.S., 1973, Underwater gravity survey of northern Monterey Bay: Naval Postgraduate School Master's thesis, 53 p. Souto, A.P.D., 1973, A bottom gravity survey of Carmel Bay: Naval Postgraduate School Master's thesis, 68 p. Spikes, C.H., 1973, A gravimetric survey of the Santa Cruz-Ano Nuevo Point continental shelf and adjacent coastline: Naval Postgraduate School Master's thesis, 114 p. Telford, W.M., Geldart, L.O., Sheriff, R.E., and Keyes, D.A., 1976, Applied Geophysics: New York, Cambridge University Press, 960 p. Woodson, W.B., 1973, A bottom gravity survey of the continental shelf between Point Lobos and Point Sur, California: Naval Postgraduate School Master's thesis, 110 p.