NEW GRAVITY AND TOPOGRAPHIC DATA OVER THE CHICXULUB IMPACT STRUCTURE

We recently, July 2002, measured gravity and 3-D position at more than 300 stations at one-half kilometer intervals and more than 400 kilometers of continuous 3-D position data over the Chicxulub Impact Structure (CIS). The gravity data collection utilized two gravimeters simultaneously, which allows us to have high confidence in anomalies measured by both. Measurement of the position data, both station and continuous, was accomplished with an autonomous differential GPS system, C-NAV. This system relies on a worldwide network of known stations to compute the clock and orbit corrections for the GPS satellites. These corrections are computed by routines developed by NASA/JPL (Real Time Gypsy, RTG) and transmitted to geo-stationary satellites, which retransmit them for receipt by the C-NAV unit. Receiving both L1 and L2 GPS signals eliminates the atmospheric delay. With C-NAV we collected continuous real time data while driving to our stations and logged both real time and raw data (for post processing) at the stations. Preliminary investigation of the real time positions indicates that they are accurate in x-y to within a few decimeters, which is more accurate than required for our gravity survey, and in elevation to within about 2 meters, which corresponds to about .3 mgal error in Bouguer gravity. Since some of the anomalies we seek are less than one mgal we hope to be able to reduce the elevation error by careful analysis of the real time data and/or post processing of the raw data.

Our data strengthen the previously recognized relationships between topography and the buried CIS i.e. the area over the crater is low, there is a distinct rim and there are shallow valleys developed over several structural features of the crater. However, the small gravity anomalies so distinct along the western and northwestern coast of the Yucatan Peninsula are not as well developed along our N-S traverse from Uman to Ticul (over the recently drilled ICDP well in Yaxcopoil) or along our E-W traverse from Telchac Puerto along the coast to Dzilam de Bravo and then inland to Yalsihon.