RECONCILING PHYSICAL-PROPERTIES WITH SURFACE SEISMIC DATA FROM A LAYERED MAFIC INTRUSION

Compositional layering of intrusive rocks is often cited as a source o f seismic reflectivity in the crystalline crust. Direct evidence for t his is based primarily on synthetic seismograms calculated from labora tory measurements of rock velocity and density, or estimates of aggreg ate rock properties from measurements of mineral velocities and densit ies. Little is known about how in situ effects such as fracturing, che mical alteration, or anisotropy, may change the reflectivity character of the surface seismic data. An ideal dataset for examining the hazar ds of using physical properties data for estimating crustal reflectivi ty occurs at the Nellie intrusion, a middle Proterozoic-age layered ma fic intrusion that lies beneath the Permian Basin of west Texas. Due t o a well that penetrated 4.5 km of the intrusion, we have a dataset th at spans the scale from compositional data on the intrusion derived fr om well cuttings, to log data, to surface reflection data. Sonic and d ensity log data from the well measure velocities and densities 10 to 1 5% below the values calculated from modal mineralogy. We attribute thi s difference primarily to pervasive post-emplacement alteration and fr acturing of the rock mass in situ. Thus, this unique dataset provides a quantitative estimate of the magnitude of difference between observe d and theoretical velocities that can be expected for crystalline rock s in the upper crust and its cause. Despite the difference, analysis o f synthetic seismograms shows that primary compositional variation of the intrusion is still responsible for sub-horizontal layered reflecti vity in both the log-based and petrology-based synthetic seismograms. Boundaries of intrusive cycles are characterized by large negative ref lection coefficients due to the juxtaposition of mafic-rich basal laye rs against plagioclase-rich layer tops. In detail, only the log-based synthetic provides a good match to the surface seismic reflection data . This suggests that physical properties from modal mineralogy are use ful in obtaining a generalized model for reflectivity, but that differ ences in bulk physical properties of the rock mass preclude the possib ility of obtaining a one-to-one match with surface seismic data.