Premium
The effects of mylonitization and fractures on elastic wave velocities in crystalline rock — Examples from the Cajon Pass Scientific Drillhole
Author(s) -
Moos Daniel
Publication year - 1988
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl015i009p01053
Subject(s) - geology , gneiss , shear (geology) , shear zone , seismology , mineralogy , porosity , petrology , metamorphic rock , geotechnical engineering , tectonics
Sonic compressional and shear wave velocities were calculated from full waveforms recorded in the Cajon Pass drillhole from 250 to 1829 m depth. A complete suite of Schlumberger logs was also recorded over the same interval. Compressional velocities in the sediments (250‐495 m) range from 3.3 to 4.0 km/s; shear velocities are unreliable. Shallow crystalline basement has low velocities (Vp < 4.8; Vs < 2.8 km/s) and resistivities due to minor alteration. Below 708 m Vp is 5.5 to 6.0 km/s and Vs is 3.2 to 3.5 km/s. Velocities are generally lower in more felsic rocks. In gneisses sonic waveforms are more distorted, semblance coherence is lower and physical properties are more variable. Mylonitized zones and fractured intervals have distinctly lower velocities but different aggregate log response. Lower velocities are found within a zone of fractures within the gneiss at about 840 m. Porosity increases only at one large fracture within the interval. In a mylonitized shear zone at 1440 m the velocity reduction and the porosity increase occur across the zone.