Sound Velocity-Density Parameters of Sediment and Rock from DSDP Drill Sites 315–318 on the Line Islands Chain, Manihiki Plateau, and Tuamotu Ridge in the Pacific Ocean

Abstract

Compressional sound velocity, wet-bulk density, water content, porosity, and acoustic impedance and anisotropy were determined for Quaternary through Cretaceous sediment and rock recovered from 0 to 1000 meters below the sea floor at Deep Sea Drilling Project Sites 315-318 on the Line Islands chain, Manihiki Plateau, and Tuamotu Ridge in the western Pacific Ocean. Regional correlations of these properties versus depth below sea floor are good for the Line Islands chain, the Manihiki Plateau, and the Tuamotu Ridge. Measurements were made under laboratory conditions, and comparison of laboratory interval velocities to those from the seismic reflection and drill depth records shows some discrepancies, suggesting that temperature-pressure corrections of up to +15% might be needed. In order to properly determine the in situ corrections needed will require future research of the porosity and velocity rebound characteristics, when in situ pressures are released, for a variety of sedimentary types with a wide variation in porosity and cementation. Velocities through Pleistocene through Oligocene chalk are typically 0 to 3% greater horizontally than vertically, while Eocene and older sedimentary rocks typically have 5% to 15% anisotropy. Velocity versus its corresponding impedance is an approximately linear relationship for a given mineral makeup, with an impedance error of ±0.

Site 415, Agadir Canyon, Deep Sea Drilling Project Leg 50

“…In general, the Archie (1942) and Boyce (1968) equations compare well to the porosities of 40 and 51 per cent derived from the wet-bulk densities from the FDC log (2.04 and 1.84 g/cm 3 ; see Boyce, this volume). In summary, the FDC log appears to record true formation density.…”

“…For Leg 50 analog GRAPE data (particularly Site 416), all rock diameters were measured by hand, usually one measurement per 5 cm core segment. These core segments are very rough and irregular; therefore, when these diameters (and assumption of offset from the gamma-ray beam as described by Equation 36 in Boyce, 1976) are applied to the raw GRAPE data, the resulting adjusted data (dotted lines) are subject to huge errors, particularly when small, irregular-diameter core segments are scanned and the calculated (Equation 38) offset is incorrect, thus causing extremely bad data. As a result, the unadjusted GRAPE data are plotted as a solid line, with "diameter-adjusted" data presented as a dotted line.…”

“…If "HumbleV formula (Winsauer et al, 1952) is used, F = 0.62 Φ~2 A5 , and the resulting porosities are 37 and 45 per cent. Boyce's (1968) …”

“…In the Cretaceous section, sound travels faster horizontally by 0 to 20 per cent (12% being typical). Acoustic anisotropy may be created by any combination of (1) hydrostatic pressure, and temperature, as discussed in Boyce (1976); however, because there is no correction for grain-to-grain overburden pressure, these velocity values are minimal. Details of the calculations are in Boyce (this volume).…”

“…Investigators interested in the density of a specific layer or rock piece should check the sample diameter from the core photographs and make the appropriate diameter corrections, as discussed in Boyce (1976 Section 1, 80-140 cm -DOLOMITE, grayish yellow green (5GY 7/2) to grayish olive green (5GY 3/2), with irregular silty laminae. Crystals of anhydrite!?)…”