The oceanic crust formed at the East Pacific Rise (EPR) and subducted offshore Costa Rica (Fig. F1) has an anomalously low heat flow of only one-third of the expected value (Fisher et al., 2003). This is explained by fluid circulation in the basement of the downgoing plate, which effectively cools the crust (Kimura, Silver, Blum, et al., 1997).

How and where this fluid flow takes place was one focus of Ocean Drilling Program (ODP) Leg 205. Therefore, a ~170-m-thick interval of oceanic crust was cored and logged at Site 1253, ~200 m seaward of the Middle American Trench (MAT; Figs. F1, F2). Logging comprised density, porosity, and resistivity measurements (Shipboard Scientific Party, 2003c), Formation MicroScanner (FMS) electrical imaging of the borehole walls, and Dipole Shear Sonic (DSI) measurements. DSI records allow for full waveform processing of compressional (P-wave), head (S-wave), and Stoneley waves.

Stoneley waves are a useful indicator for possible fluid pathways because permeability increases their attenuation and decreases their velocity. Our research was intended to validate the usefulness of such DSI logs for assessment of hydrological properties of oceanic basement rocks. If possible, we aimed to derive a qualitative permeability estimate from Stoneley waves in crystalline basement at Site 1253. A quantitative analysis is beyond the scope of this paper because it would require comparison with in situ permeabilities not yet available from circulation obviation retrofit kit (CORK) data or a sophisticated inversion technique (i.e., Brie et al., 2000).

Based on a database search (, it seems that not many studies have been performed on Stoneley wave data collected during ODP, although full waveform acoustic logging has been performed as a standard for several years. In contrast, the exploration industry uses Stoneley wave attenuation to detect and characterize permeable zones based on hydrocarbon production (Mikhailov et al., 2000; Hornby et al., 1989). Their main interest, however, is focused mainly on boreholes in sedimentary rocks.

It is therefore sensible to evaluate whether processing Stoneley waves recorded in crystalline basement boreholes yields additional information on permeability distribution. This information could complement core measurements and other methods to derive permeability and could be of special interest in boreholes where core recovery is small. Our research could indicate that Stoneley wave processing could be implemented into standard ODP data processing to add information without requiring more measurement time.

In this paper, we present our attempts to use Stoneley waves to identify permeable zones in the basement logged at Site 1253. In order to derive their velocity and energy profiles with depth, we applied a semblance analysis to detect coherent arrivals in the full waveform records. Although velocity was already a result of onboard processing by the Schlumberger engineers and helped us in controlling the performance of our algorithm, Stoneley energy is not derived in standard ODP processing. We identified zones of higher attenuation and lower velocity in the profiles, but this effect can be also induced by borehole breakouts. Therefore, we restricted our further investigations to smooth borehole sections, assuming that the observed velocity and energy variations are mainly permeability induced in those sections. We then correlated Stoneley wave attenuation with core and FMS fracture observations.

We will present an overview of acoustic logging and then introduce the Stoneley wave. A short description of the semblance analysis concept will be given before we present calculated velocities and energy estimates for Stoneley waves. Finally, we will discuss the results, including FMS measurements and fracture observation on cores.