Maya Elkibbi

An outsider's review of the astronomical theory of the climate: Is the eccentricity-driven insolation the main driver of the ice ages?

Abstract Although the astronomical theory of the climate (also known as the “Milankovitch” theory) explains a number of key features of the paleoclimate time series (especially frequency and phase distribution) in terms of orbital influence on the waxing and waning of ice sheets, it still faces several important unresolved challenges. In this review, we discuss five main challenges we believe the theory must resolve in order to survive: (1) The large amplitude of the ∼100 ky ice age cycles; (2) the switch from 41-ky- to 100-ky-long glaciations around 0.9 Ma BP; (3) the absence of significant spectral power at 413 ky over the past 1.2 Ma; (4) the variation in glacial cycle duration in the last 800 ky; and (5) the presence of non-orbital spectral peaks in the climate record. But in spite of these problems, we conclude that the Milankovitch theory may still be a viable one; at least in the original sense of Hays et al. [Science 194 (1976) 1121], that orbital changes somehow influenced the ice ages. Many investigators, ourselves included, appear to see the impact of orbital forcing in the data, clear evidence of astronomical forcing, as well as evidence which suggests that the climate system responds nonlinearly to all Milankovitch (orbital) frequencies.

Shear-wave splitting and reservoir crack characterization: the Coso geothermal field

Abstract This paper aims to improve current understanding of the subsurface fracture system in the Coso geothermal field, located in east-central California. The Coso reservoir is in active economic development, so that knowledge of the subsurface fracture system is of vital importance for an accurate evaluation of its geothermal potential and day-to-day production. To detect the geometry and density of fracture systems we applied the shear-wave splitting technique to a large number of high-quality seismograms from local microearthquakes recorded by a permanent, 16-station, down-hole, 3-component seismic array running at 480 samples/s. The analysis of shear-wave splitting (seismic birefringence) provides parameters directly related to the strike of the subsurface fractures and their density (number of cracks per unit volume), and, consequently, is an important technique to outline zones of high permeability. Three major fracture directions N10–30W, N0–20E, and N40–50E, of which the first and the second are the most prominent, were identified from the seismograms recorded by the 16-station down-hole array. All orientations are consistent with the known strike of local sets of faults and fractures in local wells and at the surface, as well as with previous analyses of seismic anisotropy in the region. The high quality of the recordings has allowed us to launch an unprecedented investigation into the characteristics of the temporal variations in crack polarization and crack density in a producing geothermal environment. Preliminary results point to significant temporal changes in shear-wave time delays, probably influenced by temporal changes in crack density within a period of 5 years (1996–2000). They are tentatively interpreted as due to a local ∼3% increase in shear-wave velocity in the southwestern part of the field during 1999.