(Table 2) Estimated basement temperatures at DSDP Hole 69-504B@en

The achievement of deep penetration (562 m) of seafloor basalts at Hole 504B, near the Costa Rica Rift (1°13.63'N, 83°43.81'W), on DSDP Legs 69 and 70 presented a rare opportunity to examine the structure of young (6 m.y.) oceanic crust. In addition to the recovery of samples for laboratory studies, an extensive suite of downhole logs and experiments was carried out at this site, for two main purposes: (1) to allow reliable deductions about the nature of the entire section of penetrated crust, because recovery of samples was far from complete (-25%); (2) to probe the physical state of rock around the drilled hole on a scale of tens of meters to kilometers. Information on the latter large-scale phenomena at Hole 504B were provided mainly by the oblique seismic experiment, utilizing a bore-hole seismometer (Stephen 1983), and by the large-scale-electrical- resistivity experiment described below.在位于哥斯达黎加裂谷附近（北纬1°13.63'，西经83°43.81'）的504B孔，深海钻探计划（Deep Sea Drilling Project, DSDP）第69与70航次实现了海底玄武岩562米的深穿透，这为研究年轻（6百万年）洋壳的结构提供了难得的契机。除采集用于实验室研究的岩样之外，该钻孔位点还开展了成套井下测井与实验工作，主要出于两个目的：（1）因样品回收率仅约25%、远未实现完整采集，故而能够对已穿透的地壳全段性质做出可靠推断；（2）以数十米至千米级尺度探究钻孔周围岩石的物理状态。针对504B孔的此类大尺度现象的相关信息，主要通过两项实验获取：一是借助井中地震仪（bore-hole seismometer）开展的斜向地震实验（Stephen 1983），二是下文将详述的大尺度电阻率实验。