By Mark Zoback, Stephen Hickman, William Ellsworth, and the safod science Team doi: 10. 2204/iodp sd. 11. 02. 2011

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Scientific Drilling, No. 11, March 2011

Science Reports

carried out during the summer of 2004, involved rotary drill-

ing vertically to a depth of ~1.5 km, then steering the well at 

an angle ~60° from vertical toward the repeating microearth-

quakes described above (Fig. 3). Note that these earthqua-

kes occur to the southwest of the surface trace of the San 

Andreas, which indicates that at this location the fault dips 

steeply to the southwest. By design, Phase 1 ended just 

outside the San Andreas Fault Zone so that relatively 

large-diameter (24.4 cm) steel casing could be deployed and 

cemented in place prior to drilling through the active fault 

zone where substantial drilling problems might be encoun-

tered. Results from a number of scientific studies carried out 

during Phase 1 were needed to establish key engineering 

parameters (such as the optimal density of the drilling mud) 

for drilling through the San Andreas Fault during Phase 2 

(Paul and Zoback, 2008). 

Phase 2 was carried out during the summer of 2005.  

A relatively large-diameter (21.6 cm) hole was rotary drilled 

across the San Andreas Fault Zone (Fig. 3). While many of 

the key scientific objectives of SAFOD require recovery of 

core samples from the fault zone, we decided to rotary drill 

through the fault zone for several reasons. First, rotary drill-

ing is far more robust than core drilling. If the borehole 

turned out to be unstable due to the rock being highly bro-

ken up and chemically altered by faulting (which turned out 

to be the case), and/or high pore pressure was encountered 

in the fault zone at depth (which was not the case), it would 

be much easier to deal with such problems and ensure that 

we would make it all the way across the fault zone with rotary 

drilling rather than core drilling. Second, rotary drilling pro-

duces a larger diameter hole than core drilling. This was 

needed to carry out a wide range of sophisticated geophysi-

cal measurements (especially well logs) in the fault zone 

with equipment developed for the petroleum industry. When 

drilling problems are encountered during coring, it is com-

mon for the drill rod to get stuck in the hole. When this hap-

pens, the sizes of drill bit and coring rods are reduced so that 

coring can continue through the bottom of the stuck coring 

rod. Consequently, the diameter of core holes start relatively 

small and potentially reduces rapidly. As illustrated below, 

these geophysical measurements proved to be critical for 

defining the nature of the overall fault zone as well as the 

active shear zones within it. The final reason for maintaining 

a relatively large-diameter hole was related to deployment of 

the observatory instrumentation in the fault zone after drill-

ing. It was important to complete the well with sufficiently 

large-diameter casing (17.8 cm) to allow a suite of seismo-

meters and accelerometers to be deployed in the borehole.

Phase 3 was carried out during the summer of 2007; it 

involved drilling multi-lateral holes which start by milling a 

hole in the side of the steel casing in the Main Hole. By using 

multilateral drilling to create secondary holes at optimal 

locations (a technology that is now commonplace in the 

petroleum industry), we could direct coring efforts within 

the most important intervals identified during Phase 2. By 

addition, stress measurements in the Pilot Hole were found 

to be consistent with the strong crust/weak fault model 

discussed above (Boness and Zoback, 2004; Hickman and 

Zoback, 2004). In other words, stress differences in the crust 

1.8 km from the San Andreas were high and consistent with 

Byerlee’s law, whereas the direction of maximum horizontal 

stress in the lower part of the hole was nearly orthogonal to 

the San Andreas Fault. Furthermore, heat flow measured to 

2.2 km depth (Williams et al., 2004) was found to be consis-

tent with shallower data in the region, confirming that the 

shallow measurements are not affected by heat transport 

and thus indicate no frictional heat being generated by slip 

on the San Andreas Fault. Hence, the Pilot Hole confirmed 

that the SAFOD site was indeed an appropriate site for 

examining possible explanations for the San Andreas stress/

heat flow paradox. 

After drilling and downhole measurements were complet-

ed, the Pilot Hole was used for deployment of a vertical seis-

mic array to record naturally occurring microearthquakes 

and to image some of the large-scale structures at depth in 

the vicinity of the San Andreas (Chavarria et al., 2003; Oye 

and Ellsworth, 2007). This array was also used to record sur-

face explosions as an important part of the effort to constrain 

seismic velocities in the vicinity of the borehole for achiev-

ing the best possible locations of the target earthquakes 

(Roecker et al., 2004). Use of the Pilot Hole for experiments 

such as cross-hole monitoring of time-varying shear velocity 

(Niu et al., 2008) will continue to produce interesting results 

for years to come. From an engineering perspective, by es-

tablishing the depth to basement and the conditions affect-

ing drilling in the upper sedimentary section, the Pilot Hole 

helped establish key aspects of the engineering design of the 

upper part of the SAFOD Main Hole.

SAFOD Main Borehole

A great deal of engineering and operational planning went 

into SAFOD, since drilling, coring, and scientific measure-

ments in the hostile environment of an active, plate-bounding 

fault zone had never been attempted before. A number of sci-

entific workshops were held on drilling and downhole 

measurements, fault zone monitoring, and core handling. In 

addition, a formal advisory structure was established to take 

advantage of the knowledge and experience of scientists 

from universities, the USGS and U.S. Department of Energy

 

(DOE) national labs, and the petroleum industry. A Scientific 

Advisory Board provided high-level scientific guidance for 

the project. Technical panels on drilling, coring, and safety, 

downhole measurements, core handling, and downhole 

monitoring provided invaluable advice on literally hundreds 

of issues affecting how the project was eventually carried 

out. 

One of the most important aspects of the SAFOD opera-

tional plan that came out of this planning process was to 

carry out the project in three distinct phases. Phase 1, 

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