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Supplementary material to “Scientific Drilling Into the San Andreas Fault Zone”

1 June 2010

Mark Zoback, Department of Geophysics, Stanford University, Stanford, California, USA

Stephen Hickman and William Ellsworth, U.S. Geological Survey, Menlo Park, California, USA

Citation:

Zoback, M., S. Hickman, and W. Ellsworth (2010), Scientific drilling into the San Andreas fault zone, Eos Trans. AGU, 91(22), 197–199. [Full Article (pdf)]

Table S1. Accessing On-line SAFOD Data

An Important Note About SAFOD Depths

Unless otherwise noted, all depths reported for SAFOD are measured depths (MD) along the deviated borehole trajectory relative to the drill rig floor (i.e., Kelly Bushing, or KB), which is 9.45 m (31 ft) above ground level. While drilling or coring, measured depths were determined using a drill-pipe tally prepared for each SAFOD Phase based upon precise measurements of the length of each section of drill pipe, which was run in and out of the hole in a fixed, predetermined order. When logging the hole with geophysical instruments, electronic depth counters measure the amount of wireline going in and out of a hole. Different depth counters were used in the different logging operations.

Measured depths discussed in this paper are derived from (or synchronized to) depths from the open-hole geophysical logs shown in Figure 2, which were acquired in the Phase 2 SAFOD borehole on August 11, 2005, by Baker-Atlas, Inc. Conversions between MD and true vertical depth for SAFOD Phases 1, 2 and 3 can be obtained using the gyroscopic directional surveys available at the ICDP web site (see URL in Table S1 above).

SAFOD depths have been synchronized to relate repeated measurements of casing deformation in the Phase 2 hole with the geophysical data shown in Figures 2 a, b and the cores recovered during Phase 3 (e.g., Figures 2 c-f). Following standard industry practice, depths for SAFOD wireline logs were synchronized to each other using sensors on each tool that record the intensity of natural gamma radiation as a function of depth. Depending upon the clarity of the signals obtained, the accuracy attainable with this type of inter-log synchronization is about ±0.3 m. Depth synchronization of geophysical logs along a single continuous borehole is relatively straightforward. The repeated casing deformation measurements made using a 40-arm caliper tool (a Schlumberger PMIT tool) during the two-year period between Phases 2 and 3 were synchronized in depth to the Phase 2 open-hole logs (Figure 2 a, b) using distinctive natural gamma anomalies visible in both logs. In this manner, we determined that 1.52 m should be subtracted from depths in the final PMIT log collected in the Phase 2 hole (on June 6, 2007) to synchronize its depths to the Phase 2 Baker-Atlas open-hole logs collected on August 11, 2005. Highly repeatable natural gamma logs are routinely used in the petroleum industry to align geophysical logs obtained by different service companies. Prior to alignment, small errors in depth measurement accumulate over the depth of a well. This 1.52 m correction factor applies both to the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). Once this 1.52 m depth shift is applied, both the SDZ and CDZ as identified in the PMIT logs coincide with the distinctive low-velocity zones identified in the Phase 2 open-hole logs (Figure 2b).

Since the Phase 3 SAFOD borehole was drilled as a multilateral branching off of the Phase 2 hole, synchronizing depths between the Phase 3 core and the Phase 2 geophysical logs that define the location, physical properties and deformational behavior of the San Andreas Fault Zone is more complex. Accordingly, a variety of techniques were employed to map the SDZ and CDZ as revealed in the Phase 2 open-hole and PMIT logs into the Phase 3 core:

1. Using open-hole natural gamma and resistivity logs acquired across the SDZ in both the Phase 2 and Phase 3 holes. Owing to hole stability problems, these logs could only be run in the Phase 3 hole across the SDZ, as access to the CDZ was blocked by hole collapse following coring.
2. Using open-hole natural gamma logs acquired in the Phase 2 hole across both the SDZ and CDZ, in concert with high-resolution spectral gamma scans run in the laboratory on the entire Phase 3 core.
3. Using offsets in distinctive methane peaks identified near the CDZ during drilling of Phases 2 and 3 through real-time analysis of gasses dissolved in the drilling mud (T. Wiersburg, pers. comm.).

The first and second techniques take advantage of the fact that: 1) both the SDZ and CDZ are associated with an anomalously low total natural gamma count in the open-hole geophysical logs and 2) both intervals of foliated fault gouge in the Phase 3 core — the 1.6 m associated with the SDZ and the 2.6 m associated with the CDZ — have a distinctively low total natural gamma signature relative to the rest of the core. In this manner, for the SDZ we determined that 5.03 m should be subtracted from the depths indicated for the Phase 3 core to synchronize it to the Phase 2 Baker-Atlas open-hole logs collected on August 11, 2005 (Figure 2 a, b). In contrast, for the CDZ, 3.96 m should be added to the depths indicated for the Phase 3 core to synchronize its depths to the Phase 2 Baker-Atlas open-hole logs. Once these depth shifts are applied, both the SDZ and CDZ show a clear juxtaposition of: 1) the actively deforming zones identified in the repeat casing deformation (PMIT) logs, 2) the low P- and S-wave velocity zones identified in the Phase 2 open-hole logs (Figure 2 b), and 3) the foliated fault gouge recovered in the Phase 3 core.

SAFOD Observatory

The deployment of instrumentation within seismically active crustal rocks in the San Andreas Fault at SAFOD has opened a new window for the study of the earthquake source and fault zone structure. Over the course of the Major Research Equipment and Facility Construction (MERFC) phase of the EarthScope project, a total of nineteen experimental deployments of seismic and tilt sensors (MH001 through MH019) were made in the SAFOD main hole to 1) gather data to guide the drilling of Phases 2 and 3; 2) obtain information on the structure of the fault; and 3) test instrumentation systems before the installation of the SAFOD Observatory at the conclusion of the MERFC project. This research and development work was done in cooperation with Pinnacle Technologies and Oyo Geospace. In addition to the main hole deployments, eight instrument deployments were made in the SAFOD pilot hole, beginning with the installation of the 32-level 3-component pilot hole array in 2002 and including a year-long deployment of a broad band seismometer in cooperation with Guralp Systems, Inc. in 2008-2009.

Digital 3-component borehole seismometers were operated at measured depths up to 3417 m in the SAFOD main hole during the breaks between drilling Phases 1, 2 and 3. These instruments recorded numerous local earthquakes, including those in the SF, LA and HI target zones. An 80-level 3-component seismic array was installed by Paulsson Geophysical Services, Inc. (PGSI), in cooperation with Geometrics, Inc., in late April and early May 2005. During the 2-week deployment, over 2 TB of seismic data were collected by the PGSI array.

The SAFOD main hole also contains a fiber optic strainmeter designed and installed by Mark Zumberge of UC San Diego. The strainmeter was cemented into the annulus between casing strings set at the end of the Phase 1 drilling in 2004 and is still in operation today.

In September 2008 the SAFOD Observatory (MH020) was deployed to the bottom of the Phase 3 drill hole, with the instruments located at measured depths between 3069-3174 m. This placed them about 150 m above the Hawaii target earthquake (see Fig. 1a). The sensors in the SAFOD observatory include three 3-component seismometers, three 3-component accelerometers and two 2-axis tilt meters. An electromagnetic sensor was also installed at the bottom of the array in partnership with NASA.

Unfortunately, MH020 failed in October 2009, apparently due to water leakage into the control lines. Following the completion of the MREFC phase of the EarthScope program, the responsibility for operation and maintenance of the observatory was transferred from Stanford University to UNAVCO. UNAVCO, in consultation with its SAFOD Advisory Committee and the National Science Foundation is developing plans to reestablish the observatory.

The SAFOD Observatory was designed to permit wire line access to the top of the observatory instrumentation at a measured depth of 3063 m through the inside of the 2 3/8 in EUE tubing on which it was deployed. Following the failure of MH020 the U. S. Geological Survey in cooperation with the Institute of Earth Science and Engineering, University of Auckland, New Zealand, deployed a simple 3-component seismometer to bottom in December 2008 that continues to record dozens of nearby microearthquakes each week.

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