Drill String Vibration: A Proxy for Identifying Lithologic Boundaries while Drilling (2002)

During Ocean Drilling Program (ODP) Leg 179, we recorded drill string vibration data to investigate the subseafloor environment as part of two seismic-while-drilling experiments in May 1998. Holes 1105A and 1107A were drilled in 714 and 1660 m water depth, respectively, in the Indian Ocean, where these experiments were conducted. To our knowledge, such measurements have never before been recorded by ODP or on other deepwater drilling rigs. By comparison of vertical and horizontal drill string acceleration with wireline logs and core data, variations in the formation properties are correlated to drilling parameters. Drill string acceleration signals vary inversely with porosity measured from logs and core data. The signal amplitude in sediments is roughly half that in either basalt or gabbro. These signals illustrate characteristics of the advancing drill bit— significant energy radiates through the seafloor and differently through various formations; thus, it may provide a useful tool to evaluate drilling conditions and formation properties encountered at the bit. Determining these properties while drilling, therefore, could assist both drillers and geologists in identifying lithologic contacts and sediment/ rock interfaces, even in cases where core and log data are not available. INTRODUCTION In the course of everyday operations aboard the JOIDES Resolution, the drill string vibrates continuously. These vibrations are characteristic 1Myers, G., Goldberg, D., and Rector, J., 2002. Drill string vibration: a proxy for identifying lithologic boundaries while drilling. In Casey, J.F., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 179, 1–17 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades NY 10964, USA. Correspondence author: gmyers@ldeo.columbia.edu 3University of California at Berkeley, 330 Hearst Mining Building, Berkeley CA 94720, USA. Initial receipt: 16 October 2000 Acceptance: 9 January 2002 Web publication: 30 May 2002 Ms 179SR-015 G. MYERS ET AL. DRILL STRING VIBRATION: IDENTIFYING LITHOLOGIC BOUNDARIES 2 and can be acquired at the rig floor or at the drill bit, although the latter provides a more accurate signature of the environment being drilled. Our goal is to evaluate whether these reverberations generated from the formation and environment encountered at the bit, by ship heave motions, and from other extraneous noise sources that can be used to improve operations and provide some understanding of the rock properties while drilling. Drill string acceleration data were acquired at two sites reoccupied during Ocean Drilling Program (ODP) Leg 179 (Pettigrew, Casey, Miller, et al., 1999). Figure F1 shows the location of our drill sites. Our primary location, Site 1105, is situated in the southwestern Indian Ocean on the eastern flank of the Atlantis II Fracture Zone. The rocks encountered at this site are characterized as anisotropic metagabbro containing alternating layers of olivine and Fe-Ti oxide (Pettigrew, Casey, Miller, et al., 1999). Numerous intervals of increased porosity associated with fractures are observed in the log data acquired at Site 1105 during Leg 179. The increased porosity zones are associated with intense regional uplift and deformation along the fracture zone (Robinson, Von Herzen, et al., 1989). Core recovery at this site was exceptional, on average >87%, and the structure of these igneous and metamorphic rocks may be readily observed. These features were oriented using borehole image logs (Goldberg et al., 1991). The foliation and fracturing that crosscut these cores dip steeply and strike north-northwest. Site 1107 is located on the Ninetyeast Ridge in the eastern Indian Ocean, near ODP Site 757 drilled in 1988. At this location, coring demonstrated, subaerially emplaced basaltic lava flows are overlain by >350 m of unconsolidated silt, clay, volcanic breccia, and tuff (Peirce, Weissel, et al., 1989). In this paper, we describe the drill string acceleration recorded during Leg 179 and investigate its correlation with the available log and core data at these sites. Seven key data sets are used in the analysis presented in this paper: 1. Stacked spectra of drill string acceleration data (The sampling interval of the accelerometers is 0.1 m.); 2. Autocorrelations of drill string acceleration to check for the presence of drill string multiples; 3. The root-mean-square (RMS) value of the acceleration vs. depth at Holes 1105A and 1107A (Depth smoothing of 1.5 m was applied.); 4. Drilling depth (Depth resolution of the driller's log is roughly 0.5 m.); 5. Logs acquired at Site 1105 (Vertical log resolution is ~0.15 m.); 6. Core data from Site 757; and 7. Analog drilling parameter data. METHODS AND OPERATIONS In typical seismic-while-drilling (SWD) experiments, the force of a roller-cone drill bit impacting the formation generates energy, which radiates axially and is received by sensors located on the seafloor (Rector and Marion, 1991). The energy generated at the bit is translated into the drill string in the form of vibrations with horizontal and vertical components. During Leg 179, two geophones were placed on the seafloor adjacent to the borehole to record the energy transmitted into the formation. The drill string vibrations are recorded uphole with a rig 735B and 1105A 1107 and 757 Indian Ocean 0° 20° N