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Publications > Expedition Publications > Logging Summaries

Logging Summaries

IODP Expedition 344:

Costa Rica Seismogenesis Project, Program A Stage 2 (CRISP-A2)

Expedition 344 Scientific Party

Introduction

    Figure 1. Location map of CRISP program sites, IODP Expedition 334.

    Integrated Ocean Drilling Program Expedition 344 (CRISP-A2) is the second expedition in the Costa Rica Seismogenesis Project, which started with Expedition 334 (Figure 1). The CRISP program was designed to understand the processes that control nucleation and rupture of large earthquakes at an erosional convergent margin. The Costa Rica location was selected because of its relatively thin sediment cover, fast convergence rate, abundant seismicity, subduction erosion, and change in subducting plate relief along strike. CRISP drilling complements other deep-fault drilling (San Andreas Fault Observatory at Depth and Nankai Trough Seismogenic Zone Experiment) and investigates the first-order seismogenic processes common to most faults and those unique to erosional margins. The primary goals of Expedition 344 were to estimate the composition, texture, and physical properties of the décollement zone and upper plate material; to assess the rates of sediment accumulation and margin subsidence/uplift in slope sediment; to evaluate fluid-rock interaction, the hydrologic system, and the geochemical processes in the upper plate; to measure the stress field across the updip limit of the seismogenic zone; and to study the Cocos Ridge subduction and evolution of the Central American volcanic arc.

    Figure 2. Wireline logging tool strings used during IODP Expedition 344. 

    The downhole logging program of Expedition 344 was designed to complement the core sample record by measuring continuous, in situ profiles of physical properties such as bulk density, porosity, resistivity, and natural gamma ray radiation. In addition to these formation properties, downhole logging provides oriented images of the borehole wall useful to determine the directions of bedding planes, fractures, and borehole breakouts. As the logging-while-drilling used in the previous CRISP Expedition 334 was not available due to budget constraints, in Expedition 344 we conducted wireline logging operations, where downhole measurements are taken by tool strings lowered in a previously drilled borehole (Figure 2).

Logging Operations

    Figure 3. Logging data showing the inferred top depth of the 10¾ inch casing string in Hole U1380C.
    Figure 4. Logging data showing the inferred base depth of the 10¾ inch casing string in Hole U1380C.

    Wireline logging was attempted in Holes U1380C, U1412B, and U1413C, drilled in the upper plate of the Costa Rica margin, and in Hole U1414A on the subducting Cocos plate. Logging in the upper plate sites proved challenging because the holes were unstable after drilling, making it difficult for the tool string to enter the open hole interval. Despite repeated attempts, we were unable to exit the drill pipe and log in the open hole at Holes U1380C (middle slope) and U1412B (base of the slope near the toe of the frontal prism). Nonetheless, in Hole U1380C the log data were useful to image the 10 3/4 inch casing string, whose final depth was uncertain because the string fell into the hole during deployment (Figures 3 and 4).

    Hole U1413C was drilled in the upper slope at 540 m water depth and is a “pilot hole” for proposed deep riser drilling. Hole U1413C was cored to 582 mbsf, and the first logging string deployed was a slick Triple Combo that measured borehole diameter, natural gamma ray, and resistivity. Because of a borehole obstruction, the tool string could not descend below 187 mbsf, and we logged an 84 m open hole interval to the base of the drill pipe (104 mbsf). Two more tool strings focused on imaging borehole breakouts were successfully deployed in this interval: a UBI tool string (ultrasonic imaging) and a FMS tool string (resistivity imaging; for explanations of tool acronyms and tool descriptions see http://iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html).

    Hole U1414A was located on the flank of the subducting Cocos Ridge, ~1 km seaward of the deformation front at 2469 m of water depth. The two logging strings deployed in Hole U1414A, a Triple Combo and a FMS-Sonic combination (Figure 2), could not reach the bottom 50 m of the cored interval because of a borehole obstruction. Logging data were successfully collected in the interval between 421 mbsf and the base of the drill pipe (94 mbsf).

     

Logging Results

    Hole U1413C

    Figure 5. Summary of wireline log data acquired in Hole U1413C.

    A summary of the downhole logging measurements collected in Hole U1413C is in Figure 5. Three logging units were defined on the basis of the log data. Logging Unit 1 (93-148 mbsf) is characterized by total gamma ray values of 38-46 gAPI, by a low U content of 1.4-2.7 ppm, and by relatively low resistivities just above 1 ohm.m. In this unit, the UBI images display vertical bands with large reflection traveltimes and one pair of the caliper arms in the FMS tool measures a large borehole diameter, up to the maximum aperture of the arms (15 inches) In contrast, the borehole is almost circular and nearly in gauge through Logging Unit 2 (148-169 mbsf). Compared to Unit 1, Unit 2 displays a higher total gamma ray (~60 gAPI), higher U content (about 3 ppm), and higher resistivity. As the resistivity of sedimentary formations is mostly controlled by porosity, the increase in resistivity implies a decrease in porosity in Logging Unit 2. This decrease in porosity and the circular, in gauge borehole of Logging Unit 2 suggest a more consolidated formation than in Unit 1. The borehole seems to be washed out in all directions in Logging Unit 3 (169-184 mbsf), and the low values of natural radioactivity and resistivity measured around 169 mbsf are likely artifacts caused by a pronounced borehole enlargement.

     

    Hole U1414A

    Figure 6.. Summary of wireline log data acquired in Hole U1414A. 

    A summary of the downhole logging measurements collected in Hole U1414A is in Figure 6. We established four logging units in Hole U1414A. Logging Unit 1 (94-259 mbsf) is characterized by low values of total gamma ray, bulk density, resistivity, and elastic wave velocities that show a decreasing trend toward the base of the unit, where the total gamma ray value is ~10 gAPI, bulk density is 1.5 g/cm3, resistivity is 0.5 ohm.m, and compressional and shear velocities are 1.6 km/s and 0.4 km/s, respectively. The low and variable densities observed above 185 mbsf are likely unreliable because of an enlarged borehole. Logging Unit 2 (259-335 mbsf) displays an increase with depth of bulk density (1.6 to 1.8 g/cm3), resistivity (0.5 to 1 ohm m), compressional velocity (1.7 to 2.1 km/s), and shear velocity (0.5 to 0.7 km/s). Natural gamma ray values are generally 10-30 gAPI. Logging Unit 3 (335-375 mbsf) contains larger variations in physical properties, with gamma ray values ranging between 20-60 gAPI, bulk densities 1.8-2.2 g/cm3, resistivities 1-10 ohm m, compressional velocities 2-4 km/s, and shear velocities 0.5-2.7 km/s. Logging Units 1, 2, and 3 correspond to sediments that become progressively more consolidated with depth, and logging Unit 4 (375-410 mbsf) is the volcanic basement at Site U1414. This basalt interval features very low natural radioactivity (10 gAPI or less) and high bulk density (2.3-2.8 g/cm3), resistivity (2-100 ohm m), compressional velocity (3.2-6.7 km/s), and shear velocity (1.7-3.8 km/s).

     

Scientific Highlights

    Borehole Breakouts in Hole 1413C

    Figure 7. Downhole log images obtained in Hole U1413C.

    Borehole breakouts are sub-vertical hole enlargements that form on opposite sides of the borehole by local failure due to non-uniform horizontal stresses. In a vertical borehole, the breakout direction is parallel to the minimum principal horizontal stress orientation and perpendicular to the maximum principal horizontal stress orientation. Therefore, borehole breakouts are key indicators of the state of stress in the subsurface.

    Clear evidence for breakouts was collected in Hole U1413C by the UBI ultrasonic tool and the FMS microresistivity tool (Figure 7). The UBI images show an irregular, large radius borehole in Logging Unit 3 (below 169 mbsf) and a borehole that is smooth and has a nearly constant radius in Logging Unit 2 (148-169 mbsf). Within Logging Unit 1 (above 148 mbsf), the images show two nearly vertical bands of high rugosity (low amplitude) and large borehole radius (large traveltime). These two bands are breakouts that formed on opposite sides of the borehole. The FMS images in Logging Unit 1 show low resistivity values (dark) in the two pads that are in the same direction as the high rugosity/large traveltime bands in the UBI images. The FMS caliper measurements in this interval show that the pads measuring low resistivities also measure the larger borehole diameter. As the FMS tool is pulled up, the 6 cm-wide microresistivity pads get stuck in these large-diameter borehole sectors, and the measured low resistivity is likely caused by the rough borehole surface that prevents close contact with the pad. The azimuth of the pair of pads measuring the larger borehole diameter is also plotted in Figure 7, showing that the UBI and FMS measurements of these large-diameter borehole sectors are entirely consistent. Notably, the approximately N-S direction of the borehole breakouts imaged in Hole U1413C is the same as that detected by logging-while-drilling in Hole U1379A, which was drilled in a similar upper slope setting during Expedition 334.

       

    Borehole Breakouts in Hole U1414A

    Figure 8. Downhole log images obtained in Hole U1414A.

    Borehole images of FMS microresistivity and UBI reflection amplitudes (related to the small-scale roughness of the borehole wall) from Hole U1414A are in Figure 8. The images in Figure 8 span the lowermost interval logged in Hole U1414A, containing the consolidated sediments in Logging Unit 3 and the basalt of Logging Unit 4. In the sediment interval (340-375 mbsf) the FMS and UBI show a sedimentary formation with generally horizontal layers, except for an interval between 360 and 370 mbsf where there is evidence of a westward dip. The UBI image also shows vertical fractures oriented ESE-WNW, which could be drilling-induced tensile fractures The top of a steep fracture intersecting the borehole is also visible at 354-355 mbsf. The base of the sediment column at ~375 mbsf is marked by a thin (<0.5 m) borehole enlargement, which was also measured by the caliper logs. In the basalt (375-410 mbsf), the image logs show a complex set of fractures. Intervals of higher resistivity and reflection amplitudes (e.g., at 375-380 and 385-396 mbsf) correspond with intervals of higher core recovery, suggesting a more competent formation.

Summary

While downhole log data acquisition was challenging in the upper plate sites of Expedition 344 because of collapsing boreholes, wireline logging measured profiles of gamma-ray radioactivity, density, sonic velocity, and electrical resistivity together with ultrasonic and resistivity images of the borehole wall in Holes U1413C (upper slope) and U1414A (Cocos Ridge flank). The borehole images in Hole U1413C show clear evidence of borehole breakouts, which form when there are differences in the principal horizontal stresses. These images provide key data to estimate the state of stress in the upper plate, one of the major objectives of the CRISP program. Log data and borehole images collected in the lower part of Hole U1414A will complement core observations in intervals of incomplete recovery.

 

    Alberto Malinverno: Logging Staff Scientist, Borehole Research Group Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA
    Email: Alberto Malinverno

    Saneatsu Saito: Logging Scientist, Institute for Frontier Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061 Japan
    Email: Saneatsu Saito

     


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