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Geologging Imagery, Applications and Geological Interpretation

Shea Altadonna1, Jim Fulton2, E.I.T. 1 Geologist, Advanced Construction Techniques Inc. 1000 N. West St. Ste 1200, Wilmington, DE 19801; saltadonna@agtgroup.com 2 Civil Engineer, Advanced Construction Techniques Inc. 1000 N. West St. Ste 1200, Wilmington, DE 19801; jfulton@agtgroup.com ABSTRACT: Geologging imagery has many advantages when used in support of drilling and grouting projects. In particular, the use of acoustic and optical sondes, as well as borehole televiewers, allows for the gathering and interpretation of data to better understand in-situ or as-built conditions of underground civil engineering projects. These sondes allow the visual observation of actual conditions in the subsurface, which traditionally has been a problem in this field. The analysis of accrued data allows for faster problem solving and the development of drilling and grouting protocols that may be better suited to meet the site specific needs of the client. These factors make geologging imagery a technologically advanced, yet cost effective, addition in supplementing a successful project. INTRODUCTION Geologging imagery is an advantageous tool for underground civil engineering projects. The nature of the work inhibits the ability to observe the in situ conditions and post-work results using traditional methods. However, the increasing use of borehole televiewers and other imaging technology provides an opportunity to better understand and interpret these subsurface conditions. This paper details some of the different sondes that can be used in these types of projects, and some of the advantages to their use in support of a well rounded drilling and grouting program in the seepage mitigation industry. APPLICATIONS Geologging has been used in the underground civil engineering industry since the mid-1900s. The use of geologgers began in the oil industry, with the early development of acoustic-based televiewers to identify changes in stratigraphy and identify potential petroleum-bearing formations. The basis of this technology has since been further applied to other sectors of underground civil engineering, such as tunnel and mine shafting, subsurface investigations, and remedial grouting projects. Acoustic sondes are used to obtain a near-borehole, 3-D representation of the subsurface. They can be used when the explored interval is located within saturated groundwater conditions or when highly turbid fluids prevent the use of optical sensors. Borehole enlargements related to structures such as fractures, foliation, and bedding

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planes scatter energy from the acoustic beam, reduce the signal amplitude, and produce recognizable features on the image (Paillet et al., 1990). The acoustic imaging also includes a magnetometer and accelerometer array to provide orientation to magnetic north. In this way, the accurate geospatial as-built position of the borehole can be determined. This ultrasonic image is attained by measuring the elapsed time of travel for waves to hit an obstruction and return to the sensor. The downhole televiewer provides downhole and sidewall views in a borehole. It provides a real-time color image feed back to the operator, who can adjust the camera orientation, the lighting, pan, and focus to examine features of interest. This type of sonde is useful when evaluating casing integrity, water flow and where blockages are encountered. This was the primary use for downhole video in production holes (Prensky 1999), and continues to be the case today. It is also used in geotechnical and environmental evaluations for shallow boreholes no deeper than 1524 meters (Prensky, 1999). The downhole televiewer produces a video file which can be observed on most standard video playing programs, or burned onto a DVD for archival or distribution. The use of media files allows for a quicker distribution of the information, which can speed the problem solving process and allows for quicker analysis of the encountered conditions High resolution optical sondes provide a 360° image of the borehole wall that can be imported into an image analysis program such as RGDip or WellCAD. The optical televiewer imaging systems use a ring of lights to illuminate the borehole (Williams et al., 2004). Lithology and structures such as fractures, fracture infillings, foliation, and bedding planes are viewed directly on the optical images. Optical images can be collected in air- or clear-water-filled intervals of boreholes (Williams et al., 2004). This sonde takes a series of pictures to create a single continuous image file. The high resolution optical sonde also records position data relative to the surface point. This sonde also uses an array of magnetometers and accelerometers to collect positional data as the sonde is lowered into the borehole. It uses the magnetic declination of the site as its reference, and collects the positional data in terms of northing, easting and azimuth. This allows for a three dimensional portrayal of the borehole in the field, for as-built purposes. With the aid of external programs such as Microsoft Excel, WellCAD, or RGDip, this data can be applied and plotted against the original design. This is particularly useful when the accuracy of the boreholes is of importance, such as when specific formations or features are being targeted for treatment.

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1. Visible water table

2. Sleeve Ports

3. Bottom of PVC/Overburden Casing and Casing grout.

4. Clay filled seam

5. Partially filled Clay seam

FIG. 1 Optical Televiewer Image (Hole UP2181 Center Hill Lake Dam, TN 12.5m – 15.2m)

Figure 1 is a section of an image produced by the optical televiewer sonde. This example shows the quality capabilities of this particular sonde by showing the color variations between the hole casing or PVC and the rock formation. Visualization of bedding, sleeve ports, casing integrity, fracture infilling and others are easily seen. The images shown in Figure 2 depict the same section of an exploratory hole in a pre- and post-pressure grouting stage. Figure 2-A shows two distinct open features prior to grouting and Figure 2-B displays the grouting results of the same interval. This visual information can be used to track specific networks of fractures and voids across an area within specific geological formations. The post-grouting image is also useful for closure analyses and visualizing the overall effectiveness of a pressure grouting program. Having visual evidence to support pressure testing and grouting results give an opportunity to better understand pre- and post-grouting conditions.

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A B

FIG. 2 Pre and Post Grouting (Hole FP3 Center Hill Lake Dam, TN 7m – 8.5m) The images shown in Figures 3 and 4 are another representation of how geologging imagery can provide a visual of in-situ geology as a substitute to observation of rock core. Images such as these can supplement geological investigations where core loss may be experienced. Acoustic and optical images also provide critical information for placement of inflatable packers for testing and monitoring and for interpretation of resulting hydraulic and water quality data (Williams et al., 2004).

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FIG. 3 (CP-1, Center Hill Lake, TN FIG. 4 (CP-2, Center Hill Lake, TN 41.2m-45.2m) 51.8m-53m) Geologging data collected during imaging can be used to calculate the borehole deviation. Data can be imported in to a program to generate a graphical representation of the borehole path. The deviation is plotted and displayed to show the difference between the design and survey, normal to the bearing, parallel to the bearing and a plan view. Figures 6, 7, 8, and 9 are plots generated from verification hole VDP-1950 at Center Hill Lake Dam, TN. Hole VDP – 1950 was drilled at a 10° angle, with a bearing

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of 223.66° or S 43.66°W. Figure 6 shows the image survey data, the design data as well as the delta between the survey and design or plan data.

FIG. 5 Note: This log is a sample from a job that used Imperial Measuring Units Figure 5 deviation log shows the survey and plan data plotted normal to the bearing.

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Figure 6 deviation log shows the survey and plan data together in a plan view perspective.

FIG. 6 Note: This log is a sample from a job that used Imperial Measuring Units These data plots provide an as-built position of boreholes. This is a useful tool where drilling precision is of paramount importance, especially in drilling pilot and guide holes. Furthermore, this capability can aid in more accurate mapping of particular lithologies and features of interest.

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CONCLUSIONS The acoustic televiewer allows for identifying and evaluating borehole conditions where groundwater conditions prevent attaining a clear video or optical view. By mapping the travel time of acoustic waves, the profile of a borehole wall can be identified. Any distinct open fractures, conduits, or voids can be found using this method, which can be coupled with pressure testing and grout take results, to form a complete closure analysis to demonstrate the effectiveness of a grouting program. Using the borehole televiewer sonde gives a live, real-time display of downhole conditions, and the option to examine specific sidewall features up-close by using the side viewing camera. This allows for examining water carrying fractures, clay filled seams, grout casing integrity, and post grouting verification. This is a cost effective and quick alternative that provides visual evidence to reinforce drilling and pressure testing results obtained in the field. Optical sondes provide a combination of high resolution images and borehole surveying, which optimizes efficiency when both sets of information are required. The images collected can be studied individually, or plotted together to provide a view of conditions across an entire job site. Using this technique can aid in identifying troubling formations, as well as mapping fracture networks and features encountered. Coupling these capabilities with field surveying results means that fracture networks can be pinpointed in space, dramatically increasing the precision of feature elevations and the further understanding of specific fracture networks. An advantage to borehole imaging is the real-time 360° display of the borehole and storage and sharing of data with the owner/client. The acoustic, optical, and video sondes will display 2D or 3D portrayals of existing conditions of the hole. The data acquired during imaging can be used to calculate borehole deviation relative to the surface location as well as provide additional information to a rock core log. Geologging imagery has become increasingly viable when working on subsurface construction projects such as, grouting programs, geological investigations, and general borehole troubleshooting. When using geologging imagery in a downstage drilling and grouting program, the opportunity exists for multiple views of pre and post-grouting conditions of bedrock. The ability to review and monitor subsurface construction can be used to verify grout refusals on specific stages, fractures, joints, and lithological contacts. The wealth of modeling and interpretive data that is attained with imagery, as well as the speed in which this data can be produced and presented, makes the practice of geologging imagery an important component of this industry’s ever-expanding technological future. REFERENCES Prensky, S.E. (1999). “Advances in borehole imaging technology and applications” Geological Society, London, Special Publications V. 159: 5-16 Williams, J.H., and Johnson, C.D. (2004). “Acoustic and borehole-wall imaging for fractured-rock aquifer studies” Journal of Applied Geophysics 55: 151-159 Paillet, F.L., Barton, C., Luthi, S., Rambow, F., and Zemanek, J.R. (1990) “Borehole imaging and its application in well logging – an overview” Borehole Imaging, Society of Professional Well Log Analysts pp. 3-23