Borehole instrumentation and initial broadband surface seismic and walkaway VSP test results at the Priddis Geophysical Observatory, Alberta, Canada Kevin W. Hall*, Kevin L. Bertram, Malcolm B. Bertram, Joe Wong, Peter M. Manning, Eric V. Gallant, Kristopher A.H. Innanen, Don C. Lawton and Gary F. Margrave Summary Two new geophysical test holes were drilled and completed at the University of Calgary's Priddis Geophysical Observatory (PGO), south of Calgary, Alberta, Canada in 2013. The test holes were drilled to a depth of 146 m and were cased with schedule 80 polyvinyl chloride (PVC) casing. Testhole 1 (Figure 1) was permanently instrumented with 45 28 Hz three component (3C) geophones at a nominal 3 m spacing, in addition to single and multi-mode optical fibers. All instrumentation was strapped to the outside of the casing upon insertion into the borehole and was then cemented into place. Immediately following installation two approximately 300 m long 2D surface seismic lines with 10 Hz 3C geophones at a nominal 6 m spacing were acquired at approximately right angles to each other centered on the instrumented borehole. All surface and borehole receivers recorded linear sweeps from an IVI EnviroVibe source, 0.125 kg dynamite test shots, and United Service Alliance accelerated weight-drop test shots. The weight drop source was used to hit the ground vertically, at +45O from vertical and at -45O from vertical at each source point that it was used. In 2014, CREWES and INOVA conducted a high-resolution broadband multi-component surface seismic and walkaway vertical seismic profile (VSP) at the PGO. Before beginning the 2014 seismic survey, natural gamma-ray logs and full waveform sonic logs were acquired in Testhole 2 using equipment described by Wong et al., 2009. Four dynamite source lines (E-W, NE-SW, N-S, NW-SE, 0.125 kg, 5 m deep, 6 m spacing) arranged in a star pattern centered on Testhole 1, and one Vibe source line (E-W, 6 m spacing) were recorded by the permanent three-component 28 Hz geophones (3 m vertical spacing) in Testhole 1, clamping 3C optical sensors (20 m vertical spacing) in Testhole 2, as well as single-component accelerometers (6 m spacing), single-component high-sensitivity 10 Hz geophones (6 m spacing), three-component 10 Hz geophones within ground-screws (6m spacing) and three-component 10 Hz geophones (3 m spacing) on the surface. The data are of generally good quality, but are contaminated with acoustic and electrical noise from generators powering various recording systems, as well as from regional power lines.

Introduction The University of Calgary has surface rights to land located at legal subdivisions (LSD) 3 through 6 of 13-22-3W5, near Priddis, Alberta. The Rothney Astrophysical Observatory (RAO) is located on the center-east part of LSD 6. Geophysical test hole locations are shown as red and white rings in Figure 1. The northernmost borehole (Testhole 4) is an existing test hole that was drilled in 2007 and well-logged (Wong et al., 2009). Testhole 1 and 2 locations were picked based on relatively flat locations that a drill rig could access, and were further constrained to be 50 m apart in the regional dip direction (70O azimuth; Geological Survey of Canada, 1941), with Testhole 1 approximately centered on the property. The Testhole 3 location is intended to be roughly equi-distant between Testholes 2 and 4 along approximate regional strike. Testholes 1 and 2 were drilled in late September and early October of 2013. Testhole 1 was cored from 31.5-124.0 m depth, and seismic instrumentation was strapped onto the outside of the casing before cementing.

Testhole 1 instrumentation Twelve downhole geophone cables originally intended for a different project were available for deployment in Testholel 1. Each cable has 4 geophones at nominal 10 m spacing, with the exception of cable 12 which only has one geophone. The cables were interleaved as shown in Figure 2 to achieve a 3.06 m geophone spacing in the well with a

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Figure 1: Test hole locations. Background image © 2012 DigitalGlobe, 2013 Microsoft Corporation (Bing Maps, 2013).

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minimum of slack cable. Geophones and optical fibers protected by stainless steel tubing were strapped to the outside of the PVC casing as it was inserted into the well. Geophone resistances were provided by the cable manufacturer, and geophones were tested by measuring resistance with a multi-meter at the University before deployment and after cementing was completed in Testhole 1.

Surface seismic and walkaway VSP test (2013) A seismic program was conducted two weeks after completion of Testholes 1 and 2. Linear Vibroseis 10-120 Hz sweeps, shear wave weight-drop hits (vertical, +45O and -45O thumps) and dynamite test shots were recorded by the

single-mode optical fiber and 28Hz 3C geophones in Testhole 1, as well as by single surface 10 Hz 3C geophones that were laid out at a 6 m receiver spacing (black dots, Figure 1). Dynamite charge sizes were all 0.125 kg. Six shotholes placed single charges at 10 m depth and equi-distant from Testhole 1, with two of the shotholes drilled vertically, two shotholes drilled at a 30O angle from vertical oriented radially away from Testhole 1, and two shotholes drilled at a 30O angle from vertical oriented radially towards Testhole 1. The seventh dynamite shothole had charges vertically spaced from 20 meters depth to almost the surface, which were fired separately from bottom to top. A sample of the data from the first of the dynamite shots is shown in Figure 3. This is the vertical component of the four surface lines 1 (N-S), 3 (E-W), 5 (NW-SE) and 7 (SW-NE) and the first horizontal (X) component of the downhole array. This shot was 0.125 kg at 10 m depth near Testhole 1. After applying component rotation to the downhole geophones, it is possible to examine the near offset shots in more detail. Figure 4 shows a gather from a 45O northward thump near the well. The separation of energy between axes is very good, and the near surface velocity information is easy to determine. The measured values from this plot are: P-wave (right panel) 3060m/s; S-wave (center panel) 623m/s to a depth of 40m then 1530m/s to TD.

Figure 2: Geophone placement in Testhole 1.

Figure 3: Example of a dynamite shot gather with 200 ms AGC applied. Lines 1, 3, 5 and 7 are the vertical components of the surface receivers; line 9 shows the X component of the downhole array before re-ordering and component rotation.

Figure 4: The downhole gather from a northward thump at a location 2.5 m west and 3.7 m south of Testhole 1.

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Broadband surface seismic and walkaway VSP (2014) The 2014 field program at the Priddis Geophysical Observatory (PGO) had many objectives: • Obtain many closely spaced dynamite shots from many azimuths into the permanent geophones in Testhole1 • Acquire high-resolution three-component 2D surface seismic lines at a variety of azimuths • Deploy our new USSI fiber optic system in Testhole2. This is the first time we have deployed this system. • Source tests (Dynamite vs. Vibe) • Receiver tests (Geophone vs. Accelerometer) • Receiver tests (planting methods) Layout Four surface lines were laid out (E-W, NE-SW, N-S and NW-SE), where the E-W and N-S lines are co-located with the 2013 lines. All four lines intersect at Testhole1 (Figure 5). Testhole 2 is located between the NE and E arms of the E-W and NE-SW lines 50 m away from Testhole1 (not shown). While this survey was intended to be processed as four separate 2D lines, almost every shot was recorded by almost every receiver, which means that it is possible to process the downhole and surface data as a 3D, although the resulting azimuthal coverage in each bin may not be of the best quality (Figure 5).

Recording systems Five separate recording systems were used for this survey. Four cabled systems (two Inova Aries SMP Lites, one Inova G3i and one USSI optical) were triggered by Pelton VibePro’s, and one nodal system (Inova Hawk) recorded

continuously. All systems sampled at 1 ms with the exception of the USSI recorder, which sampled at 0.25 ms. Sources Two source types were used, dynamite and Vibroseis. Dynamite shotholes were drilled and loaded at every second station from 101-181 of the E-W, NE-SW, N-S and NW-SE lines. There were a total of one hundred and fifty 0.125 kg dynamite shots spaced 6 m apart, with the charges placed 5 m below ground level. An Inova Univib operated by Geokinetics acquired the E-W line with a vibe point every 6 m. A linear 1.5-180 Hz sweep was used for production shooting. Downhole Receivers The 45 permanent GS-14-L9 28 Hz three-component geophones cemented into Testhole1 recorded all shots. In addition, a retrievable six level three-component fiber-optic system from USSI was deployed in Testhole 2 from 15 m to 115 m depth, with a node every 20 m. Surface Receivers Three kinds of receivers were deployed on the surface (Figure 6). These included SM-7 three-component 10 Hz geophones in nail-type cases and within ground-screws, SM-24HS single-component high-sensitivity geophones and AccuSeis single-component SL11 accelerometers on the surface. The SM-7 nail-type cases were planted in augered holes and were oriented to magnetic north with the pigtails pointing to the south. These geophones were planted on all four lines at every station from 101 to 180 at a 3m geophone spacing. The SM-7 groundscrews were screwed into augered pilot holes from stations 143-165 at a 6 m spacing on the E-W line only. The SL11 and SM-24HS vertical component sensors were planted at stations 83 to 271 at a 6 m spacing on the E-W line only. These sensors were planted with just the sensor spike in the ground or with the entire case in the ground in an alternating pattern, so that a particular planting method had a 12 m spacing down the line.

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Figure 5: Dynamite CMP fold for SM-7 geophones binned with 1.5x1.5 m bins. Colour bar extent has been limited to a maximum of 10 fold.

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Figure 6: Surface sensors; SM-7 in nail-type case (top), SM-7 in ground-screw, SM-24HS and SL11 (bottom).

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Initial results (2014) Figures 7 and 8 show an example of the vertical and radial component surface data from the SM-7 geophones in nail-type cases for SP 2101 and VP 2101 on the E-W line. The component rotation from horizontal one (H1) and two (H2) to radial and transverse was a purely geometric one, assuming that all of the geophones were planted perfectly with H1 oriented towards magnetic north. The transverse component is not shown. A correction for the 10 Hz geophone response from 0-10 Hz has been applied. This brought up a lot of very low frequency noise (less than 1 Hz), which has been removed by applying a 1-2-180-200 Hz Ormsby bandpass filter. It is visually apparent that the geophone correction has enhanced data below 10 Hz. Note that most of the energy appears to be in the 0-40 Hz band, which is disappointing. This story may improve after ground-roll removal and stacking. Narrow horizontal bands in the amplitude spectra indicate the presence of electrical noise, likely due to the presence nearby powerlines, as well as to acoustic and electrical noise from the variety of gas and diesel generators powering recording systems within and near the survey. The repetitive nature of the bands indicates the data also contain harmonics of the generator/powerline noise. Conclusions and Future Work The well-logging and seismic programs were successfully carried out, meeting all of the initial objectives. However, there are some issues with electrical noise in the data that still need to be addressed. The geophones that were cemented into Testhole 1 have, for the most part, survived the installation and are providing good data. The new shear wave weight-drop (thumper) source is providing a good source of shear wave energy for near surface investigations. To date only a small amount of weight-drop data has been acquired, and more projects are planned at the Priddis Geophysical Observatory to further evaluate the source in terms of available offset range and frequency content, as well as attempting shear wave reflection acquisition. The horizontal data need to be more carefully rotated to radial and transverse components. Some of the dynamite shots will be used in order to determine the actual orientation of the permanent geophones in Testhole1 for future surveys. Electrical noise issues need to be dealt with, possibly by modelling and subtraction. Ground-roll needs to be carefully removed in order to preserve shallow reflections. All data volumes need to be stacked, migrated, inverted and interpreted.

Acknowledgements The authors would like to thank (in alphabetical order): Austin Powder, Carbon Management Canada (CMC) CREWES staff and students, Geokinetics, GroundForce geoDrilling Solutions, Halliburton, Inova, OutSource Seismic, SITE, Schlumberger, US Seismic Systems (USSI), and Val’s Drilling. We thank the sponsors of CREWES for their support. We also gratefully acknowledge support from NSERC (Natural Science and Engineering Research Council of Canada) through the grant CRDPJ 379744-08.

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Figure 7: SP 2101, 0.125 kg Dynamite @ 5m, SM-7 in nail-type case.

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Figure 8: VP 2101, Linear 1.5-180 Hz sweep, SM-7 in nail type case.