Seismic Reflection Imaging across the Johnson Ranch, Valley County, Idaho

Transcription

1 Seismic Reflection Imaging across the Johnson Ranch, Valley County, Idaho Report Prepared for the Skyline Corporation Lee M. Liberty Center for Geophysical Investigation of the Shallow Subsurface (CGISS) Boise State University Boise, Idaho 8372 and Ed Squires Hydro Logic, Inc W. Franklin Street Boise, Idaho Technical Report BSU CGISS June 2003

2 Seismic Reflection Imaging across the Johnson Ranch, Valley County, Idaho Lee M. Liberty and Ed Squires Boise State University/Hydro Logic, Inc. 1.0 Summary We have acquired approximately 3/4 mile of seismic reflection data across portions of the Johnson Ranch in Valley County, Idaho for the purposes of geothermal exploration. More specifically, we acquired a seismic reflection profile to image the sedimentary section, calculate depth to bedrock, and locate faults and/or fractures in the subsurface that may indicate a favorable location to place a geothermal producing well. Results from the seismic survey suggest that the granite basement dips approximately 32 degrees to the east from surface outcrop exposures to roughly the intersection of West Mountain Road and Smylie Road. The basement rocks appear flat-lying across the eastern portion of the seismic profile at a depth of approximately 1800 feet from West Mountain Road east to Johnson Ranch driveway. In the transition zone between the dipping bedrock along the western portion of the profile and the flat bedrock section to the east, a fault zone appears that may represent the best target to locate a production geothermal well. Reflections from overlying sedimentary layers appear relatively flat lying across much the seismic section suggesting that major faulting does not extend to the surface in this region. However, small offset faults likely propagate through portions of the overlying sedimentary section and this may suggest hydraulic connectivity with the underlying aquifer. Depths to bedrock within the fault zone range from feet. 2.0 Site Location We located our seismic reflection profile across the inferred orientation of the geologic structures for the region (Schmidt and Mackin, 1970), on the property of the Johnson Ranch (Figure 1). The site is located along the western portion of Long Valley, a north-south topographic depression that extends for more than 30 miles in west central Idaho. The generalized geology of the region con- 2

3 Site McCall Donnelly Well Cascade Seismic Line Johnson Ranch Start of Line=0 ft west shoulder of West Mtn Road=2383 ft FIGURE 1. Topographic map showing the seismic reflection profile across the Johnson Ranch property, west of the Payette River in Valley County, Idaho. Inset map shows the site location with respect to Donnelly, McCall, and Cascade Lake in Idaho. sists of Quaternary-age coarse- to fine-grained glacial deposits, colluvium, and alluvium that overlie a Cretaceous-age granite and perhaps a Tertiary-age basalt basement. Volcanic rock (Columbia River Basalt) outcrops appear on the regional geologic map (Schmidt and Mackin, 1970), but it is not clear from their study whether these rocks are present in the subsurface below our study area. North-south trending faults are interpreted along the range-front boundaries in the region based on gravity measurements (Kinoshita, 1962) and geologic mapping (Schmidt and Mackin, 1970). However, the geological and geophysical evidence for the precise fault locations from these two studies is not convincing. We identified granite on the surface near a slope break and inferred location of a range-bounding fault and placed the start of our seismic profile here. If a fault appears west of our seismic line (as mapped), this fault is contained within the Cretaceousage granite and not likely a major component to basin formation or geothermal production. A 300 ft deep borehole was previously drilled through granite approximately 0.5 miles northwest of the 3

4 western end of the seismic line (Figure 1) further constraining the location of major basin-bounding faults to the east. Fine-grained sediments appear on the surface along much of the length of the seismic profile, likely alluvium and colluvium from nearby stream sources. These sediments are fully saturated along much of the seismic profile, providing a favorable medium for seismic exploration. We terminated our seismic reflection profile at the western boundary of the Johnson Ranch driveway. 3.0 Seismic Reflection Method The benefits of the seismic reflection method compared to other geophysical or geologic methods is the ability to simulate a geologic cross section (or side cut) across a survey area. The seismic reflection method works by bouncing sound waves off boundaries between different geologic or hydrogeologic interfaces (Figure 2). In the case of this survey, we used a trailer-mounted accelerated weight drop (Figure 2) device to excite ground motion. Due to the physics of the earth, we must acquire seismic reflection data at a wide variety of offsets in order to accurately image the subsurface and interpret the seismic waveforms. We recorded the seismic response using a line of sensors or geophones spaced 16 feet apart and connected to a digital seismograph. We recorded soundings every 16 feet across the length of the profile to accurately image reflections up to 3000 feet depth. The seismic reflection method requires considerable computer processing time after data collection to include the following: 1) remove seismic modes and signal noise that are not important to this study (for example, ground roll energy that you may feel during an earthquake appears on our seismic records, but this energy is not useful to interpret the geologic structures of interest), 2) account for changes in surface elevation at different source and receiver positions, 3) account for changes in seismic velocities across the profile, and 4) bin the seismic waveforms at the same subsurface imaging point. Once these steps are complete, we can compile a seismic reflection section that can be interpreted for geologic information. It is important to note that the seismic reflection methods record changes in physical properties of the subsurface and cannot predict the actual lithologies present in the subsurface. 4

5 A) B) 300 Seismic Shot Record offset (m) C) time (ms) FIGURE 2. A) Schematic showing seismic waves and raypaths propagating through an earth model. The seismic energy is generated by a hammer source and recorded by geophone sensors on the earth s surface. B) The seismic source for this survey. C) A field seismic shot record from this survey located along the eastern portion of the seismic profile. 4.0 Results Mapping stratigraphy and identifying fractures and/or faults are critical to accurately placing a geothermal production well. Seismic reflection methods help identify geologic boundaries where changes in rock density and/or seismic velocity occur. A change from soft sedimentary rocks to hard granitic basement is an ideal seismic boundary and identifying laterally changing structures on this seismic boundary will often point to a favorable geothermal production target. Figure 3 5

6 shows the results from the seismic survey represented as two different images. The first image is an unmigrated seismic section and the second is a migrated seismic section containing a basic geologic interpretation. We present both section because each image provides slightly different geologic information that may be useful for interpretation. First, the unmigrated seismic section shows a more clear image of the seismic reflection boundaries that appear in the subsurface, but if geologic dip is present (as it is here), the actual depth and orientation of geologic and seismic boundaries do not precisely match. Also, unmigrated seismic images more clearly define fracture or fault characteristics in the subsurface by showing diffractions or edge effects at laterally terminating boundaries. The migrated section places the seismic reflections and diffractions in the proper spatial position, but sometimes the quality of each reflection on a migrated section may degrade. The migrated image is best used for interpretation with reflections and diffractions that appear on the unmigrated section as a guide for interpretation. Figure 3 shows a relatively flat lying package of reflections that increases in thickness to the east. The reflection package forms a wedge that ranges in thickness from a few feet along the western edge of the seismic profile to a maximum thickness of 1800 feet along the eastern portion of the profile. The character of this reflection package is typical of unconsolidated to loosely consolidated saturated sediments that are mapped in this region (Schmidt and Mackin, 1970). Within this reflection package, a seismic boundary at approximately 500 feet depth is the largest and most laterally consistent seismic boundary in the section. The actual geologic change that is represented by this seismic boundary can only be confirmed by drilling, but likely represents a lithology change characteristic of a large change in bulk grain size (e.g., change from fine-grained clay dominated sediments to a more coarse-grained sedimentary section). Although this reflection is flat lying along much of the profile, it dips eastward along the western portion of the profile and shows some evidence for lateral discontinuities near the center of the seismic image. Below the interpreted sedimentary section lies a large-amplitude reflection (Figure 3; Horizon A) that likely represents the top of the granitic or volcanic basement. We base this interpretation on the connection to the granite outcrop immediately west of the seismic profile and also the observed seismic amplitudes. Horizon A dips approximately 32 degrees to the east from within 100 feet of the surface at position 0 to at least 2000 feet distance to the east with no evidence for large-offset faulting along this tilted plane. We interpret the same seismic boundary as a flat-lying 6

7 Unmigrated Seismic Reflection Section Elevation (feet) west Distance (feet) creek West Mountain Road Smylie Road east Approximate Depth (feet) no data zone west Migrated Seismic Reflection Section east Approximate Depth (feet) no data zone Horizon A ~32 degree dip Horizon A FIGURE 3. Seismic reflection section across the Johnson Ranch showing a) unmigrated seismic section and b) migrated and interpreted seismic section. We interpret the strong amplitude, continuous reflection (Horizon A) that roughly dips 32 degrees from the land surface as the top of the granitic basement. East of West Mountain Road, Horizon A appears flat lying. We interpret the zone between the two contrasting dips on Horizon A to represent a fracture or fault zone that may be a favorable location to site a geothermal production well. Vertical exaggeration is roughly 2:1. 7

8 plane along the eastern portion of the seismic profile (position ), again with no identifiable vertical offsets that may suggest faulting. The major change in reflection dip that appears on Horizon A across the seismic survey is within a single 300 ft zone, centered roughly 250 feet west of the West Mountain Road intersection (western road shoulder = 2383 ft position). The 300 ft wide zone that separates the dipping basement seismic reflection to the west from the flat-lying seismic reflection to the east (at position ) contains a low seismic amplitude response compared to the same contact both to the east and west. The lower amplitude response suggests the density and/or seismic velocity contrasts of the geologic boundaries in this zone are much lower, consistent with a fault zone. Additionally, diffractions on the unmigrated image also suggest faulting is present. Depths to bedrock within the inferred faulted region range from feet, although small offset faults may extend into the overlying sedimentary section. The relatively flat lying and consistent amplitudes of reflections overlying Horizon A suggest that the interpreted fault zone has not significantly affected the structural style of the overlying reflections associated with sedimentary deposits and that the sedimentary deposits were deposited horizontally on the preexisting tilted plane. However, small offsets that appear in the overlying reflections suggest faulting may propagate toward the surface, and if a major geothermal production zone appears below, may represent a conduit for upward migration of geothermal water. This interpretation of faulting extending toward the land surface may be further evidenced by a slope break that appears on the surface (Figure 3), but the influence of faulting on water temperature and pressure in the sedimentary section can only be confirmed by drilling. 5.0 Summary Although the gravity and geologic maps suggest near-vertical normal faults appear along the western margin of the seismic profile, results from our seismic survey suggest that a major fault boundary appears further east, below the sedimentary section, roughly 2000 feet east of the exposed granite and immediately west of West Mountain Road. The 32 degree dip observed on Horizon A along the western portion of the seismic profile ties to outcrop exposures of granite and is consistent with observed dip of nearby exposed granite that has resulted from either tectonic tilt or erosion (Schmidt and Mackin, 1970). The flat-lying basement reflection along the eastern portion of the seismic profile either represents a leveled granite surface (perhaps scoured from glacia- 8

9 tion) or a basalt cap that was deposited on the older granite (and perhaps removed by erosion to the west). Between the two distinct bedrock reflections lies a 300 ft wide fault zone that is characterized by at least 500 ft of vertical relief. Reflections from overlying sedimentary rocks do not contain significant vertical offsets, but may contain small offset faults or fractures. Based on the seismic results, geothermal well placement would best appear within the interpreted fault zone, roughly centered 250 ft west of West Mountain Road at a depth of 1300 ft or greater. This fault zone is the best candidate along the seismic profile to encounter water that is in hydraulic contact with deeper geologic structures, thereby potentially providing the pressures and temperatures necessary for a geothermal production well. 6.0 References Kinoshita, W.T., 1962, A gravity survey of part of the Long Valley district, Idaho: U.S. Geological Survey Oper File Report, 11 p. Schmidt, D.L. and Mackin, J.H., 1970, Quaternary geology of Long and Bear Valleys, West-Central Idaho: U.S. Geological Survey Bulletin 1311-A, 22 p. 7.0 Acknowledgments The authors wish to thank the field crew, consisting of Boise State University students: Ed Reboulet, Scott Hess, Suwimon Udphuay, Chris Paul, and Hydro Logic employee Loren Pearson for participating in this survey. The authors also wish to thank the generosity of Ted Johnson and family for providing the use of their personal cabin and food supplies to the field crew during seismic acquisition. 9