Geological evaluation of subsurface features near Ithaca, NY: interpretations of seismic reflection profiles collected by the petroleum industry
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To provide background geological information with which to assess some of the technical and environmental risks of a Cornell Earth Source Heat (ESH) project in Ithaca, NY, this report describes the geological features below Tompkins and easternmost Chemung counties that are revealed by approximately 150 km (94 miles) of 2D hydrocarbon-industry seismic reflection profiles. Details of subsurface features near the Cornell campus are presented on maps, and also described. Such data were not previously available in any publicly available reports, and therefore the analysis presented is a major step forward in documentation of subsurface features near Cornell.
The vertical distribution of sedimentary rocks known from deep hydrocarbon boreholes was used as the basis for the approximate sedimentary unit identification of packages of seismic reflections to a depth of about 3 km (10,000 ft). The lower limit of sedimentary rocks, which overlie a crystalline basement, can be identified readily in only a minority of the seismic profiles; in most of the data, there is a large uncertainty on position of the basement contact. Sedimentary units with possible interest as geothermal reservoirs are expected within the lowest 300-600 m (1000 to 2000 ft) of sedimentary rocks near Cornell. These data allow tentative identification of a unit of sedimentary rocks with favorable reservoir potential immediately overlying the basement in paleovalleys near campus.
Disruptions to the positions or continuities of these reflective sedimentary rock units are identified as either folds, which are smooth undulatory waveforms of the rocks, or faults, which are breaks in the units. Two classes of faults, one sub-vertical and one sub-horizontal (i.e., thrusts) are differentiated such that their different roles in technical and environmental risk can be individually evaluated. The seismic profiles reveal five categories of structural deformation, two of which were not expected based on publicly available reports for Tompkins and neighboring counties.
Among the three expected categories of structural deformation, folds of the uppermost sedimentary layers are widespread; these should impact an ESH project only by making predictions of depths to horizons of interest slightly more uncertain. Folds and thrusts within the Syracuse and Vernon Formations are highly likely to occur under the Cornell campus region and plausible ESH project sites, in a depth range of 750-1200 m (2500-4000 ft). These widespread features are not shown in the executive summary illustration, yet they are detailed in the report. Standard practices exist in central New York for drilling through and isolating this interval of deformed, weak rocks.
The seismic reflection profiles reveal that the third expected category, sub-vertical faults known by the hydrocarbon industry as “Trenton-Black River” (TBR) structures, occurs in some sectors of Tompkins County. A Trenton-Black River fault cluster is not expected near the Cornell campus (see summary figure). An uncertain individual TBR-type fault is located about 1.4 km (0.9 mi), and a more reliable single fault about 3.4 km (2.1 miles), north of the Palm Drive area. The first unexpected category of structures is a widespread set of sub-horizontal thrust faults within the Cambrian and Ordovician sedimentary rocks, in an interval of rocks predicted from boreholes to be about 350 m (1150 ft) thick at Cornell’s campus. The near-Cornell industry- quality seismic reflection profiles reveal thrust faults in these sedimentary units (see summary figure). A seismic depth model with high uncertainty implies that these thrusts may be as shallow as 2.1 km (6900 ft) or as deep as 3.0 km (10,000 ft) near the east end of campus. Because of their sub-horizontal disposition, these disruptions may have more relevance to analyzing reservoir potential than to seismic hazard analysis.
The second unexpected category of structures is of greater uncertainty than any of the other features described here. Within Tompkins County, there are a small number of fold-forms in the deepest well-imaged sedimentary units. There is a significant degree of uncertainty that some or all of these fold-forms are physically real parts of the rocks. Conventional geological wisdom suggests that these undulations may be associated with faults that are not imaged by the seismic reflection data. Hypothetically, either of two markedly different types of faults could be related to deep folds: near-vertical faults (like the TBR faults) that offset rocks in the basement, or sub-horizontal thrust faults within the poorly imaged deeper sedimentary rocks or at the sedimentary rock-basement contact. I recommend that additional modeling and analyses of the seismic reflection data be considered, in efforts to reduce the uncertainty on the fold-forms near the base of the sedimentary rocks, to improve estimates of the depth to reflectors, and to learn whether more useful information about the crystalline basement can be extracted. This study, supplemented by the Vibroseis survey collected in 2018 by Professor Brown and students, has illuminated relatively well the nature of the sedimentary rocks near the eastern edge of the Cornell campus development. Because sub- vertical faults projecting down toward the basement are not revealed close to campus, it is my opinion that investments in future geophysical studies should focus on extracting information about the crystalline basement rather than about the sedimentary rocks. Perhaps the best designs for further geophysical studies will involve instrumentation within a pilot borehole that complements instrument deployments across the land surface.