Integrating data : Quaternary stratigraphic analysis using

fully cored boreholes and downhole geophysics Jessica M. Slomka

1, Andrew N. Brennan

1, and Carolyn H. Eyles

1

1School of Geography and Earth Science – McMaster University, Hamilton, Ontario, Canada

ABSTRACT Groundwater exploration programs commonly employ both sedimentological and geophysical techniques but some studies rely more heavily on one method due to financial or personnel constraints. This study applies sedimentological and geophysical analytical techniques to the interpretation of complex Quaternary stratigraphies in the Georgetown region of southern Ontario. Each method provides valuable information about subsurface conditions; however, the integration of both data sets greatly enhances the quality and reliability of the subsurface stratigraphic analyses and interpretations. RÉSUMÉ Les programmes d‟exploration pour l‟eau souterraine emploient couramment des techniques sédimentologiques ou géophysiques, cependant plusieurs études s'appuient davantage sur l‟une des méthodes en raison de contraintes financières ou de connaissances personnelles. Cette recherche applique ensemble les techniques sédimentologiques et géophysiques à l‟interprétation d‟assemblages complexes de sédiments Quaternaire à Georgetown dans le sud de l‟Ontario. Chaque méthode donne des informations utiles concernant les conditions souterraines, cependant l‟intégration des deux ensembles de données améliore l‟exactitude et la fiabilité des interprétations stratigraphiques. 1 INTRODUCTION Both sedimentological and geophysical data are common sources of subsurface information used in groundwater, petroleum and mineral exploration programs. Although both data types are generally available and each provides valuable information about subsurface they are most effectively used in combination. Integrating sedimentological information from core and outcrop with geophysical data from borehole logs allows more confident interpretation of subsurface materials and their spatial correlation between individual data points. This enhances the quality of subsurface geological models produced from these data that may be used in a range of mineral and hydrogeological exploration programs.

This study applies both sedimentological and geophysical methods to the analysis of 2 fully-cored and geophysically logged boreholes (BH1 and BH2). The objective of the study is to identify where and how interpretations of the two data sets differ, and how appropriate corrections can be made to produce the most accurate interpretation possible. To begin, sedimentological and geophysical data from each borehole were analyzed independently and used to identify and delineate subsurface stratigraphic units. The stratigraphic interpretations made from each of the data sets were then compared to identify areas of dissimilarity in interpretation, named „query intervals‟ in this study, which required further investigation. Two types of query intervals were recognized based on differences in geophysical and core data: 1) intervals of discrepancy, where both data methods provided varying or conflicting interpretations, and 2) intervals of uncertainty, where data provided by either method was insufficient to make a stratigraphic interpretation or boundary pick with a high degree of confidence (e.g. poor core recovery or an ambiguous geophysical log signature).

The two boreholes used in this study were drilled in the Georgetown region of southern Ontario as part of a regional groundwater exploration program. The boreholes were drilled 3028m apart. The regional Quaternary stratigraphy of southern Ontario consists of multiple till sheets separated by thick successions of coarse-grained outwash and glaciolacustrine deposits, which typically infill buried bedrock valleys and host regional aquifers (Barnett 1992; Karrow 2005; Meyer & Eyles 2007). 2 METHODOLOGY 2.1 Sedimentological analysis Each of the two continuously cored boreholes examined penetrated Quaternary age sediments up to 48 m thick. The recovered core was subjected to sedimentological analysis using standard sedimentary logging techniques that involved identification of grain size and sorting, sedimentary structure, colour, clast size, shape and lithology (Eyles et al., 1983). These sedimentological observations allowed the identification of 11 lithofacies types (Figures 1, 2).

These individual lithofacies types were grouped into 8 distinct stratigraphic units on the basis of their sedimentological characteristics, associations with under- and overlying facies types, their interpreted depositional origin, and the nature of bounding surfaces separating sedimentary units (Figure 3). The 8 stratigraphic units are found in various combinations in each borehole (Figure 3). Lithostratigraphic correlation was then made between the stratigraphic units identified in each borehole.

Figure 1. Lithofacies types observed in BH1 and BH2 cores (codes based on Miall, 1977 and Eyles et al., 1983).

Figure 2. Lithofacies types encountered in BH1 and BH2 cores (see Figure 1 for lithofacies codes and descriptions).

Figure 3. Lithostratigraphic units identified in the two cored boreholes and interpretation of depositional origin. (Note: boreholes are hung relative to approximate surface elevation).

2.2 Geophysical log analysis Downhole geophysical logs were collected in the two boreholes (BH1 and BH2) using natural gamma, conductivity, resistivity, magnetic susceptibility, caliper and full-waveform sonic probes. Geophysically-based lithological interpretations and characteristic log responses (electrofacies) were identified by visual analysis of log signatures, principally on variations in the natural gamma, resistivity and conductivity logs (Figure 4; Boyce et al., 1995; Greenhouse & Karrow, 1994). Downhole changes in p-wave velocity were important for discriminating more compact sedimentary units such as subglacial till (Pullan et al, 2002).These observations allowed the identification of 10 electrofacies units (EUs) within the two boreholes (Figure 5).

Figure 4. Electrofacies units determined from log responses

Figure 5. Electrofacies units defined on the basis of geophysical log responses for A) BH1 and B) BH2 2.3 Data integration process Initially, sedimentological and borehole geophysical data were analysed independently to identify variations in facies characteristics and delineate major stratigraphic units (Figures 3, 5). During the integration process (Figure 6), differences between sedimentological and geophysical interpretations of the same unit and in the delineation of major bounding surfaces were noted and identified as „query intervals‟ (areas of discrepancy and uncertainty) which are sections of the borehole that required further investigation (Figures 7- 9). Data provided by both methods at each query interval were analyzed to determine the causal effect of the initial discrepancy or uncertainty in interpretation (Figure 10). By simultaneously considering data produced by both methods at each query interval, it was possible to

modify unit and boundary picks to produce a more reliable stratigraphic interpretation (Figures 7- 9).

Figure 6. Integration methodology work flow used in this study 3 DATA INTEGRATION 3.1 Borehole 1 (BH1) Sedimentological examination of core BH1 allowed the identification of 9 lithofacies (Dc, Ds, Df, Fl, Fm/d, Sm/d, Sr, Sc, Gm) and 7 lithostratigraphic units, whereas geophysical analysis identified 10 electrofacies units (Figures 1- 5). 6 major „query intervals‟ were identified in BH1 where core and geophysical log picks were at variance. These intervals were examined in closer detail to determine why the inferred boundaries were different and to re-evaluate the locations of the stratigraphic boundaries (Figure 7).

The first query interval (Q1; Figure 7) is an area of uncertainty that was encountered at the top of U2 in the core, a transition zone where highly deformed sand is incorporated into the base of the overlying sandy diamict (U3), interpreted as a subglacial till. The incorporation of sand from U2 into the overlying till (U3) makes the sedimentological delineation of a contact between U2 and U3 extremely difficult. However, the distinct increase in velocity and resistivity signatures on the geophysical logs at 42.76m clearly identified a major lithological change at the U2-U3 boundary, nearly 3m deeper than initially identified in core (Figure 7).

Query interval 2 (Q2; Figure 7) was an area of uncertainty within U3. Core from this interval consisted of poorly sorted, coarse-grained sands and gravels interbedded with sand-rich diamict. The sedimentological data were insufficient to allow division of U3 into distinct subunits but the geophysical data clearly identified a discrete sand-rich subunit indicated by high conductivity and gamma signatures and a low velocity response.

The third query interval in BH1 (Q3; Figure 7) is an area of discrepancy regarding the position of the upper bounding surface of U4. Geophysical data identified a transition to finer grained electrofacies at 14m depth. However, core data showed a distinct change in lithofacies from poorly sorted sand and gravel facies to well-sorted sand and fine-grained facies at 17.5m depth. The latter is more likely to represent the upper surface of U4 but was not immediately apparent in the downhole geophysical data.

The final query intervals (Q4-Q6; Figure 7) in BH1 were areas of uncertainty involving poor core recovery near the top of the borehole that prevented the identification of stratigraphic unit boundaries. However, these boundaries were clearly identified by rapid transitions to lower gamma values and an increased variability in the caliper response in the geophysical well logs.

Figure 7. Strip logs geophysical and sedimentologic picks and query interval (Q) locations in BH1. The upper 3m was not recovered in core. See Figures 3 and 4 for colour codes.

3.2 Borehole 2 (BH2) Sedimentological analysis of BH2 identified 10 lithofacies (Ds, Dc, Fl, Fm/d, Sm/d, Sc, Sh, Sr, Gm) and 6 lithostratigraphic units, whereas geophysical analysis identified 7 electrofacies units (Figures 1-5). During integration of the core and geophysical data, 8 query intervals were identified in BH2, 4 of which involved areas of uncertainty as a result of poor core recovery at stratigraphic boundaries (Figure 8).

The first query interval (Q1; Figure 8) was an area of discrepancy encountered in the bedrock. Initial examination of geophysical data suggested the bedrock surface was located at 43m depth based on a sharp transition in gamma and velocity signatures. However, analysis of core data at this interval revealed highly fractured shale bedrock with fractures infilled with calcite mineralization occurring at 1 -20cm intervals. The bedrock surface was clearly identified in core at 41.65m depth, which is the most likely location of the true bedrock surface.

The second query interval (Q2; Figure 8) was an area of discrepancy, regarding the location of the upper contact of U2 and the identification of an overlying fine- grained electrofacies (U3). Core data revealed a continuous fining-upward succession of gravel, sand and fine-grained facies from 41.40m up to 29.11m depth, but no distinct fine-grained stratigraphic unit. At 29.11m, an erosional bounding surface marks the contact between U2 and overlying sand and gravel facies of U4. This contact was identified on the geophysical logs at slightly lower depths (29.5m) where a sharp increase in conductivity and decrease in magnetic susceptibility is observed. However, the clear lithological contact observed in core was chosen as most appropriate position for the upper boundary of U2.

The third query interval (Q3; Figure 8) was an area of discrepancy regarding the interpretation of U4. Geophysical data contains a dual peak resistivity signature and increased velocity response (EU3-EU5; Figure 5B) which are interpreted to record the presence of a subglacial till complex divided into three subunits. However, analysis of core samples indicates a thick succession of sand and gravel and an absence of subglacial till within U4. Cemented gravel is found within this interval in core from 24.6m- 25.6m and from 26.8m- 27.2m. The high resistivity peak records the presence of cemented gravel horizons rather than till. The high velocity observed from 24m - 29m indicates possible increases in consolidation, cohesion and/or lithification of sediment, consistent with the presence of gravels and cemented intervals.

The remaining query intervals (Q4-7; Figure 8) are classified as areas of uncertainty involving the delineation of stratigraphic unit boundaries. Poor core recovery limited the ability to resolve bounding surfaces of U4-U6d which resulted in imprecise lithostratigraphic boundary picks. Consequently the electrofacies picks were used to determine the formation tops.

Figure 8. Strip logs of geophysical and sedimentologic picks and query interval (Q) locations in BH2. See Figures 3 and 4 for colour codes. 3.3 Summary

Integration of downhole geophysical data and sedimentological core data in the two case studies brought to light four common query types that related to loss of data, anomalously high values in the geophysical signatures, the location of unit boundaries and the subdivision of stratigraphic units. The recognition of query intervals helped to identify areas that required further investigation. Detailed examination of these intervals in turn helped to explain the cause of the discrepancy or uncertainty which allowed more accurate stratigraphic interpretations to be made (Figure 9). Possible explanations for discrepancies between the core and geophysical data are presented in Figure 10.

Figure 9. Stratigraphic interpretations based on geophysical, sedimentological and integrated data sets. Note differences in unit geometry, thickness and lateral continuity between boreholes. See Figures 3 and 4 for colour codes.

Figure 10. Query types encountered in this study and possible explanations for their occurrence.

4 CONCLUSION Downhole geophysical logs and sedimentological data were integrated to delineate stratigraphic units within two fully cored boreholes. Examination of the subsurface stratigraphies created independently from each of the data sets showed several sections within each borehole that were interpreted differently by each of the data methods and which required further investigation. These „query intervals‟ were identified as either areas of uncertainty, where data provided by either method was insufficient, or areas of discrepancy, where both data methods provided varying or conflicting interpretations. The four query sub-types encountered during the integration process related to:

1. Loss of data; 2. High counts forming peaks in geophysical

signatures; 3. Identification of stratigraphic boundaries; and 4. Subdivision („lumping‟ and „splitting‟) of

stratigraphic units Three types of modification/ enhancement of the

stratigraphic interpretation were made to resolve each query:

1. the geophysical interpretation was modified based on sedimentological information available from detailed core analysis;

2. the sedimentological interpretation was modified as a result of physical information provided by geophysical data; and

3. both the geophysical and sedimentological interpretations were adjusted and refined.

Our results show that careful integration of sedimentological and geophysical data sets creates a comprehensive database that significantly enhances the quality and reliability of the stratigraphic interpretations. Reliable subsurface stratigraphic delineation is imperative for the creation of accurate groundwater flow and contaminant fate and transport models. Hence, optimizing the potential of both sedimentological and geophysical data, as demonstrated by this study, is a key component of ongoing and future groundwater exploration programs.

ACKNOWLEDGEMENTS The authors would like to thank the Regional Municipality of Halton for financial support and access to core samples and boreholes. Partial funding was also provided by NSERC Discovery Grant to C. Eyles. The many fieldwork assistants in the Glacial Sedimentology Lab and Environmental Geophysics Lab at McMaster University are also acknowledged for their support. REFERENCES Barnett, P.J. 1992. Quaternary Geology of Ontario, In

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Greenhouse, J.P. and Karrow, P.F. 1994. Geological and geophysical studies of buried valleys and their fills near Elora and Rockwood, Ontario. Canadian Journal of Earth Science, 31: 1838-1848.

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