APPLICATION OF GEOPHYSICAL METHODS TO LOCATE BURIED TUNNEL CHANNELS BENEATH THE GLACIAL DRIFT DEPOSITS IN TEXAS TOWNSHIP, KALAMAZOO COUNTY, MIArthur Obiadazie , Dept. Of Geosciences, Western Mich. Univ., Kalamazoo, MI

ABSTRACTThe objective of this research was to delineate and map the network of gravel and sand –filled channels beneath the glacial drift of Texas Township in SW Kalamazoo County, MI. These channels are excellent local aquifers that are prized by water agencies. The delineation was done by mapping the depth to the interface between the glacial drift and the much older Mississippian Coldwater Shale, which has higher density. Tunnel valleys produce gravity anomalies on the order of -0.2 to -0.4mGal. The gravity method was used to create a Bouguer gravity anomaly map of the township. A total of 360 stations were surveyed, mostly along roads, but did include off-road profiles. Station elevations were obtained using differential GPS techniques, and the data were reduced using an average crustal density of 2.67g/cm3. A regional trends map from a much larger area was subtracted from the Bouguer anomaly map to produce a residual Bouguer gravity map. Theresidual anomaly contour map revealed a low gravity anomaly trending NE-SW and another set of anomalies trending in the N-S direction. In general, the low gravity anomalies showed a complex network of tunnel channels but for the most part, corresponded strongly with the occurrence of a linear alignmentof small lakes and wetlands. A seismic survey was conducted in the axis of the valley based on access andproximity to water wells for depth-to-bedrock control. The deepest reflector obtained by interpretingthe seismic reflection data was located 107 meters below the surface, while the deepest adjacent productionwell (B-3) was 104 meters deep and does not make contact with the shale bedrock. Careful mapping of the tunnel valleys is very important for future water supplies of the Kalamazoo-Portage urban area, as this area is totally dependent upon ground water.

ACKNOWLEDGEMENTSSpecial thanks to the City of Kalamazoo Water department and to John Pacquin for providing the permit to conduct the seismic surveys and driller’s logs at the Al Sabo Land preserve. Thanks are given to Dr. William Sauck and Benjamin Hoyt for assistance with the seismic surveys and to Dr. Sauck for providing technical assistance and advice with the gravity survey. Additional thanks are given to Dr. Kehew for providing resources and materials relating to previous studies of the tunnel channels in the area.

GLACIAL GEOLOGY OF SOUTHERN MICHIGAN AND TEXAS TOWNSHIP

The area was subject to two periods of Pleistocene glaciations; Illinoisan and Wisconsin which deposited the main body of sediments. These glacier lobes were part of the Laurentide ice sheet that covered much of the North America and at its maximum extent, it spread as far south as latitude 37◦N and covered and area of more than 13,000,000 sq. km (5,000,000 sq. miles). The landscape of Southern Michigan Texas township in particular, was greatly influenced by the glacier cover of the Pleistocene epoch (Figure 1).

Fig. 1: Three Lobes of the Laurentide Ice Sheet in Southern MI (Kehew& Kozlowski, 2007).

AREA OF STUDYTexas is one of 16 townships within Kalamazoo County(T 3S, R 12 W) and is located near the Southwestcorner of the County (Figure 2). Over 10,000 people reside within the township. The township is completely covered with glacial sediments ranging in thickness from less than 50 feet to 650 feet (15.24m to 198.12m). The thickest sediments lie in the west of Texas township were the Kalamazoo moraine overlap bedrock channels.

Fig.2: Map of Texas Township Kalamazoo MI. Image from Kalamazoo County Roads Commission.

ORIGIN OF BURIED TUNNEL VALLEYS

Tunnel valleys are elongated depressions cut into unconsolidated sediment or bedrock, with typical lengths of 1-100km, widths of up to 4km and depths of up to 400 meters. Tunnel valleys can either be in filled with sediments or remain unfilled.

Formation of Tunnel valleys( Clayton et al. 1999)

Fig.3: Formation of tunnel valley without collapse of ice and debris into sub-glacial valley.

Fig.4: Collapse of ice and debris into sub-glacial valley, leading to formation of partially filled, more subdued linear depression containing lakes and wetlands.

Fig.5: Formation of palimpsest tunnel valley through burial of sub-glacial valley filled with ice and debris by outwash fan sediment from later advance.

FOCUS OF THE RESEARCH

This research involved the mapping and delineation of the interface between the glacial drift and the much older Mississippian Coldwater Shale. The aim is to detect the network of buried tunnel channels and valleys.

GEOPHYSICAL METHODS USED

The gravity method was used to create a Bouguer gravity anomaly mapof the township, from which a residual map was derived.

Fig. 6: The Lacoste & Romberg Gravimeter similar to the one used for the gravity survey.

The seismic survey was conducted to provide local depth to bedrock for the much broader township-wide gravity survey.

REGIONAL GRAVITY ANOMALY

A Regional gravity anomaly map was extracted from a state-wide gravity survey to produce a residual gravity anomaly map.

Fig. 7: Map of the regional gravity anomaly of the 9 – township area used in the regional anomaly grid.

SIMPLE BOUGUER GRAVITY ANOMALYA total of 360 gravity readings were collected using the Lacoste-Romberg gravimeter.A GPS unit was used to measure the station position and elevation for data reduction.

Fig.8: The simple Bouguer gravity anomaly map of Texas Township. A broad gravity low can be observed with a trend NE-SW across the township.

RESIDUAL GRAVITY ANOMALY

The residual gravity anomaly was obtained by subtracting the Regional gravity anomaly grid from the Simple Bouguer anomaly grid. The residual gravity represents the gravity field after the near surface noise and the regional have been removed. It presumably represents effects of the intermediate zone of interest.

Fig.9: The residual gravity anomaly map of Texas Township. A broad gravity low can be observed that has a trend NE-SW direction. Another gravity low runs N-S along the eastern part of the township. Line A-A’ marks the cross section used in the GM-SYS interpretation.

2D GM-SYS MODEL OF BEDROCK TOPOGRAPHY FROM INVERTED GRAVITY DATA.

The GYM-SYS is a software program used for calculating gravity response from a geologic model And vice versa. The program was used to invert the residual gravity data along cross section line A –A’ in Fig. 9 to create a bedrock model that could produce the observed residual anomaly.

Fig.11: GM-SYS model of inverted gravity anomaly along cross section line A – A’. The bedrock topography shows a broad 2D profile of a tunnel valley with an esker located at the mid profile.

The area (Al-Sabo preserve) is located along the major tunnel valley that cuts across the township. The location of the seismic surveys were conducted along the major valley axis based on access and proximity to water wells for calibration control. This was done for depth-to-bedrock control in collaboration with Driller’s log of the production wells at the preserve.

SEISMIC DATA COLLECTION

Fixed 72-m, 96-m and 48-m long receiver arrays were surveyed using the single fold coverage method. The seismic energy was generated both by hammer and by a Betsy gun. The Geometrics Strata-View X 24 Channel seismograph and twenty-four 40 Hertz geophones were used to record the wave arrivals for all 3 lines surveyed.

The reflection arrivals from Line 2 were successfully analyzed to determine the velocity of the seismic waves in the subsurface and the depth to the reflectors. This area also was the location of well B – 3 of which the driller’s log data for the producing water well was provided by the Water Department of the city of Kalamazoo.

Fig. 13: Time-Distance plot of seismic wave arrival times along times along Line 2.

Fig. 14: SIP Inversion results showing velocity of seismic waves in Layer 1 along Line 2.

Fig.15: SIP Inversion results showing velocity of seismic waves in Layer 2 along Line 2.

Fig. 16: SIP Inversion result of the topography below the subsurface along Line 2.

Fig. 17: Seismic record along Line 2. The picks are on the 2nd reflector’s 1st breaks.

X2 - T2 plot of seismic record 2-08.dat and 2.09.dat

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Summary of results from Velocity analysisSlope Vel2 (m/s) Vel (m/s) Intercept (t0)2 Depth below Datum (m)

5.02E-07 1.99E+06 1411.9 2.17E-02 104.0

Summary and interpretation of resultsThe average surface elevation of the geophones was 270m, which is 3m above the datum. This places the depth to the reflector at 107m. From the driller’s log records, the depth of the production well B – 3 located at the SW corner of Line 2 is 104m.The lithology of the sedimentary layer at that depth is composed of sand, little gravel and clay.

CONCLUSIONSIn general, the low gravity anomalies showed a complex network of tunnel channels, but for the most part corresponded strongly with the occurrence of a linear alignment of small lakes and wetlands. The seismic survey conducted along the axis of the tunnel valley at the Al-Sabo land preserve was performed for depth-to-bedrock control. The deepest reflector obtained by interpreting the seismic reflection data was located 107m below the surface. This is presumably the depth to the glacial drift/bedrock interface.

SIGNIFICANCE OF RESEARCHCareful mapping of the tunnel valleys will be very important for future water supplies of the Kalamazoo-Portage urban area as this area is totally dependent upon groundwater.The tunnel valleys often contain abundant fluvial gravel and coarse sands, and thus are excellent high-yield aquifers. They can also guide deeply flowing contaminants, so their locations and orientations must be known in order to take appropriate remediation measures in the unfortunate case of a chemical spill.REFERENCESAbdelwahid Ibrahim, (1970), The Application of the Gravity Method to Mapping bedrock topography in kalamazoo County, Michigan, PhD Dissertation, MSU Dept of Geology, 1 – 21.

Farrand, W.R. and D.F. Eschman, (1974), Glaciation of the Southern Peninsula of Michigan: a review: Michigan Academician 7(1): 31 – 56.

Kehew, A.E. & Kozlowski A.L., (2007), Tunnel Channels of the Saginaw Lobe, Michigan, USA. Applied Quaternary research in the central part of glaciated terrain, Geological Survey of Finland, Special paper 46,69 – 78.

Michigan DNR, Geological Survey Division, (1987), Bedrock Geology of Southern Michigan. Michigan Sesquicentennial 1837 to 1987.

Fig:2: Geometrics Strata-View X24 Channel Seismograph.

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