Andy F. Bajc

A three-dimensional hydrostratigraphic model of the Waterloo Moraine area, southern Ontario, Canada

Aquifers of the Waterloo Moraine play a key role as the main source of drinking water for the Region of Waterloo. For the effective management of this water source, a sound understanding of the aquifers contained within and below the Moraine is essential. Critical knowledge required for this understanding includes the definition of the sediment facies distribution, architectural elements and geological origin of the Quaternary-aged deposits. A basin analysis approach has been applied to geologic data collection and interpretation to unravel the paleogeographic history of the study area and to provide a predictive framework for understanding its geological variability. Coarse (sand and gravel) sediment within the Waterloo Moraine was deposited during a series of high-energy meltwater discharge events from several sediment input corridors (eskers), into a deep, large, ice-supported glacial lake. This depositional setting led to a complex three-dimensional architecture comprising sand-gravel and mud units that are increasingly interbedded away from the multi-directional influx sources around the perimeter of the Moraine. A recently completed digital, three-dimensional geologic model of the area provides details of the various geological units that help refine the understanding of the hydrostratigraphy. This information has improved the understanding of groundwater flow (including interaction between surface and groundwaters) and has provided valuable information critical for source water protection. Information on the distribution, thickness, geometry and properties of these units has resulted in a better understanding of the potential linkages between near-surface recharge areas and deep aquifers across the region. This geological information is important in developing predictive models, for example, determining the location of high transmissivity zones within the moraine. Derivative products such as aquifer vulnerability and recharge maps may help inform policy makers in developing land use and nutrient management plans in the vicinity of well fields and sensitive lands.

Mapping the Subsurface of Waterloo Region, Ontario, Canada; An Improved Framework of Quaternary Geology for Hydrogeological Applications

Abstract Please click here to download the map associated with this article. The Ontario Geological Survey (OGS) has embarked on a pilot project of 3-dimensional mapping of Quaternary deposits within the Regional Municipality of Waterloo in southwestern Ontario, Canada. This project is part of a broader OGS initiative designed to provide basic geoscience information for the protection and preservation of the provincial groundwater resource. The main objective of this project is to develop a series of protocols for detailed 3-dimensional mapping of Quaternary deposits. These protocols shall be used as standards for similar surveys to be undertaken in other areas of the province. 3-dimensional mapping involves the characterization of the geometry and inherent properties of subsurface deposits. This information can: 1) aid in studies involving groundwater extraction, protection and remediation; 2) assist with the development of policies surrounding land use and nutrient management; and 3) help to better understand the interaction between surface and groundwater systems. This paper briefly summarizes the main sources of information used for the creation of the 3-dimensional block model and the key processes involved in its generation. The attached map is an exploded representation of the main Quaternary units present within the Regional Municipality of Waterloo. The regionally-based block model is created with cells measuring 100 by 100 m. The structural contour and isopach depictions can: 1) assist with the identification of windows that hydraulically connect upper and lower aquifers; 2) help to define areas where aquifers are intrinsically more susceptible to contamination; and 3) aid in unravelling depositional environments which can ultimately be used to help one predict sediment variability in the subsurface.

Multicomponent vibroseismic profiling over high velocity glacial ground: an example from Southern Ontario

A 3-D Quaternary mapping project conducted by the Ontario Geological Survey (OGS) in the southern part of Simcoe County involves borehole drilling, airborne geophysics, such as TDEM and magnetics and ground gravity surveys. Geophysical surveys are necessary to define the top of bedrock, including buried bedrock valleys and the architecture of overlying sediments for evaluating groundwater resources. In support of this project, the Geological Survey of Canada (GSC) carried out a three-line 21.2 km seismic reflection survey. Geophysical logging in two deep boreholes was undertaken to assist with the calibration of the seismic sections. The seismic survey was performed using an IVI “Minivib 1” source with a “landstreamer” three-component geophone array built by the GSC. The landstreamer consists of 72 - 3 kg metal sleds, spaced at 1.5 m, towed using low-stretch belts. Data were acquired with shot points every 4.5 m. The source vibrates a 140 kg mass in in-line (H1) horizontal mode, using a 7 second nonlinear logarithmic sweep of -2 DB/Oct from 20 to 300 Hz. This type of sweep increases the time spent in the low end of the sweep which has the effect to increase the low frequency energy to enhance shear body wave energy. Data were recorded using seven 24- channel Geometrics Geode engineering seismographs operated in the cab of the Minivib. Only the vertical component of the 24 geophones, furthest from the source, was recorded in order to obtain a better coverage of the P-wave data acquisition window. Uncorrelated records were collected to allow pre-whitening of the data and careful choice of the correlating function was the first step in the data processing sequence. P-wave sections were derived from processing the first 0.5 sec. (after correlation) of data acquired on the vertical geophones, while S-wave sections were produced using the in-line, H1, component over a correlated window of 2 seconds. Seismic sections were then correlated with borehole geophysical data. Interpretation of the equivalent compressional (P-) wave section permits delineation of seismic facies sequences. The P-wave velocity is an order of magnitude higher than the shearwave velocity and as a result, the vertical resolution of the section is lower. However, the acoustic impedance contrast with underlying materials (coarser sediments, tills or bedrock) is lower than in the case of shear-wave. The shear-wave data produce remarkably detailed sections over buried valleys down to 150 m.

Identifying SAR permeability zones on groundwater recharge areas

High-resolution multidate SAR spring images with similar incidence angles were used to update permeability maps over recharge areas on glacial aquifers. From a difference image produced from two dates in early spring we interpreted high medium and low permeability zones. These SAR permeability zones are related to the distribution and behavior of soil moisture on different surficial deposits and slopes. Permeability thematic maps will be useful to identify nutrient infiltration patterns and accumulation on farmed recharge areas.

Near Surface Seismic Reflection and Refraction

Every reliable interpretation method of shallow refraction seismic traveltime data (also known as first arrivals or head waves) has to include criteria for distinguishing between vertical and horizontal variations of velocities. The Common Refractor Element -CREmethod presents a simple approach to interpret shallow refraction seismic data especially in cases of piecewise lateral changes along the refracting interfaces. In this method, a common linear traveltime element, (or part of it), which corresponds to a linear common refractor element is used for inversion of these first arrivals. Linear traveltime elements are defined as linear parts of the traveltime curve with equal slopes and consequently equal apparent velocities. Traveltime parameters such as the layer reciprocal time, the apparent refractor velocity and the intercept time values are used to distinguish between vertical and horizontal variations. Layer reciprocal time means that the forward and reversed traveltime refracted from one layer must be equal at the two ends of the traveltime curve. Lateral changes in dip and/or velocity along the refracting interface create a traveltime curve of different linear segments and the extrapolation of corresponding traveltime elements for reversed profiles are not equal at both ends of the traveltime curve, except for the last two refractor elements that represent the last two elements from each side of the refractor. Apparent refractor velocity can be used in several ways as another parameter for lateral variation or structure identification. In-line reversed profiling technique is essential for the required data.

Aquifer Sensitivity Map using SAR and GIS Techniques

High-resolution multi-date SAR spring images with similar incidence angles and other hydrogeological information were used in a Geographic Information Systems (GIS) to produce aquifer sensitive map over sensitive recharge areas. SAR difference image produced from two dates in early spring correspond to sensitive permeability zones. Information on land use, surficial materials and the depth of the aquitard and SAR permeability allow us to produce an aquifer sensitivity map. We identify nutrient infiltration patterns and accumulation on the farmed recharge areas that will assist in planning future land uses for groundwater protection in the area.