Advanced Geophysics

Technician handling geophysics probe in the field
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G360 develops and applies advanced borehole, surface, and airborne geophysical techniques as well as novel hydrophysical techniques to help characterize physical aquifer structures and properties, and the position and magnitude of groundwater flow. Identifying active groundwater flow and quantifying flow rates in bedrock aquifers is critical for developing a robust conceptual or numerical site model but is a persistent challenge for hydrogeologists due to lack of suitable field methods and tools. Our key advancements in the field include the development of the Active Line Source (ALS) wireline temperature logging, Fibre Optic Active Distributed Temperature Sensing (A-DTS) and high sensitivity transducer deployments in temporarily sealed boreholes, as well as advanced surface and airborne geophysics to assess aquifer geometry and structure.

Active Line Source Temperature Logging
Active Line Source (ALS) temperature logging was developed at G360 to identify the position and relative magnitude of active groundwater flow in the subsurface (Pehme et al. 2007, 2013). A continuous line source heater in a borehole temporarily sealed with a FLUTe liner is used to heat the water within the borehole and force thermal disequilibrium. Once the heater is turned off, a highly sensitive (+/- 0.001 C) probe is trolled down the sealed borehole to measure detailed temperature profiles. Active groundwater flow accelerates cooling, which can be measured with the sensitive wireline probe. This allows specific flow zones or fractures to be identified, and the relative magnitude of flow to be assessed. The ALS technique is enhanced with the use of the custom designed Thermal Vector Probe (TVP) which provides a 3-D measurement of the thermal gradient in the borehole, providing insights on groundwater flow direction and thermal stratigraphy (Pehme et al 2014). These novel methods have been applied at dozens of research sites around the world.

Fibre Optic Active Distributed Temperature Sensing
Fibre Optic Active Distributed Temperature Sensing (A-DTS) in sealed boreholes builds off the ALS method and is a field method developed and advanced by G360 since 2011. It allows efficient identification of depth discrete hydraulically active fractures along the full length of a borehole (Coleman et al. 2015).  The method involves installing fibre optic cables in open bedrock coreholes, and then sealing the borehole with a FLUTe liner. The FLUTe liner serves several purposes: (1) to push the cable against the borehole wall to ensure good contact with the formation, and (2) to seal the borehole to restore natural gradient hydraulic conditions and to prevent vertical flow within the borehole. The A-DTS test involves heating the fibre optic cable using electrical resistance heating of two copper wires that are part of the cable assembly, and continuously monitoring the temperature along the full length of the cable using a Distributed Temperature Sensor (DTS).  Active groundwater flow causes preferential cooling to occur at discrete flow intervals during the heating of the cable, which can be identified in the resultant temperature data. Recent advances by Maldaner et al. (2019) and Munn (2019) at G360 have allowed the quantification of volumetric flow rates and groundwater flow direction from A-DTS tests. Most of the development of the technique was conducted at local field research sites in the Silurian dolostone aquifers, but the technique has been applied in ‘real-world’ field sites with industrial partners including sites in France, the United States, and Sweden.

Temporary Deployments
The ‘Temporary Deployment’ technique is a removable and reusable installation of sensors placed at numerous discrete depth intervals along a borehole length, and then sealed with a FLUTe liner. Typically, rock core, geophysical and hydro-geophysical data are used to design 20-30 sampling intervals, wherein pressure transducers and high sensitivity (0.0001 C°) thermistors are deployed at targeted, depth-discrete intervals, each hydraulically isolated with a continuous seal created by the liner. Subsequently responses to either natural or artificially induced hydraulic stresses are used to identify hydraulically active fractures, their connectivity, as well as the vertical components of hydraulic and thermal gradients that provide insights into hydrologic unit boundaries and infer groundwater flow directions, respectively. These data create an improved basis for a permanent MLS design and avoid cross-connection of units with distinct hydraulic, hydrochemical and/or contaminant conditions. The temporary deployment methodology also supports multi-borehole data collection techniques (e.g. cross-hole testing).

Surface and Airborne Geophysics
Borehole, surface and airborne geophysical techniques are used to tackle a broad range of groundwater resource and water quality issues. Much of our research has been focused on highly detailed characterization of discrete flow features within bedrock aquifers and aquitards. Applications of surface and airborne geophysics include assessment of geologic structures such as fractures, dissolution features and buried bedrock valleys, groundwater-surface water interactions along fractured rock rivers, and multiphase contaminant migration. These geophysical techniques are often combined and interpreted together with geologic and hydrogeologic information that we obtain from continuous core records and multi-level groundwater monitoring systems.

Surface Geophysical Techniques
• Electrical Resistivity
• Seismic Refraction
• Frequency-Domain Electromagnetics
• Time-Domain Electromagnetics
• Gravity
• Ground-Penetrating Radar

Frequently Applied Borehole Techniques:
• Acoustic Televiewer
• Borehole Video Camera
• Formation Conductivity
• Natural Gamma
• Full Waveform Sonic
• Magnetic Susceptibility
• Thermal Vector Probe
• Heat Pulse Flowmeter
• Optical Televiewer

Advancing Technologies:
• Active Line Source
• Active Fiber Optic DTS
• Temporary Deployments
• Magnetic Nuclear Resonance

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