Fluid Column and Flow Borehole Methods

Most borehole geophysical methods induce a signal within the borehole and receive responses from borehole-intersected formations and their interstitial fluids. Conversely, fluid logging techniques measure characteristics of the fluid column (i.e., water within the borehole), and certain fluid properties (e.g., temperature, conductivity, solute concentration) are used to infer formation properties. Though each has inherent informational value, fluid methods are often used to better understand groundwater flow structure, which is vitally important to numerous hydrogeological and environmental studies.

Driven by hydraulic head (i.e., pressure) gradients, groundwater intricately moves through the subsurface via transmissive fractures and within the pore space of permeable earth materials. The net flow that occurs at a point within the subsurface is a result of the combination of numerous structural, compositional, and physical properties of the earth materials. Thus, this subsurface transportation process is complex, and numerous methods and techniques have been developed to map various aspects of groundwater flow networks.

A borehole that intersects a saturated aquifer with a hydraulic gradient will contain fluid that experiences some type of net flow through the borehole. Fluid can enter and exit through various transmissive zones, and such flows can be representative of large-scale flow mechanisms and aquifer properties. Thus, fluid column measurements can aid the estimation of certain hydraulic parameters (e.g. transmissivity, hydraulic head gradients, and hydraulic conductivity) that are useful to conceptual and numerical site models.

The determination of flow within and between a borehole is extremely useful for characterizing groundwater hydrology. Measurements of vertical flow within a single well are most commonly and easily determined, but horizontal flow through a single well may also be assessed using certain techniques (e.g., active line source, single borehole tracer). Flow is generally analyzed using flow-measuring probes (e.g., vertical flowmeter), tracer methods (e.g., single borehole tracer, fluid temperature and -Conductivity), and/or thermal methods (active line source, fluid temperature) (Keys, 1990).

Active line source logging induces a thermally unequilibrated borehole environment, monitors fluid temperature, and characterizes flow by considering groundwater flow effects on thermal energy transport mechanisms. Single-borehole tracer methods consider how a concentration of a tracer (e.g., dye, salt, heat) changes over time and space to estimate horizontal and/or vertical flow velocities. Vertical flowmeters collect nearly direct measurements of the direction and magnitude of the vertical component of flow velocity to aid in volumetric flow rate calculations.

Fluid-temperature and -conductivity logs can provide useful information on fluid properties and movement within a borehole and are often combined with other data sets. Thus, it is typical for fluid methods to be interpreted in conjunction with one another and/or other borehole techniques. With such an approach, hydrologically active zones within a fractured-rock or unconsolidated aquifer can be more comprehensively identified and characterized. The methods useful to numerous hydrogeologic and environmental studies include:

• Fluid-Temperature and Fluid-Conductivity Logging
• Active Line Source (ALS) Logging
• Vertical Flowmeter Logging
• Single Borehole Tracer (SBT) Logging

References

Keys, W.S., 1990, Borehole Geophysics Applied to Ground-water Investigations, in Techniques of Water-Resources Investigations: Denver, Colorado, United States Geological Survey, Book 2, Section E2, 150 p.