Camera inspections are used for locating pipe joints and damages, and serve to access the direction of filter screen orientation. Incompletely screwed pipe joints and structural defects in the well pipe may indicate the ingression of extraneous water. Camera inspections can reveal free-phase LNAPL and obstructions and provide information on sediment accumulation at the bottom of the well. They can also provide evidence of stationary vertical flow within the monitoring well by reading characteristic staining patterns and streak marks along the well screen.

Older monitoring wells frequently show floating flocculated material, biofouling, incrustation, and deposits along the casing walls. These obstructions are often unevenly distributed along the screened intervals. They may clog the well screen and backfill material, leading to a significant influence on passing groundwater flow in the affected well section.

For horizontal flow measurements, it is important to define suitable measurement depth, as measurements near pipe junctions an in clogged screen sections may lead to erroneous results. As depth positions of pipe junctions are typically not documented in well construction plans, a preliminary camera inspection provides the required information and supports quality assessment.

As additional measure, we can restore older wells or partly clogged screens to meet the requirements to perform reliable measurements.

Measurements conducted by simultaneous hydraulic stimulation in the well allow to identify preferential inflow zones and to gain information on relative hydraulic conductivity. Due to the high sensitivity of the measurement probes, only low pumping rates are required to induce hydraulic stimulation and to acquire measurement data. At contaminated sites, this minimizes the amount of groundwater that must be handled and disposed of.

As a standard, we conduct vertical flow measurements in well diameters ranging from 2.5 to 8 inches and down to a depth of 100 meters. The measurements can be supplemented by temperature and electrical conductivity profiling.

Depth-Resolved Groundwater Flow Measurements

Groundwater contour maps generally draw a stationary, two-dimensional picture of groundwater flow. For large-scale assessments this is usually sufficient. However, groundwater often moves spatially through an aquifer and along preferential flow paths controlled by sedimentary structures and interconnected sediment networks. This information is often crucial for understanding groundwater flow on a local scale.

Consequently, horizontal groundwater flow measured within a monitoring well may vary significantly with depth. A reliable interpretation therefore generally requires measurements at multiple depths. PHREALOG addresses this challenge by performing horizontal multilevel in-situ flow measurements at several depths within the same monitoring well. This approach enables a higher-resolution characterization of groundwater flow throughout the aquifer profile, improves the reliability of interpretation, and can reduce the overall field effort.

It is to consider that in-situ flow measurements provide only point-or line-specific information. Depending on the sedimentary architecture and monitoring well construction, the measured flow may differ from the local groundwater flow regime. Therefore, the results of in-situ flow measurements should, whenever possible, be interpreted and evaluated in conjunction with accompanying site investigation results.

Depending on the length of the well screen section, vertical flow measurements can be performed for closer aquifer characterization and to check on vertical flow rates that might influences horizontal flow measurements. Based on these initial surveys and on borehole log data, measurement depths for horizontal flow are defined.

When used as a stand-alone investigation method, the significance of in-situ flow measurements can be substantially improved by correlating depth-resolved flow data with local sedimentary structures. To gain meaningful results, in general, flow measurements at different depths of the monitoring well are required.

Limitations of Analytical Correction Factors and the PHREALOG Approach

Our field experience shows that flow direction and velocity naturally vary, even within homogeneous sediment layers. Variations in flow direction and velocity within a monitoring well result from small-scale lithological heterogeneities, irregular well construction, partial clogging of screen openings or filter pack material and influences due to the orientation of well screen slots. These interactions can produce considerable well-specific flow artefacts (PHREASIM). Understanding the underlying flow processes is therefore essential for distinguishing natural groundwater flow characteristics from measurement effects caused by the monitoring well itself. Comparing flow data obtained at multiple depths significantly improves this interpretation.

The mentioned circumstances have an impact on the measured flow rates and their use for further calculations. Conventional analytical approaches commonly use a correction factor, known as “α-factor”, to describe the relationship between flow velocity measured within a monitoring well and the Darcy velocity in the aquifer. Based on the measured in-well velocity and an assumed or calculated α-factor, the Darcy velocity in the adjacent aquifer is estimated.

However, our experience indicates that such analytical approaches do not account for all factors influencing the hydraulic response of a monitoring well. In addition to the uncertainties already mentioned, these calculations generally do not fully account for hydraulic effects associated with well screen geometry such as the orientation of screen slots and supporting ribs relative to the direction of groundwater inflow.

For this reason, PHREALOG follows a measurement-based, statistically supported approach. By systematically collecting flow data at multiple depths, we characterize the distribution and variability of groundwater flow throughout the investigated aquifer profile, identify prevailing flow directions and velocity ranges, and calculate representative mean flow velocities where justified by the measurement data.

This approach combines extensive depth-resolved measurements with an assessment of well-specific hydraulic effects to provide a more robust interpretation of local groundwater flow conditions while reducing field effort.

Over the past decades, a wide range of methods for measuring in-well flow have been developed in response to the growing demand for high-resolution information on groundwater flow and an improved hydraulic understanding of aquifer systems. Typical in-situ flow measurement methods include techniques based on artificial tracers used as flow markers, methods relying on physical stimulation, and optical techniques utilizing natural particles as flow markers. PHREALOG® employs such an imaging-based measurement approach.

Our measuring approach visualizes naturally occurring fine suspended particles transported by groundwater flow and serving as natural flow tracers. Within a laser-illuminates field of view these particles become visible and their light scattering is recorded as particle patterns. A specialized optical setup is used to reliably detect suspended particles even under challenging visibility conditions, as high turbidity and high particulate loads—and track them as a pattern (EP2989471B1). Typical particles are biocolloids and biological aggregates of various compositions, humic substances, sediment particles, aquatic fauna, contaminants, and mixed forms. Within a sequence of images, the displacement and direction of these patterns relative to magnetic north is determined that allows to calculate flow vectors every few seconds. The resulting vector data are displayed in time-resolved diagrams.

Imaging-based measurement technologies provide significant advantages for in-situ flow investigations. By utilizing naturally occurring particles as flow tracers, artificial tracer-related measurement errors do not arise and continuous long-term data acquisition becomes possible. Changes in flow direction and flow velocity can be detected with high precision, enabling detailed real-time tracking of transient flow processes. Due to the contactless sensing principle, the technology remains highly robust even in chemically aggressive environments. On-site calibration is not required and measurements can be performed even under varying optical disturbances and over a wide range of velocities. Operational effort, setup time, and technical susceptibility to failure remain low.

Imaging-based techniques allow for maintenance-free, time-resolved monitoring of groundwater flow dynamics with high directional accuracy and a continuous monitoring and analysis of particle freight. Two live videos of particle flow are shown below: on the left at a flow velocity of 1 × 10⁻⁴ m/s, and on the right at 5 × 10⁻³ m/s.

Depending on the length of the screened interval, groundwater flow in a monitoring well frequently occurs in both horizontal and vertical directions. This can result in complex flow conditions and the formation of irregular eddies and flow convection within the screened section. Eddies occur particularly at the inflow into the well.

In order to perform measurements under comparable hydraulic boundary conditions, it is necessary to decouple the flow components and create a controlled, turbulence-free flow environment within the measurement section of the well screen. This enables a more reliable comparison of measurement data from different depth intervals and enhances the reliability of the measurement results.

To meet these requirements, we have developed flow guiding devices designed to decouple and harmonize horizontal flow within the measurement compartment and to minimize disturbances caused by well screen geometry and vertical flow movement. These measures help to guide and quickly stabilize horizontal flow, while also reducing measurement time, as accurate datasets can be obtained in less time.

The bypass function reduces hydraulic resistance during deployment of the measurement chain, allowing multiple modules to be lowered rapidly to the required depth while minimizing the hydraulic recovery time after installation.

Individually designed for different well diameters, the arrangement, number, and diameters of the struts used for the flow guiding devices are based on extensive simulation results. The development is based in part on findings obtained within our BMBF-funded research project FLIMSYS (BMBF grant number 02WQ1192) and is the subject of an international patent application.

During a field campaign, we focus on the following aspects:

1. Camera inspection: Does the recorded monitoring well construction match the well completion plan? At which depths are pipe junctions located? Is the condition of the well screen suitable for conducting in-situ flow measurements?
2. Vertical flow measurements: Can hydraulic short-cuts be identified, and may they influence horizontal flow measurements? Are hydraulically active depth sections present that may serve as target depths for horizontal flow measurements? Are convection effects noticeable that may affect the measurements?
3. Horizontal flow measurements: Which flow directions and flow velocities occur at which depths? How consistent are horizontal flow direction and flow rate within the measured depth profile, and how uniform is the flow over time? Are additional measurements required to compare, confirm, and substantiate the results? If temporal changes in flow rate are identified, can possible causes be determined? Does the amount and type of transported particles vary within the depth profile? Are hydraulic disturbances identifiable, and what are their possible causes?

    

   This systematic approach ensures reliable results and makes data acquisition transparent and traceable. In addressing these issues, we draw in particular on our extensive practical experience.