Shining new light on ground water measurements
NIWA’s LWL6001-S, is putting groundwater levels under the spotlight. The new laser sensor accurately measures groundwater surface levels to sub-millimetre precision. This sensor is ideal for anyone who needs the continuous, reliable data needed to effectively manage groundwater resources.
If you want to accurately manage groundwater resources you need access to continuous, precise measurements from a number of often remote sites. NIWA has designed a new laser-based water level sensor to deliver precisely that quality data.
Through our Environmental Information innovation programme, NIWA has developed and built 20 production-prototype, laser-based groundwater SDI sensors. The sensors meet the NEMS ‘top-shelf’ QC600 quality code (for groundwater <±10 mm) and are ideal for accurate, non-contact groundwater-level monitoring at remote sites.
NIWA is also planning to extend their use to surface water measurement.
How does it work?
When the data-logger requests a measurement from the sensor, a red laser light pulse is transmitted downward, inside the groundwater bore casing. This light pulse is reflected from a retrievable plastic target disc that floats on the water surface. A separate lens and light sensor detect the reflected light and the ‘time-of-flight’ of the pulse is used to determine the distance from the reference position at the top of the bore casing down to the target. On site visits, any difference between the reference and sensor may simply be added as an offset to the data-logger.
- The LWL6001-S is compatible with all Unidata data-loggers and most other loggers that have SDI-12 capability.
- The sensor is powered by the logger via the SDI connection so doesn’t require a battery.
- It has built-in diagnostics, in the form of error codes, to identify problems.
- The sensor uses an eye-safe, low-power red laser using ‘illuminated pointer mode’ to assist aiming.
- There are no moving parts.
How does it compare with submersible pressure transducers and bubblers?
Method: The LWL6001-S laser measures distance so its output is ‘closer to the primary standard’ than pressure sensors that weigh the overhead water as a surrogate for depth.
Calibration: The output of the LWL6001-S does not drift over time and doesn’t need to be calibrated, unlike submersible PTs and bubblers,
Price: The LWL6001-S costs less than precision PTs currently used for continuously monitoring groundwater, and offers significant savings on bubblers. It does not need to be removed for annual recalibration and ongoing operational costs are minimal.
Stability: The LWL6001-S is digital, from end to end. It doesn’t have the analogue drift and hysteresis problems of PTs that must convert the sensed analogue pressure to a digital output, or may even have an analogue output.
Range: The LWL6001 works over a very wide range and unlike PTs does not need to be matched to an expected water-level range. This ‘one size fits all’ approach minimizes the number of variants that need to be held on inventory, as spares, and simplifies related field work.
Resolution: The LWL6001-S has excellent repeatability and can resolve changes as small as 0.1mm.
The LWL6001-S can reveal time-series features that would go unnoticed by lower resolution sensors. For example, at one of our trial sites near Nelson, we recorded the aquifer response to a magnitude 7.1 earthquake off East Cape. At the time, we were measuring and recording the groundwater level every 10 seconds.
In association with the Tasman District Council (TDC) we are testing two LWL6001 sensors alongside a standard high-quality reference Pressure Sensor in various groundwater bores at locations in an around Nelson. You can see the laser comparison with the TDC pressure transducer in the accompanying graph.
LWL6001 sensor data plotted against data from a co-located Pressure Transducer at the Edens site, near Nelson. [Image: NIWA]
Condensation on the window can occasionally cause short periods of missing data but a number of steps can be taken to mitigate this including:
- An electric demister
- A small solar-charged fan
- Super-hydrophobic (water hating) compounds that shed water droplets
- A polished quartz window to minimize fogging.
Where to from here?
We have now handed the LWL6001-S design to Unidata for mass production. Following this, Unidata will create a variant; a self-contained, solar-powered, Neon groundwater-to-web monitor that drops directly onto a bore head so automatically self-references… significantly reducing operational cost.
These products will significantly improve measurement efficiency and accuracy and increase our understanding of bore water level changes by increasing the frequency of readings ‘from months to minutes’.
NIWA is also trialling further applications for the LWL6001-S beyond groundwater-level measurements including:
- monitoring leakage in farm effluent ponds
- measuring levels in feed silos
- monitoring surface water level.
If you have a groundwater-level monitoring need that you think might be resolved by this type of application, we’d be pleased to help.
Jeremy Bulleid – NIWA Instrument Systems