Deploying Optical Sensors Through Ice-Covered Lakes

Winter fieldwork on an ice-covered lake demands more than lowering an instrument through a hole and recording readings. Ice thickness, snow cover, wind, changing water levels, surface reflections and the movement of the sensor cable can all influence measurements from turbidity monitors and suspended-solids sensors. A carefully prepared deployment hole gives the instrument a stable route into the water, while thoughtful positioning helps distinguish genuine sediment changes from installation artefacts.

For Australian teams, this work is usually concentrated in alpine areas rather than broad, permanently frozen lakes. Lake Jindabyne and Lake Eucumbene in New South Wales can experience winter ice near sheltered margins, while high-country tarns and reservoirs in Victoria and Tasmania may develop short-lived surface ice. Access can involve a long walk from a locked gate, a four-wheel-drive trip, or a remote camp, so equipment selection and field procedures need to be practical, repairable and safe in rapidly changing conditions.

Assessing ice and site conditions

Before drilling, assess the lake edge, ice surface and surrounding weather rather than treating the ice as a uniform platform. Clear, dark ice is generally stronger than white, porous ice formed from refrozen snow, but thickness can vary sharply around inflows, outlets, reed beds, springs and areas affected by current. Snow can hide cracks and weaken the surface through repeated melt and freeze cycles. A site that appears solid in the morning may become unsafe after sun, rain or warmer wind.

Use a staged approach to access. Work from firm ground, wear a flotation suit or personal flotation device where appropriate, and use a throw line, ice picks and a buddy system. Probe ahead with an ice chisel or auger, keeping personnel and heavy equipment away from untested areas. Record ice thickness, snow depth, weather, water level and the distance to shore. These observations help explain later changes in the optical record and provide a repeatable basis for returning to the same location.

In Australian alpine settings, freeze-up may be patchy rather than continuous. A reservoir arm exposed to wind can remain open while a sheltered bay is iced over, and a sunny northern-facing margin can behave differently from a shaded southern one. Local knowledge from rangers, dam operators and experienced field crews is valuable, but it should supplement current measurements rather than replace them.

Designing the ice-drill deployment hole

The hole must be large enough for the sensor body, mounting hardware and cable connector to pass through without scraping. Its dimensions depend on the optical instrument and deployment frame, but a neat, round hole is preferable to an irregular opening made by chopping. An ice auger usually produces a cleaner entry and leaves a core that can be measured or retained for inspection. For larger housings, drill overlapping holes and trim the bridge between them carefully.

Keep the hole clear of loose chips. Ice fragments can strike the optical window, settle on the sensing face or become trapped around a protective cage. Remove slush with a scoop, bailer or small net, then inspect the opening for sharp edges. A collar or short section of pipe can stabilise the perimeter, reduce edge chipping and provide a defined point for a pulley, lowering line or temporary tripod.

Position the hole where it supports the measurement objective. A location close to shore may be convenient but can overrepresent shallow, wind-sheltered water. A hole near an inflow may capture a sediment plume, while a hole in the deepest basin may better represent lake-wide conditions. If the aim is process monitoring, such as detecting a storm-driven resuspension event, site the hole where the process is expected to occur and document that choice.

The hole should also accommodate safe retrieval. Leave enough room for a gloved operator to guide the instrument without placing hands directly beneath a suspended load. Keep the cable away from the cutting area and protect it from sharp ice. Never leave an unattended hole open where other workers, visitors or wildlife could approach it; mark it clearly and restore or guard it when the deployment ends.

Selecting the sensor depth and orientation

Optical turbidity and suspended-solids measurements are strongly affected by where the sensing volume sits in the water column. Near-surface placement can be influenced by frazil ice, bubbles, floating organic matter and light entering from the opening. Near-bed placement can be disturbed by sediment stirred up by the instrument frame, anchor, cable or deployment crew. A mid-water position often provides a cleaner representative signal, but the correct depth depends on lake stratification and the question being investigated.

Use a rigid frame, weighted cage or fixed mounting bracket when the sensor must remain at a known depth. A free-hanging instrument can swing in currents, especially when the cable passes through moving water beneath the ice. That movement changes the sampled water and may cause intermittent spikes as the sensor approaches the bed or faces into suspended particles. Add a bottom weight below the sensor where appropriate, while ensuring the weight cannot disturb sediment within the measurement zone.

Orient the optical window away from the ice hole, frame members and cable. A horizontal or slightly downward-facing arrangement can reduce direct light entering the path, but the best orientation depends on the instrument’s optical geometry. Keep the sensing face clear of the seabed or lakebed and avoid placing it directly in the wake of a support line. Where the water is very clear, stray light may matter more than expected because small background signals become significant.

For profiling, use fixed depth marks on the cable or a graduated lowering line. Pause at each target depth long enough for the reading to stabilise, while recording depth, time and any visible disturbance. A profiler should be lowered smoothly rather than dropped through the hole, since turbulence around the instrument can temporarily elevate turbidity and make a vertical profile look more variable than the lake itself.

Managing light, bubbles and fouling

An optical sensor measures how particles interact with light, so conditions around the optical path matter. A freshly drilled hole may admit sunlight that was absent under the intact ice, producing a difference between open-hole and under-ice observations. Shade the opening without sealing it against ventilation or trapping equipment. A dark collar, cover or short opaque sleeve can reduce direct illumination and help keep snow from falling into the water.

Bubbles are another common source of unstable readings. They can form when meltwater runs down the hole, when a sensor is lowered too quickly, or when wind pushes air through slushy ice. Let the instrument settle below the disturbed layer before accepting data. If readings oscillate, inspect the optical window and lower the sensor gradually. Do not interpret every rapid spike as a sediment event until bubbles and cable movement have been ruled out.

Ice crystals and organic material can also collect on the sensing face during long deployments. Use a wiper or protective system where the instrument supports one, but recognise that mechanical cleaning does not remove every source of bias. Inspect the window during retrieval, rinse it with clean water and document any film, scratches or deposits. In freshwater lakes, low temperatures may slow biological fouling without eliminating it, particularly where nutrients or plant material are present.

A field blank or reference check can expose contamination from the deployment process. Measure a clean reference water or a known standard before installation and again after retrieval when practical. Keep calibration standards within their specified temperature range and prevent freezing. Temperature compensation may improve comparability, but it does not correct a blocked optical path or a sensor positioned in a cloud of disturbed sediment.

Securing cables, frames and telemetry

The cable route should be treated as part of the deployment design. A line rubbing against the ice edge can abrade its jacket, while repeated movement from wind or wave action can fatigue connectors. Use a smooth protective sleeve, a rounded guide or a temporary pulley at the hole. Provide strain relief above the sensor so the connector does not carry the weight of the cable or frame.

Anchor the support independently from delicate instrument components. A tripod, crossbar or stake system can hold the line above the opening, but the anchor must remain stable if the ice softens. For longer deployments, place the logger and battery system in a weather-resistant enclosure, elevated above meltwater and protected from condensation. Lithium batteries can lose capacity in cold conditions, so specify the battery system for the expected temperature and verify voltage under load.

Remote monitoring is useful when a lake is difficult to revisit, yet telemetry does not remove the need for local checks. A sensor may continue transmitting plausible numbers after its window becomes fouled or its frame shifts. Log raw readings, diagnostic values, battery voltage, temperature, depth and timestamps. Use quality flags for installation, settling, maintenance and retrieval periods so later analysis does not treat field handling as environmental change.

For a project that combines water observations with aerial sampling, an integrated drone workflow can help compare a fixed under-ice record with spatial information from above the lake. In Australia, aviation rules, site permissions and weather limits need to be checked before operating near reservoirs, conservation areas or controlled infrastructure, especially when the field team is already working on unstable ice.

Validating measurements in cold water

Before deployment, inspect the optical window, connector seals, cable strain relief and mounting hardware. Confirm the sensor’s measurement range and output units, and verify that the data logger uses the intended sampling interval. Turbidity reported in nephelometric units is not automatically equivalent to suspended-solids concentration, so site-specific samples may be required to develop a relationship between optical response and gravimetric sediment concentration.

Cold water can change instrument response, battery performance and the handling characteristics of cables. Allow the sensor and standards to equilibrate where the manufacturer permits, rather than moving a warm instrument directly into near-freezing water and expecting an immediate stable value. Record water temperature beside each calibration or verification result. If the lake is stratified beneath the ice, temperature and conductivity profiles can also help explain why particle concentrations differ with depth.

Collect discrete water samples near the sensor when conditions allow, taking care not to stir the bed. Label each sample with exact depth, time, hole location and weather. Laboratory filtration or drying can provide a reference for suspended solids, while visual inspection can identify whether the optical signal is driven by mineral particles, algae, organic debris or bubbles. These records are particularly useful when an apparent winter turbidity event has no matching change in inflow or water level.

The best validation is repeated comparison. Deploy the same instrument at the same depth, use consistent hole geometry and maintain a record of sensor orientation. A result from a sheltered bay should not be compared casually with a deep-basin deployment, even if both are described as “under-ice” measurements. Consistency turns a difficult field exercise into a defensible time series.

Planning retrieval and field support

Retrieval should be planned from the first drilling operation. Mark the hole location with a visible, durable sign and record coordinates, bearing to shore and photographs of the frame. If snow may cover the area, use a tall marker and a backup reference from fixed shoreline features. Before returning, check weather forecasts, road access, reservoir operating notices and local restrictions. Conditions around Snowy Mountains roads can change quickly, and a track that was passable for a ute may become unsuitable after snow or thaw.

Carry tools for frozen fasteners, damaged cable jackets, connector cleaning and minor frame repairs. Spare gloves, dry clothing, a head torch and insulated containers are inexpensive compared with losing a sensor under the ice. A field team should establish who manages the lowering line, who handles data and who watches the ice and surroundings. Clear roles reduce rushed movements when the weather turns or visibility drops.

At retrieval, remove snow and loose ice before loading the frame. Lift steadily, keeping the sensor vertical until it clears the water, then place it in a padded container. Rinse off sediment with clean water, cap connectors as specified and download data as soon as practical. Preserve the original file and create a working copy for quality control, with notes linking anomalies to maintenance, weather and site observations.

Technical support and product-management information for legacy D & A Instruments equipment is available through Campbell Scientific support, which can be useful when confirming sensor specifications, compatible logging arrangements or the status of an older turbidity monitor. A disciplined record of hole dimensions, ice thickness, sensor depth, optical orientation and field checks gives that support team—and future analysts—the context needed to interpret the deployment properly.