You can measure the depth of water in a borehole primarily using a pressure sensor or a water level meter lowered into the borehole.
Methods for Measuring Water Depth in a Borehole
There are several methods used to determine the water depth, or static water level, within a borehole. The most common and accurate methods involve using specialized instruments:
1. Hydrostatic Depth Method (Pressure Transducer)
This method utilizes a pressure transducer, which is a small diameter sensor, lowered into the borehole. Here's how it works:
- Pressure Sensor: A pressure sensor (transducer) measures the hydrostatic pressure exerted by the column of water above it.
- Depth Calculation: The pressure reading is then converted into a depth measurement using a known relationship between pressure, water density, and gravity (hydrostatic pressure equation: Pressure = density gravity depth).
- Data Logging: Many modern pressure transducers are equipped with data loggers, allowing for continuous monitoring of water levels over time.
2. Water Level Meter (Electric Tape)
A water level meter, also known as an electric tape, is a manual method but still commonly used:
- Tape with Electrodes: The meter consists of a graduated tape measure with an electrode attached at the end.
- Lowering the Electrode: The electrode is lowered into the borehole until it makes contact with the water surface.
- Circuit Completion: When the electrode touches the water, it completes an electrical circuit, signaling with a beep or light on the meter at the surface.
- Depth Reading: The depth to the water level is then read directly from the graduated tape.
3. Acoustic Telemetry
This method is used in boreholes where direct access is limited or when continuous monitoring is required.
- Sound Pulse Emission: An acoustic sensor emits a sound pulse down the borehole.
- Reflection Measurement: The sensor measures the time it takes for the sound pulse to reflect off the water surface.
- Depth Calculation: Knowing the speed of sound in the borehole environment, the distance to the water level can be calculated.
4. Submersible Pressure Transducers
These are deployed to measure depth of water and water level in situations that require long term monitoring.
- Continuous Monitoring: Can be placed in situ to record data at regular intervals.
- Logging Data: Depth is automatically measured at the set intervals and information is logged.
Considerations
- Borehole Diameter: Ensure the sensor or meter is appropriately sized for the borehole diameter.
- Water Quality: In some instances, water quality can affect the accuracy of electrical conductivity-based water level meters.
- Debris/Obstructions: Be aware of any debris or obstructions within the borehole that might hinder the sensor's descent.
- Accuracy Requirements: Choose the method that meets the desired accuracy for the specific application.
