A geophone is a very sensitive contact microphone, with an amplifier and filters, that rests on the ground or on the installation itself to listen to the noise water makes as it escapes from a pressurised pipe. The technician walks the route of the pipe listening at several points and compares the intensity of the sound: the point where the noise is strongest and clearest is the one closest to the leak. It does not “see” the pipe or the leak; it interprets a sound. That is why it works very well in some installations and poorly or not at all in others, and it is worth knowing which are which before expecting miracles from it.
What noise it picks up exactly
When water escapes under pressure through a crack or a pore it produces three types of noise:
- The jet as it leaves the opening and strikes the ground or the material around the pipe. It is the most characteristic, a continuous hiss.
- The vibration of the pipe itself, which travels along it in both directions. In metal pipes it carries for tens of metres; in plastic it dampens much sooner.
- Water flowing through the ground or through voids, sometimes sounding like bubbling or dripping.
That noise reaches the surface through soil, concrete, flooring or wall, and that is what the sensor picks up. The professional geophone lets you filter by frequency to separate the sound of the leak from traffic, a pump, the fridge or the wind.
The two ways of listening
Direct listening at contact points
Before sweeping the floor, listening is done by placing a rod or contact sensor on the accessible points of the installation: stopcocks, meters, taps, hydrants, visible bends. The sound of the leak travels along the pipe to those points, which makes it possible to know which section to search before taking a single step over the floor. This is the pre-location phase.
Surface listening, along the route
The ground microphone is then placed on the paving following the route of the pipe, listening every few steps. Instead of looking for a “yes/no”, the aim is to find the maximum: where the noise is most intense. That point is marked and, if necessary, confirmed with another technique before opening.
What it needs to work
The geophone is only as good as the noise the leak produces and the path that noise has to the sensor. These are the factors that determine it:
| Factor | Helps the geophone | Hinders the geophone |
|---|---|---|
| Pressure in the pipe | Pressurised installations (cold or hot water mains, heating, service connections). More pressure, more noise. | Unpressurised pipes: drains, downpipes, pool circuits with the pump off. Without pressure there is no jet and nothing to hear. |
| Pipe material | Metal (iron, steel, copper): carries sound far and at frequencies the ear distinguishes well. | Plastic (PVC, polyethylene, multilayer): dampens vibration. The leak sounds deeper and fainter, and you have to listen closer and at more points. |
| Paving and ground | Concrete, asphalt, tiles: hard, continuous surfaces that transmit sound. | Loose soil, sand, gravel, lawn: absorb sound. Deep pipes are also attenuated. |
| Ambient noise | Quiet settings, working in low-traffic hours or at night. | Busy streets, running pumps, building works, strong wind outdoors. |
| Size of the leak | Small losses at high pressure: a pore with water jetting out under pressure whistles clearly. | Paradoxically, a large break with the pipe half empty or at low pressure can sound quieter than a small leak. |
There are two practical consequences: the geophone is normally used with the installation charged and under pressure, with consumption closed so that the only water noise is the leak's; and on plastic pipes it is often combined with another technique.
Geophone in pools: yes, but only on the pipes
This is the most frequent question that reaches this article. Pools have two very different parts for a geophone:
- The basin (lining, joints, light fittings, skimmer mouths and jets). Here the water is not under pressure inside a pipe: it seeps through a crack into the ground. A surface geophone is no use. It is inspected underwater, with coloured tracers and a camera, by a diver when the job requires it, with the pool full.
- The circuit pipes (skimmers, delivery, sweep, drain). With the pump off they have no pressure, so pressure has to be created: each circuit is isolated, plugged and pressurised with water or air to check its tightness. The circuit that does not hold pressure is the one losing, and once pressurised it can be walked with the geophone to narrow down the point in the buried pipe.
That is why a pool leak search starts with pressure tests per circuit and not with the geophone. If before calling you have done the bucket test with the pump on and off, you will already have a clue as to whether the loss is in the circuit or the basin.
When it is combined with other techniques
- Tracer gas: when the leak does not make a sound (plastic, low pressure, great depth, soft ground), the pipe is emptied and filled with a tracer gas; the gas escapes through the break, rises to the surface and a detector locates it. It is the natural complement to the geophone for silent leaks. For example: at a house in the province of Alicante, the geophone told us on the first sweep that the leak was outside, in the garden, but did not give a clear point. We emptied the installation, filled it with tracer gas and the detector marked the exit beside one side of the house. After the repair, the meter stayed still.
- Pressure test and sectorisation: before listening, it confirms that there is a leak and in which section or circuit, so that the whole installation does not have to be swept.
- Thermography: on hot water or underfloor heating circuits it helps follow the route of the pipe and see areas with an abnormal temperature, which guides where to listen. It does not replace the geophone: it shows temperatures, not leaks.
- Acoustic correlator (sector technique for long networks; not part of our usual procedure): on long network runs (service connections, distribution in communities or housing estates) two sensors at the ends measure the delay with which the noise reaches each one and calculate how far the leak is.
What precision can be expected
There is no universal figure. Precision depends on the installation (material and depth of the pipe), the pressure, the paving or surface above it and the ambient noise. In good conditions (metal pipe under pressure, hard paving, quiet) the geophone narrows the point enough to open a single opening at the right spot. In poor conditions it narrows down a section, and a confirmation with tracer gas or a second listening at another time is needed. Promising exact precision down to the millimetre is not serious in any case, which is why the point is narrowed down with different equipment depending on the type of pipe and ground.
What a geophone does not do
- It does not detect leaks in unpressurised pipes (drains, downpipes, pool circuits with the pump off) without pressurising them first.
- It cannot on its own tell a water leak from another similar noise: judgement is needed and, often, the listening has to be repeated with consumption closed.
- It does not cut through background noise well: listening in the middle of a street at noon may find nothing, while the same listening at night does.
- It does not confirm the pipe's material or depth; that information is obtained with a pipe locator or from the plans.
If you suspect a leak in a pressurised installation and want us to listen to it, see geophone leak detection to see how we work. To understand when a geophone is chosen and when another technique is, see our comparison of techniques for detection without works.
