Why are scientists honing in on telecommunications cables at the bottom of the sea?

They are tele­com­mu­nic­a­tions cables used by the sci­entif­ic com­munity. Loc­ated on the ocean floor (par­tic­u­larly in the Pacific and North Atlantic) and along the coast, they carry glob­al tele­com­mu­nic­a­tions. They are widely, but unevenly, dis­trib­uted across the globe. Each sub­mar­ine cable is made up of around fif­teen glass optic­al fibres. We are divert­ing them from their tele­com­mu­nic­a­tions use to retrieve a wide range of sci­entif­ic data. It is also pos­sible to take advant­age of under­ground ter­restri­al cables, and we have a pro­ject along these lines with the Nice met­ro­pol­it­an area.

We meas­ure the deform­a­tion along the cable every metre. This allows us to detect seismo-acous­tic waves, pre­cisely those that propag­ate dur­ing an earth­quake. In prac­tic­al terms, thanks to this tech­no­logy, with a single cable we have the equi­val­ent of hun­dreds of seis­mo­met­ers deployed on the ocean floor. We have also recently demon­strated that it is pos­sible to meas­ure tem­per­at­ure, to a sens­it­iv­ity of the order of 0.001°C¹. This cru­cial data was pre­vi­ously unavail­able at this level of detail for the seabed. It allows us to bet­ter char­ac­ter­ise ocean­ic pro­cesses such as intern­al waves and upwelling phenomena.

The poten­tial of this tech­nique is enorm­ous. It is revolu­tion­ising our vis­ion of the envir­on­ment, and its applic­a­tions are extremely wide-ran­ging. This new tool offers, for example, the pos­sib­il­ity of ima­gin­ing real-time mon­it­or­ing sys­tems, or even warn­ing sys­tems. If we look at the his­tory of sci­ence, we see that major advances are often linked to advances in obser­va­tion. With optic cables, we are tak­ing a new step for­ward, which sug­gests that we will be able to unblock many sci­entif­ic questions.

The oceans cov­er two-thirds of our plan­et. But we have very few sensors on the ocean floor: instru­ments must be deployed off­shore, then returned months later to retrieve them. This meth­od provides one-off meas­ure­ments and requires a lot of logist­ics and fin­an­cial resources. Tele­com­mu­nic­a­tions cables provide an unpre­ced­en­ted oppor­tun­ity to have many ‘sensors’ on the seabed! With a meas­ure­ment every few metres along each cable, the dens­ity of sensors is phe­nom­en­al and unpre­ced­en­ted. In con­trast, long tele­com cables – over 300 km – are equipped with repeat­ers every 70 km or so. At the moment, it is not pos­sible to exceed these repeat­ers, so we are record­ing meas­ure­ments up to 70 kilo­metres from the coast. The poten­tial is already colossal, since the eco­nom­ic stakes are con­cen­trated in this zone. In the future, I’m sure it will be pos­sible to over­come this constraint.

These cables offer a host of advant­ages. As they are already installed, there is no need to dis­turb the seabed any fur­ther. They are reli­able, avail­able con­tinu­ously and in real time. As a bonus, the sys­tem is very inex­pens­ive: it relies on the install­a­tion of an instru­ment cost­ing a few hun­dred thou­sand euros. As it is equi­val­ent to thou­sands of sensors, this works out at less than €10 per sensor.  Finally, the sens­it­iv­ity of the meas­ure­ments is com­par­able to that of tra­di­tion­al sensors such as seismometers.

It’s very easy to set up: all you must do is con­nect a box to the end of the earthed cable. The sys­tem con­sists of a laser that emits light into the cable. As the light propag­ates inside the optic­al fibre, it encoun­ters the small nano­met­ric-scale defects that are inev­it­ably con­tained in the glass of the optic­al fibre. These defects reflect the light. The box records this echo and meas­ures the rel­at­ive dis­place­ment of the defects along the fibre. This type of meas­ure­ment is called DAS, for Dis­trib­uted Acous­tic Sens­ing. Sev­er­al man­u­fac­tur­ers offer these sys­tems for sale. There are oth­er tech­nic­al solu­tions for using optic­al fibres as sensors, but DAS tech­no­logy is by far the most widespread.

In the 2010s, the first to imple­ment the DAS sys­tem were oil com­pan­ies: these cables are very use­ful for equip­ping bore­holes, because they are thin and strong. But at that stage, the cable was spe­cific­ally deployed for meas­ure­ment. The first to come up with the idea of test­ing DAS on exist­ing tele­com­mu­nic­a­tions cables were an Amer­ic­an team from the Uni­ver­sity of Cali­for­nia. In 2017, they pub­lished a paper² that revolu­tion­ised our approach: they showed for the first time, using the tele­com fibre on the Stan­ford cam­pus, that it was pos­sible to use exist­ing tele­com cables to track earth­quakes. With­in our team, we then rap­idly launched the first tri­als on a tele­com cable off Toulon: we con­firmed the rel­ev­ance of these meas­ure­ments for meas­ur­ing region­al seis­mi­city and wave dynam­ics³.

They are reli­able, avail­able con­tinu­ously and in real-time.

For the moment, most aca­dem­ic users work in the field of seis­mo­logy, prob­ably because seis­mo­lo­gists are very close to the pet­ro­leum geo­phys­ics com­munity. But oth­er dis­cip­lines are begin­ning to take up the tech­no­logy, and the num­ber of sci­entif­ic pub­lic­a­tions men­tion­ing DAS tech­no­logy is explod­ing: from less than 20 in 2016 to more than 150 in 2022.

We’re still in an explor­at­ory phase, so we can­’t say that any major sci­entif­ic advances have been made thanks to DAS (yet!) How­ever, we’re very quickly demon­strat­ing the bene­fits of DAS for decrypt­ing sig­nals. We have proved the rel­ev­ance of DAS for record­ing earth­quakes and we are now build­ing new cata­logues of pre­vi­ously undetec­ted earth­quakes. For example, we have a pro­ject in south-east France and Chile to equip tele­com cables and bet­ter char­ac­ter­ise the seis­mic risk in the region. Thanks to these meas­ure­ments, it will be pos­sible to improve our under­stand­ing of earth­quakes that take place at sea – which can be very destruct­ive – and even to detect them in real time.

We now know that the sys­tem is also very use­ful for study­ing ocean and storm dynam­ics⁴. Sur­face waves, for example, gen­er­ate vibra­tions that we can detect on the ocean floor, and we can also record deep ocean cur­rents. These meas­ure­ments can be sup­ple­men­ted by DAS tem­per­at­ure meas­ure­ments. Finally, a Nor­we­gi­an team has just demon­strated the value of DAS in bioacous­tics⁵.They are record­ing whale songs and estim­at­ing the 3D pos­i­tion of the anim­als. There are enorm­ous oppor­tun­it­ies for gain­ing a bet­ter under­stand­ing of the inter­ac­tions between ceta­ceans and their envir­on­ment: how they are affected by anthro­po­gen­ic noise, the move­ment of bod­ies of water, etc. Since DAS can also be used to detect boats, it is entirely pos­sible to ima­gine set­ting up anti-col­li­sion systems.

Anaïs Maréchal