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