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