As Distributed Acoustic Sensing (DAS) gains traction in the rail industry, it is often surrounded by as much mystery as excitement. While the idea of "listening" to tens of kilometers of track from a single location sounds like science fiction, the reality is grounded in sophisticated physics and practical engineering. However, as DAS technology adoption increases, several misconceptions seem to have taken root. To help you better navigate this landscape, we tackle five common myths about DAS.
1. The Eavesdropping Myth:
Can DAS Hear Your Private Conversations?
As with many myths there has to be a grain of truth to make the myth credible. In optimum conditions, it is possible to interrogate a fiber to detect voice vibrations. However those optimum conditions are something you can demonstrate in a lab, but bear little relation to what is achievable in a commercial railway installation.
In commercial DAS systems, particularly railway ones, fiber coverage is much, much longer and the fibers are often buried or below ground level. The frequencies the systems are tuned to listen to are also significantly different with most useful DAS data coming from subsonic frequencies, i.e. those below the range of human hearing.
So can you listen to conversations – in a lab yes. With a commercial system operating at range – no.
2. Specialist Fibers:
Do You Really Need New "Golden" Infrastructure?
DAS is based on measuring reflections from light pulses, and those reflections come from microscopic scattering sites within the optical fiber. If you engineer a fiber to have an increased number of scatter sites (a specialist fiber), you can increase its sensitivity. However, unsurprisingly, if more signal gets scattered the distance the fiber can monitor is vastly reduced, often by orders of magnitude.
For rail applications we typically wish to monitor long lengths of infrastructure (tens of kilometres) and so we are usually looking to maximise the range monitored, rather than minimise it. For this reason, Sensonic DAS systems are designed to use standard single mode telecoms grade fibers, e.g., G652-D.
You don’t have to, and indeed in most cases should not use specialist fibers for Sensonic DAS monitoring applications. A single dark fiber in a telecoms bundle is often all you need to monitor your track to mitigate multiple risks.
3. The Price of Precision:
Is DAS Actually Expensive?
Cost is always a tricky subject. DAS systems aren’t cheap, or at least the ones that are any good aren’t. However, we must also as far as possible make a fair comparison, i.e. compare apples with apples.
A remote tilt monitor (IoT) device might cost a few hundred dollars/pounds/euros and seem like good value. But what does it monitor? Perhaps 2m of a slope meaning you might need 1000 sensors to monitor a 1km rail cutting. Suddenly that few hundred suddenly spirals into 6 figures. Oh, and you’ll need to power them all or replace their batteries every year or so as well. In contrast a single Sensonic DAS unit might cost 5 figures but monitors 80km of rail route for decades requiring no trackside maintainance.
Suddenly the bargain IoT sensor looks more like a long-term liability. Now I’m not knocking IoT sensors, but if you have long lengths of track to monitor then they aren’t usually the most cost-effective answer. The flip-side is that (outside some niche applications), DAS isn’t likely to be the most cost-effective solution for monitoring single short distances, e.g., 50m of unstable slope.
You can plug in some numbers on this simple landslide calculator to compare
4. Comparing Apples to Apples:
Why All DAS Systems Are Not Equal
Not all DAS suppliers and their equipment are the same. We’ve tried and tested various equipment from many sources over our 14 years, and of course we are going to say we are the best – that goes without saying! However, we’ll go further by explaining what to look for in a DAS system, why it is (or isn’t) important for your project to allow you to make fair comparisons between suppliers.
There are different application requirements too, which often affect the choice of equipment; Monitoring 80km of ballasted railway for security has different requirements from monitoring a 100m long bridge for impact damage for an example. We’ve got a guide to what you should be looking for in a DAS supplier and why – contact us with your project details to find out more.
5. The Cry of "Wolf":
Mastering the Balance Between Sensitivity & False Alarms
Strictly in purely technical terms, DAS doesn’t generate any false alarms. It doesn’t hallucinate a vibration signal that doesn’t exist. However, what can happen is to incorrectly classify a signal (A false positive, or a false negative), and that is what most end users will regard as a false alarm. I explain more about balancing sensitivity vs false alarms here.
First generation DAS devices often worked by setting alarm thresholds, where excess vibration levels then triggered an alarm, indeed some suppliers still sell systems that work this way. This approach is simple and works reasonably well for very quiet environments like security fences on remote facilities, secure pipelines and some parts of isolated international borders. However, the railway environment is rarely accused of being quiet. This led to some suppliers applying quiet logic to noisy railways with perhaps the predictable result of excess false alarms leaving dissatisfied customers.
Sensonic has grown from the rail environment (Our original parent company has over 35 years of dedicated rail experience), so we built our systems and algorithms with petabytes of real rail data and inbuilt knowledge of typical track conditions and some of the challenges they cause. We won’t often claim zero false alarms, but we can advise what is likely to be possible for your application, so contact our team to find out more.
Five Fiber Fallacies Found Out
We hope this short article has helped dispell a few of the myths of fiber sensing. Ultimately, Distributed Acoustic Sensing is a powerful, scalable solution that transforms existing "dark fibers" into a high-value assets for railway safety, operations and maintenance. While it isn’t a magic cure-all for all rail woes, its ability to provide long-range, 24/7, insights makes it valuable tool for modern rail operators.
By moving past the myths - from eavesdropping concerns to the perceived need for specialist fibers - the real value lies in intelligent signal classification to provide rail-specific insights. If you're ready to cut through the noise and see how Sensonic can protect your network, get in touch with our team today.
