Underwater Multi-influence Measurements as a Mean to Characterize the Overall Vessel Signature and Protect the Marine Environment

The overall signature of a vessel comprises acoustic, magnetic, electric-field, pressure, and seismic radiations. Over the past years, the international community’s efforts have mainly centered on reducing the acoustic influence with the dual aim of decreasing the vessel’s detectability and reducing the levels of acoustic pollution generated in the marine environment. Nowadays, the need to act not only on the acoustic radiation but against the overall set of the vessel’s radiations is becoming increasingly clear, both in the military and the civilian fields, based on aspects like vessel stealthiness, security of harbor and critical infrastructures, and environmental protection. As a key element to achieve this goal, it is greatly important to have at our disposal highly modular and adaptable measurement systems covering the overall set of the vessel’s radiations, with a high capacity of data transmission to base centers to have the capacity to make measurements in all kinds of marine environments.

All vessels, independent of their shape and size, emit onto the sea a set of radiations that make up the socalled vessel signature.This signature characterizes and identifies univocally the vessel the same way the fingerprint identifies human beings.The importance of this signature is well-known since early in the past century, mainly in the defense field and specially centered on the so-called acoustic and magnetic signatures.For example, detection of vessels based on their acoustic signatures was quite important in the naval field during World War II.
In parallel with technological improvements during the 20th century, and especially in the defense field, specific techniques have been developed to reduce the level of radiations emitted onto the sea by vessels.At the beginning, reduction techniques were limited to the acoustic radiation.Next, magnetic radiation was also taken into consideration and, more recently, the electric-field, pressure, and seismic radiations have also been considered of interest.The group of five radiations referred to above make up the so-called multiinfluence signature of a vessel.An example of the simulated electric signature radiated by a sweep gear is shown in Fig. 1.
Monitoring the multi-influence signature is greatly important in the defense field and in the case of vessels like submarines it becomes a matter of survival.Also, it is becoming increasingly important in the civilian field related to the marine environment preservation, especially in the case of the marine fauna living in this environment, due to the influence of these radiations on their behavior.Finally, it is worth noting that detection of this signature permits us to determine the presence of threats in harbors/ports, critical infrastructures or cultural assets located on the sea floor, making it possible to implement specific actions to neutralize these threats.
This paper comprises four sections in addition to this introduction section: the first section describes the main characteristics of the radiations that make up the multi-influence signature.In the next two sections, the importance of this signature in the defense and civilian fields is analyzed.In the following section, a system especially adapted to measure multi-influence signatures is described.Finally, the paper is completed with the conclusions of the study.

Acoustic radiation
Th e sound generated by a vibrating source is propagated as a wave in an elastic medium such as the sea, originating pressure changes that are susceptible of being measured.
Th e underwater sound propagation, characterized by its high performance, being the way radiation currently best propagates through this medium [1], reaching long distances in the case of low frequencies.
Vessels emit two types of generic signals: broadband and narrowband.Th e former is characterized by covering a wide spectrum of frequencies, meanwhile the latter is limited to a narrow spectrum.
Th ere are diff erent sound sources in vessels.Th e three main are: machinery noise, propeller noise, and hydrodynamic noise.A scheme of underwater sound propagation obtained from an underwater acoustic propagation model is shown in Fig. 2.

Magnetic radiation
Th e Ship's magnetic infl uence comprises two components: the static component (SM) and the alternating one (AM).Th e static component is generated by permanent and induced magnetic fi elds.
Th e ship's permanent magnetic fi eld is due to the magnetization of its construction magnetic materials by the Earth's magnetic fi eld.Besides, the Earth's magnetic fi eld, as an external magnetic fi eld, always contributes to the ship's magnetic signature.Th is component depends on the ship's course and localization of the area.
In addition to permanent and induced magnetic fi elds, the Corrosion Related Magnetic (CRM) fi eld also contributes to the static magnetic component of the signature.Th is fi eld is due tto the existence of corrosion currents through the sea water, which has an associated magnetic fi eld.
Th e alternating component of magnetic signature is generated by: • Currents in the rotating coils of ship turbines.Th ese coils perform as magnetic dipoles, which generate AC magnetic signature.• Sea water Foucault currents induced by the magnetic dipoles.Th ese currents are time varying and are also associated with alternating electric fi elds.• Electric currents fl owing through the ship's 1. Modulation of the corrosion current: galvanic current is modulated due to the spin of blades and propellers.2. Ripple in machinery power supply of the ship.
A frequency tone appears, corresponding to the power supply frequency.3. Ripple in degaussing ICCP systems, corresponding to the modulation suff ered by the ICCP system current due to variations in the resistance between the shaft and hull of the ship.Fig. 4 shows an example of simulation of the electric signature radiated by a vessel.

Pressure radiation
Hydrostatic pressure due to water depth varies slowly with atmospheric pressure changes and tide rising and falling.Besides, it can vary fast with waves and train of waves, or with a ship.Pressure variation due to a ship movement is usually very small.Th is small variation constitutes the ship's pressure signature and it is produced by the Bernoulli Eff ect of the water fl owing from bow to stern.Th is fl ow originates a pressure increase at the ship's bow and stern and a decrease in the central zone (suction), whose peak is directly proportional to ship speed and its underwater shape.Fig. 5, shows this pattern in a simulation of a ship´s pressure signature.Th erefore, induced pressure fl uctuations by the ship are superimposed with the nominal static pressure from the bottom and natural disturbances caused by tide, waves and swells.

Seismic radiation
Seismic infl uence is generated by the same sources as for the acoustic infl uence.When an acoustic wave reaches a surface, the majority of the energy is refl ected but a percentage is absorbed by the new medium.Th us, very low frequency (below 10Hz) acoustic signals propagate up to the sea fl oor and transmit through it as a seismic perturbance.Th is type of perturbance travels much faster through the sea fl oor than through sea water.
From this seismic point of view, it is obvious that the acoustic energy penetrating into the sea fl oor may sometimes contribute considerably to medium-range and long-range acoustic trans mission.One clear example that shows seismic infl uences behavior is the existence of a critical frequency.For all frequencies below this limit, absorption phenomena in incident waves appears, and this phenomena depends on the characteristics of the materials and layers of the sea fl oor.signifi cantly increasing their security.Th is fact is based on early and more accurate detection of threats coming from the marine environment, such as: divers moving autonomously, manned and unmanned underwater vehicles (SDV, ROV, UUV), mini-submarines, etc.
During recent years, growing awareness has emerged worldwide on the need to protect the marine environment, especially from human activities like: fi shing, sailing, harbor works, or seismic oil and gas explorations that convey signifi cant increase in the level of a range of pollutants such as acoustic noise and other sources of energy, including electric and magnetic sources.Th is growing awareness has entailed the development of a range of national and international regulations focused on achieving eff ective preservation of the marine environment.Among these regulations is the Marine Strategy Framework Directive, promulgated by the European Union in 2008, which introduced a set of qualitative descriptors to determine the good environmental status.One of these descriptors states that: "the introduction of energy, including underwater noise, is at levels that do not adversely aff ect the marine environment".Th e eff ective application of this Directive, and of other regulations related with the marine environment protection, implies both measurement of the level of energy radiations emitted to the sea [2] and detection of marine fauna presence in specifi c areas on which high energy levels are detected with the aim of protecting this fauna from the potential harmful eff ects of the radiated energy.
Th is detection is mainly based on acoustic sensors, although other alternative detection means are currently being analyzed, such as detection of marine fauna based on the alteration of the underwater electric or magnetic fi elds originated by their presence.In Fig. 8, two kinds of cetacean species: bottlenose dolphin and sperm whale are shown; both are endangered species.
Intelligence databases have become elements of great importance for the navies in diff erent countries.Th ese databases contain, as distinctive data, the signatures of the vessels.At the beginning, they contained acoustic data and sometimes magnetic data.Currently, they also seek to incorporate data corresponding to the rest of infl uences.Th ese databases permit discriminating not only the type and class of vessel, but also the specifi c unit of the class, providing a considerable tactical advantage.
Currently, the trend in recent decades relative to the reduction of the vessel's signature is being accentuated.Th is trend is particularly intense in the case of submarines.Th ey seek to increase their level of protection by increasingly becoming more stealth vessels as a means to counter-act the development of increasingly intelligent weapons like last-generation torpedoes.Fig. 7 shows an image of U-206 type submarines.
In parallel with that stated above, more sophisticated systems incorporating a wide range of sensors are now being developed.Th e use of these multi-infl uence systems permits vessels to signifi cantly increase their detection capacities from the combination of the data provided by their suite of sensors, enabling a considerable reduction in the number of false alarms.Also, in the defense facilities protection fi eld, the use of these multi-infl uence sensors has permitted The multi-infl uence signature in the defense fi eld The multi-infl uence signature in the civilian fi eld The multi-infl uence signature measurements Just like in the defense field, use of multiinfluence sensor systems provides an effective protection of harbors and critical infrastructures like oil refineries and thermal power stations against hostile intruders.This protection can also be extended to other fields of remarkable interests as is the case of marine reserves, ship wrecks, or underwater archaeological remains.
With regards to multi-infl uence measurements, the trend in the civilian fi eld is to advance towards a standardization process in the procedures and parameters of the measurements.Th ese standards have long existed in the defense fi eld.Th e precursor of this process is, again, the acoustic infl uence for which a standard has recently (2009) been developed by the Acoustical Society or America (ASA): "Quantities and Procedures for Description and Measurement of Underwater Sound from Ships".Additionally, within the European Union scope specifi c programs exist focused on the defi nition of a European standard.Measurements are usually normalized to common references in order to compare these coming from diff erent systems, as is the case of the 1-meter from the source reference for acoustic measurements or a common reference point for all the vessels in the case of measurements of electric and magnetic fi elds.
The fact that multi-inf luence sensors provide different detection ranges permits establishing different detection layers as a function of the sensor range.Detection ranges depend on both the specific characteristics of the marine environment and those of the vessel being tested and the sensors used.In a first approach, it can be stated that both acoustic and seismic sensors provide detection distances in the range of kilometers, being the seismic sensor especially dependent on the characteristics previously referred to, electric-field and magnetic sensors in the range of hundreds of meters and pressure sensors in the range of tens of meters.
Th e wide range of operational environments in which multi-infl uence measurements are susceptible to be taken makes it highly advisable to have at our disposal modular and portable systems with small dimensions and weights.Th ese kinds of systems permit their deployment and recovery in diff erent marine areas within a reduced time interval and without requiring complex means.Other aspects to be stressed are: the capacity of data transmission to base centers (located on shore or onboard vessels) with an adequate bandwidth to cover the characteristics of the signatures tested and the capacity of storing and processing the measured infl uences, focused on providing accurate and useful information to the system operator.An example of a system that complies with the characteristics previously described is the multiinfl uence measuring system (MIRS) developed by SAES (see underwater units deployed on the sea bottom in Fig. 9).All vessels, when sailing through the sea, radiate a set of infl uences (acoustic, magnetic, electric, pressure, and seismic) that make up their multi-infl uence signature.Some of these infl uences (acoustic and magnetic) have been measured since decades ago to characterize vessels in the defense fi eld and to monitor the acoustic pollution in the civilian fi eld.Recently, interest in the international community on having at its disposal the overall signature of the vessels has emerged, seeking to globally evaluate its impact on the marine environment.
In the defense fi eld, from the fl eet's point of view to have at our disposal multi-infl uence data from vessels permits performing specifi c tasks and studies focused on reducing its own signature to decrease the probability of being detected.On the other hand, from the point of view of threats, this data permits characterizing the vessel's signature with the aim of increasing our capacity to detect them.
In the civilian field, interest is centered on the marine environment preservation, especially of its marine fauna.In the dual defense-civilian field, protection systems based on multiinfluence sensors constitute a highly efficient means to detect hostile intruders.
Due to the variety of operational areas in the marine environment, it is highly advisable to have at our disposal modular systems with contrasted capacities of data transmission, recording the measurements and processing focused on providing relevant information to the system operator.The MIRS developed by SAES configures as a verified and in-service system than complies with the requirements established for the multi-influence measurement of all kinds of platforms or naval devices in the whole spectrum of operational environments.

Fig. 1 .
Fig. 1.Simulation of the electric field radiated by a sweep gear

Fig. 2 .
Fig. 2. Underwater sound propagation paths obtained from an acoustic propagation model

Fig. 3 .
Fig. 3. Simulation of magnetic fi eld generated by a vessel