Seafloor clearance methods based on acoustic,
direct-inspection, and single-sensor magnetic approaches suffer from limitations in controlling the target-sensor distance, and may prove ineffective when the small size or the dangerous nature of targets requires high accuracy in localization.
Moreover, random magnetic variations over time bring about spatial decorrelation phenomena, and hinder the application of
double-sensor methods in noisy harbour environments.
The new High Definition (HD) magnetic survey protocol tackles the measurement-distance problem in two ways: first, by varying the sensor depth dynamically, and secondly by backprojecting the measured field according to seafloor data and vertical incremental factors associated with the bandwidth
characteristics of targets. The method to make up for timeinduced loss in spatial localization ability exploits the local
behaviour of a coherence function, which correlates local observations to a set of spatially-stabilized reference stations. The
consequent normalization of measured magnetic signals allows one to assign the monitored areas with a specific level of
confidence in the detection results, ranging from 100% (certainty) to 0% (random events).
The principles of HD detection have been fully applied in the seafloor clearance of the firing test site located south of Cape
Teulada (Sardinia, Italy), where very weak signal sources such as cartridge cases, mines, and small objects down to 1 Kg mass
values (lobster pots) have been successfully localized, even when covered by extensive colonies of Posidonia

Di Gennaro, E.

Faggioni, O.

Gastaldo, P.

Lamberti, L. O.

Leoncini, D.

Maggiani, P. V.

Soldani, M.

Zunino, R.

Earth-prints Repository

  
Abstract — Seafloor clearance methods based on acoustic, 
direct-inspection, and single-sensor magnetic approaches suffer 
from limitations in controlling the target-sensor distance, and 
may prove ineffective when the small size or the dangerous 
nature of targets requires high accuracy in localization. 
Moreover, random magnetic variations over time bring about 
spatial decorrelation phenomena, and hinder the application of 
double-sensor methods in noisy harbour environments. 
The new High Definition (HD) magnetic survey protocol 
tackles the measurement-distance problem in two ways: first, by 
varying the sensor depth dynamically, and secondly by back-
projecting the measured field according to seafloor data and 
vertical incremental factors associated with the bandwidth 
characteristics of targets. The method to make up for time-
induced loss in spatial localization ability exploits the local 
behaviour of a coherence function, which correlates local 
observations to a set of spatially-stabilized reference stations. The 
consequent normalization of measured magnetic signals allows 
one to assign the monitored areas with a specific level of 
confidence in the detection results, ranging from 100% 
(certainty) to 0% (random events).  
The principles of HD detection have been fully applied in the 
seafloor clearance of the firing test site located south of Cape 
Teulada (Sardinia, Italy), where very weak signal sources such as 
cartridge cases, mines, and small objects down to 1 Kg mass 
values (lobster pots) have been successfully localized, even when 
covered by extensive colonies of Posidonia. 
 
Index Terms— Seafloor clearance, magnetic survey, magnetic 
detection, proton Overhauser magnetometer. 
 
 
1 Istituto Nazionale di Geofisica e Vulcanologia, Via Pezzino Basso 
2, 19025 Fezzano (SP), Italy 
{osvaldo.faggioni, maurizio.soldani}@ingv.it 
2 Defence Geophysics Group, University of Genoa, DIBE, SEA 
Lab, Via All’Opera Pia 11a, 16145 Genova, Italy 
3 University of Genoa, DIBE, Via All’Opera Pia 11a, 16145 
Genova, Italy 
{davide.leoncini, rodolfo.zunino, paolo.gastaldo}@unige.it 
4 Italian Navy, Naval Logistic Inspectorate, Piazza della Marina 4, 
00196 Roma, Italy 
edoardo.digennaro@marina.difesa.it 
5 Italian Navy, Istituto Idrografico della Marina, Passo 
dell’Osservatorio 4, 16134 Genova, Italy 
lambertoo.lamberti@marina.difesa.it 
6 Italian Navy, COMFORDRAG, Viale Giovanni Amendola 1, 
19122 La Spezia, Italy 
paolov.maggiani@marina.difesa.it 
 
I. INTRODUCTION 
In May 2007, following a request by Italian Ministry of 
Defence and Autonomous Region of Sardinia, Italian Navy 
(ITN) – prime contractor – and NURC agreed to provide the 
marine survey part of the Capo Teulada NATO firing range. 
In this job the research team used acoustic and optic standard 
methods and a new magnetic survey methodology developed 
by ITN COMFORDRAG and INGV: the “HD mag survey”. A 
proton Overhauser sensor was towed from CRV Leonardo and 
a magnetometer reference station was set ashore in site of time 
variations space coherency, to permit the use of observatory 
magnetograms [F(t)] as filter of magnetic survey [F(t,s)]. The 
contribute of HD mag was relevant in the detecting in case of 
Posidonia areas, in targets sand-covered and in hard condition 
of sea bottom topography. The operative area is shown in Fig. 
1. 
 
 
The operation was planned to be conducted within 15 days 
and more than 200 contacts were classified. The performance 
of HD mag in the Teulada operation has convinced ITN to ask 
NURC for a common development programme with the aim 
to validate at sea a fully integrated multi-sensor approach 
Harbour Sea-floor Clearance: “HD” High 
Definition Magnetic Survey Performance 
O. Faggioni(1,2), M. Soldani(1,2), D. Leoncini(2,3), R. Zunino(2,3), P. Gastaldo(2,3), E. Di Gennaro(4), 
L.O. Lamberti(5), P.V. Maggiani(6) 
 
Fig. 1 Geographic scenario of high definition magnetic survey. 
 2 
 
(sonar, magnetic and optical) in MCM short term ops. using 
USV/AUV platforms. 
 
 
II. THE HD MAGNETIC METHOD 
Teulada area is characterized by the presence of Posidonia 
in proximity to the coast, and by sandy sea bottom in off-shore 
area. These environmental conditions make very high the 
probability of presence of hidden targets. These targets are 
very small (also quasi dot-like objects) and have a high 
magnetic mark. These environmental conditions qualify the 
magnetic exploration method as the best option to integrate 
optical and acoustical exploration techniques. The 
effectiveness of our magnetic survey to detect micro-sources 
is related to three metrological conditions: high density in the 
magnetic field measurements, sensor nearness to the sea 
bottom, and high precision in the time reduction of the survey 
data. On the other hand, we project a typical HD (high 
definition) geomagnetic survey to environmental purpose. As 
well known, the gradiometric system effectiveness in micro-
sources targets in a little area is very low, because weighting 
devices (e.g. gradiometric apparatus) have not nautical 
characteristics to produce the necessary precision in their 
underwater navigation. We use a single device and the “base 
station” time reduction procedure [1]. Of course, to have the 
HD time reduction, it is strictly necessary having a good 
verify of the space coherency of observatory time variations 
over the whole survey area. 
 
 
 
 
 
III. THE SURVEY 
The survey area is included in four corners (UTM 
coordinates): A 4305849 477214.9 (SE), B B4305411 
474097.6 (SW), C 4308291 474107.1 (NW), D 4308289 
474848.2 (NE) (Fig 2). The area was covered by a proton 
magnetometer measurements over 75 standard tracks and 19 
special tracks performed to have more definition over specific 
targets. The standard tracks survey has a sampling rate of 1 
[s]; the transversal distance between tracks is related to the 
local density of targets: its range is 10 [m] - 70 [m]. The use of 
a single magnetometer allows a very low survey navigation 
speed (standard 2-3 knots) and a good control of distance 
between the magnetometer and sea bottom, controlled by a 
length variation of the magnetometer traction cable. In the 
standard condition this distance “h” is 1 [m] < h < 2 [m]; over 
the Posidonia area, the survey height h becomes more or less 3 
[m] from the sea bottom to prevent biological damages. The 
spatial position of the magnetometer is obtained by the HI-PaP 
system, that is based on a transponder towed near the 
magnetometer by the magnetometric system traction cable and 
on a master station locked to the GPS of the ship: the 
transponder measurements Δx, Δy, Δz are corrected by the 
satellite position and the bathymetry (by the ship echo-
sounder system) to have the absolute position of the 
magnetometer. 
About the measurements sensibility, we consider that a 
device sensibility of 1 [nT] is sufficient for our purpose, 
because we decide a magnetic signal with amplitude lower 
than 5 [nT] has not informative capability about our targets 
(much more if it has not a well defined dipolar form). In fact, 
the signals with amplitude lower than 3-5 [nT] can be 
produced by a lot of kind of sources, as little device shaken, 
magnetic induced signals (not ferromagnetic), etc... 
 
 
IV. THE TTS REDUCTION 
 
The reference geomagnetic observatory is located in Isola 
Rossa, near to the northern border of the surveyed area. To 
control its spatial coherence, we use the Timer Tracks (TT) 
techniques, based on the time correction of the geomagnetic 
profiles and on the correlation between the corrected profiles. 
The spatial coherence defines the spatial stability of the 
geomagnetic time variations measured in an observatory [2,3]. 
If the same profiles are performed in two different times, the 
two geomagnetic surveys have not the same results, because 
they have the same geomagnetic spatial contribution but also 
have different geomagnetic time contributions. If the profiles 
have the same values after the time correction, and then the 
time variations measured in the observatory are stable over all 
distance surveyed, then the observatory geomagnetic is named 
“in spatial coherency”. 
 
 
Fig. 2 Geomagnetic survey of Capo Teulada (South Sardinia - Italy) -
standard tracks. 
 3 
 
 
 
 
 
 
 
 
 
To test the Isola Rossa observatory spatial coherency, we 
use a profile sampled in two different times (go and back), 
then we verify the sampling phase and reverse the second 
survey (back run), and lastly we subtract the second survey to 
the first one (Fig. 3) [4]. The level of Isola Rossa observatory 
spatial coherence is defined by the difference between the 
corrected TT1 track (black marked, from SW to NE) and the 
corrected and reversed TT2 Track (red marked, before 
elaboration oriented from NE to SW) (Fig 4); this difference 
will be nearly zero. In the present experiment, the effective 
analysis of coherence is executed in the sector of tracks 
superimposition. Figure 4 shows the coherence survey 
(without the ship evolution runs) and the sectors of tracks 
superimposition. The TTs analysis shows that the 70 % of the 
samples have, after the correction, a geomagnetic amplitude 
residual of  0 < ΔF <= 2 [nT], the 25 % 2 [nT] < ΔF <= 3 [nT] 
and lastly 5% 3 [nT] < ΔF <= 5 [nT]. We consider that the 
geomagnetic observatory of Isola Rossa is in spatial coherence 
with the area of Capo Teulada survey. 
The length of coherence computation sectors is more or less 
1500 [m] and its circular surface centred in the Isola Rossa 
geomagnetic observatory covers more or less the 80% of 
survey surface; furthermore, in the external area (corner of 
SE) there are not sources of local noise able to change the 
high frequency band of the geomagnetic field. This class of 
noise sources are typical of the coast and urbanized area. 
V. THE RESULTS 
A. Figures and Tables 
The HD geomagnetic survey of Capo Teulada and the 
cleaning of its data set permits to detect and localize 258 
magnetic target signals; 134 impulses positive (52%), 93 
dipoles (37%), 28 impulses negative (11%). The maximum of 
amplitude detected is 574 [nT] and the minimum is 6 [nT]; the 
statistic of amplitude distribution is 127 signals with 5 [nT] < 
A <= 20 [nT] (49%), 21[nT] <= A < 100 [nT], 101 <= A < 
574 [nT]. In the survey of very high definition, where the 
targets are very little, the 2D and 3D maps have not very high 
effectiveness because it is not possible to have topographic 
scale fit to localize the targets with sufficient precision: a lot 
of dipoles or impulses have wavelength of few meters. It is 
standard procedure defining the targets position by the 1D 
products (profile) corresponding to the survey tracks. In the 
present case we show two maps (2D and 3D) to indicate an 
artificial anomaly crossing the entire survey area (Figure 5A 
and 5B). This anomaly corresponding to the magnetic field 
associated to two cables. The cables are a section of a control 
system of the sea area of Capo Teulada; the system is 
dedicated to traffic ships control in the Teulada area sea firing 
ground. This apparatus looks like an inductive type where the 
East cable (A external) is the active component (inductor) and 
the West cable (B internal) is the passive component (reader). 
 
 
 
 
 
 
 
A
 
 
Fig. 4 Timer Tracks for the geomagnetic time variations spatial coherence
computation (Isola Rossa geomagnetic reference station). 
 
 
TT1(s,t)
TT2(s,t) NO
YES
Gps Clk
TTcorr
F(t)
TT1(s)-iTT2(s)
Φ ctrl TT1(s,t) –iTT2(s,t) ΔTT(1-2)(s)
 
 
Fig. 3 Procedure of the reference geomagnetic time variations space 
stability. 
 4 
 
  
In figure 6 we show examples of targets. T.C. 24/01-03 is a 
sequence of dipolar anomalies due to a series of three fish pot 
built in metallic net with a iron wire skeleton (more or less 2 
kilos); T.C. 27/05: half iron barrel (more or less 15 Kilos); 
T.C. 29/06-07: the sea firing ground security system cables: 1 
inductor, 2 reader; T.C. 60/01: mine MARK3 (dummy) lose in 
a Posidonia area during a past exercitation; T.C. 75/02-03: a 
sequence of two iron shell case signals (more or less 4 kilos). 
 
Fig. 7 shows, as an example, the MK3 training version 
mine (Fig.7 A) retrieved inside a Posidonia field. A photo of 
the mine has been taken by a ROV after its finding onto the 
seafloor (Fig. 7 B). The retrieving operation has been 
performed using the HD mag technique. The magnetic 
anomaly characteristic of the MK3 is represented in Fig. 7 C. 
 
 
 
 
 
In the following some signals, grabbed during the research 
operation, have been reported for instance. The data have been 
acquired with a flying quota between sensor and sea bottom of 
about 15 [m]: in Fig. 8 have been represented the signals 
produced by three metallic cables (run 24) and by a cartridge 
case of a naval gun caliber 105 (run 27). 
B
 
Fig. 5 
A.: Total intensity field of “Capo Teulada” investigated area 2D. 
B.: 3D Representation of the total intensity field. 
 
 
A
B
 
Magnetic Anomaly
MK 3
Iron 200kg     
Ø = 50 cm      
L = 200 cm
ΔF = 365 nT
25 samples      
L = 37.5m  
Placement 
Error
C
Fig. 7 MK3 training version mine retrieved inside a posidonia field in the 
near by of Capo Teulada coast (magnetic anomaly graph by Marine 
Magnetics software). 
 
 
 
 
Fig. 6 Examples of detection of magnetic signals after processing. 
 5 
 
 
The comprehensive result of HD mag detections verified is 
reported in Fig. 9, in which white lines are the naval runs and 
red points are the points of interest (targets). 
 
 
 
VI. MAPS OF EFFECTIVENESS 
A basic element to qualify the effectiveness of HD 
magnetic survey is represented by the effectiveness maps. 
These documents show the ratio between the survey covered 
area (naval courses) and the effective covered area: greater is 
the density of the naval courses and smallest is the distance 
between the sensor and the target, greater is the target 
magnetic signature and the covered area is more similar to the 
effective covered area. 
In Fig. 10 are shown the explored areas (EA, pink) 
compared with the surveyed areas (SA, gray), in a square of 
about 10m, using a flying quota between sensor and sea floor 
of about 1.5m for A class sources type, equal to 10-20 [nT] at 
this quota, for B class sources type, equal to 20-100 [nT] and 
for C class sources type, equal to 200-250 [nT]. 
 
Applying this principle to the real survey, the explored 
areas map of Cape Teulada operation can be create (Fig. 11). 
These effectiveness maps represent the fundamental element 
to the evaluation of the HD technique in magnetic maritime 
survey for mine clearance because they give a quantitative 
statistical measurement of the ratio between detected object 
and missed target 
.
 
In case of necessity of a g ratio EA/SA, the maps show not 
only the areas to be most densely surveyed (yellow and red) 
but also the number of necessary naval courses. This features 
guarantee to the HD magnetic survey a greater reliability in 
operative applications. 
 
 
VII. CONCLUSION 
The magnetic survey in HD of the NATO firing range in 
Cape Teulada has pointed out the ability of this method to 
detect several targets that using the classical approaches based 
on acoustic and optical signal can not be detected. 
This feature has been decisive especially in areas with 
presence of Posidonia, in which also high dimensional target 
resulted to be hidden (MK3 mine), and in sandy sea bottom, in 
which targets are often buried. 
The proposed method has moreover defined with clarity the 
presence of two metallic cables, crossing all the survey area, 
which were not present in the official maritime map. 
This technique proves to be a great auxiliary method to the 
standard approaches using in sea floor survey and detection 
(acoustic, optic, etc…). Analyzing the results obtained in 
Capo Teulada ITN has decided to carry on the research on 
MCM developing a multi sensorial system. 
 A targets B targets NOT  
EXPL. 
EA = 80% 
A 
EA = 60% 
A 
EA = 40% 
A 
 
Fig. 11 Effectiveness maps of the survey for A and B classes of sources type
(distance between sensor and sources equal to flying quota of the sensor 1.5
[m] 20%). 
 
A B C
Fig. 10  
A.: EA (gray) about 20% of SA (pink) 
B.: EA (gray) about 50% of SA (pink) 
C.: EA (gray) > 90% of SA (pink) 
Gray lines represent the survey naval courses. 
 
 
Fig. 9 Map of magnetic anomaly detected and verified (red points represent
standard target, yellow points represent target due to metallic cables). 
 
 
Fig. 8 Magnetic micro-target signals (graphics by Marine Magnetics
software). 
 6 
 
ACKNOWLEDGMENT 
Thanks are due to NATO Undersea Research Centre for its 
technical support. 
REFERENCES 
[1] Hill M.N., Mason C.S., 1962, “Diurnal variations of the earth’s magnetic 
field al sea”, Nature, Vol. 195, 365-366. 
[2] Faggioni O., Caratori Tontini F., 2002, “Quantitative evaluation of the 
time-line reduction performance in high definition marine magnetic 
survey”, Mar. Geoph. Res., Vol. 33, pp. 353-365. 
[3] Faggioni O., Beverini N., Carmisciano C., 1997, “Geomagnetic time 
variations and high definition study of space magnetic effects induced 
by artificial submerged sources”, Boll. Geof. Teor. Appl., Vol XXXVIII, 
N 3-4, pp. 211-228. 
[4] E. Di Gennaro, E. Bovio, F. Baralli, O. Faggioni and M. Soldani, 2008, 
“Clearance operation of Teulada Site (Italy): a novel approach for short 
term MCM missions in seafloor hard conditions”, CD-ROM Proceedings 
of UDT Europe 2008 Conference, June 11-13, Glasgow, UK. 


Harbour Sea-floor Clearance: “HD” High Definition Magnetic Survey Performance

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Harbour Sea-floor Clearance: “HD” High Definition Magnetic Survey Performance

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