ADGEOAdvances in GeosciencesADGEOAdv. Geosci.1680-7359Copernicus PublicationsGöttingen, Germany10.5194/adgeo-43-39-2017AlpArray-Italy: Site description and noise characterizationGovoniAladinoaladino.govoni@ingv.ithttps://orcid.org/0000-0002-0017-5260BonattoLucianaCapelloMarcoCavaliereAdrianoChiarabbaClaudioD'AlemaEzioDanesiStefaniahttps://orcid.org/0000-0002-7884-8242LovatiSaraMargheritiLuciahttps://orcid.org/0000-0003-3853-254XMassaMarcoMazzaSalvatoreMazzariniFrancescoMonnaStephenhttps://orcid.org/0000-0001-9241-1322MorettiMilenaNardiAnnaPiccininiDavidehttps://orcid.org/0000-0002-1826-646XPiromalloClaudiahttps://orcid.org/0000-0003-3478-5128PondrelliSilviahttps://orcid.org/0000-0002-0851-7405SalimbeniSimonehttps://orcid.org/0000-0002-1077-5833SerpelloniEnricohttps://orcid.org/0000-0003-1822-403XSolarinoStefanohttps://orcid.org/0000-0002-9577-1347VallocchiaMassimilianoSantulinMarcothe AlpArray Working Grouphttps://orcid.org/0000-0002-0017-5260Istituto Nazionale di Geofisica e Vulcanolgia, Via di Vigna Murata, 605, Rome, ItalyIstituto Nazionale di Geofisica e Vulcanologia, Via Bassini 15, 20133, Milano, ItalyIstituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola, 32, 56126, Pisa, ItalyIstituto Nazionale di Geofisica e Vulcanologia, Via Donato Creti, 12, 40128, Bologna, ItalyA full list of authors and their affiliations appears at the end of the paper.Aladino Govoni (aladino.govoni@ingv.it)13March201743395230September201616January20178February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://adgeo.copernicus.org/articles/43/39/2017/adgeo-43-39-2017.htmlThe full text article is available as a PDF file from https://adgeo.copernicus.org/articles/43/39/2017/adgeo-43-39-2017.pdf
Within the framework of the European
collaborative
research initiative AlpArray (http://www.alparray.ethz.ch),
the Istituto Nazionale di Geofisica e Vulcanolgia (INGV) deployed overall 20 broad-band seismic stations in Northern
Italy and on two islands in the Tyrrhenian Sea (Capraia and Montecristo)
during Fall-Winter 2015.
The temporary deployment (16 stations) will run
for two to three years and 4
INGV National Seismic Network accelerometric sites are now equipped with additional permanent broad-band sensors.
The 16 temporary stations are equipped with REF TEK 130 digitizers and
Nanometrics Trillium Compact 120 s sensors, a couple have Nanometrics
Trillium 120P sensors and one a Streckeisen STS2.
For each site we describe the settings and discuss the noise levels, the
site effects and the preliminary sensitivity analysis.
Introduction
We describe here the deployment, managed by Istituto Nazionale di Geofisica e Vulcanolgia
(INGV), of 20
new
real-time broad-band seismic stations in Northern-Central Italy. The deployment contributes to the
AlpArray Seismic Network (AASN)
in the framework of the AlpArray project (www.alparray.ethz.ch),
a large European collaborative research initiative . The aim of AlpArray is carrying
out cutting edge research to advance our understanding of the deep structure, geodynamics,
tectonics and seismic hazard of the greater Alpine area (Alps-Apennines-Carpathians-Dinarides
orogenic system) and integrating seismological data and other geophysical observables with geological studies.
The INGV Z3 network deployment status in Italy (July 2016).
4 of temporary planned sites were close to accelerometric sites and are
now part of the National Seismic Network IV.
Four sub-teams (PI, BO, MI, CNT) are responsible for the different
geographical sectors: Tuscany and islands (PI), Emilia-Romagna and SE
Veneto (BO), Lombardia and neighboring sites (MI),
Veneto and Friuli Venezia–Giulia (CNT).
Last column specifies the soil type: SOFT (a – alluvial,
s – sand); ROCK (s – sedimentary, v – volcanic).
The project relies on a high-end seismological array
complementing the permanent stations coverage by national networks.
The characteristics of the array
were
designed to maximize the resolution of geophysical imaging in order to progress our knowledge on the 3-D structure and physical properties of the lithosphere and of the upper mantle.
The success of such a project, based on the deployment of heterogeneous type of instrumentation, relies on the
collection of high quality data. For this reason, each AASN station has to fulfill quality standards
(specified in the AA technical strategy document,
www.alparray.ethz.ch/organisation/documents)
concerning site selection, installation techniques, data management and noise performances.
More than 50 research institutes and seismological observatories from 18 countries are involved in the
AlpArray scientific project,
while 24 institutions are contributing
to the AASN with
high quality data from
390+ permanent and 260+ temporary
broad-band seismic stations installed in 10 countries in the great Alpine
area, to build up a unique data set of at least two years (officially
starting on 1 January 2016).
To achieve an average spacing of ∼40 km between each broad-band site,
the initial AASN plan has identified, within the Italian border, 32 temporary
sites to complement the permanent stations geometry. Due to the absence of
national calls for this type of fundamental research projects the
AlpArray-INGV deployment has been funded with internal resources up to a
maximum of 20 sites. The INGV mobile seismic stations pool, the “Rete
Sismica Mobile” (ReMo, ) of the Centro Nazionale
Terremoti (CNT), has provided most of the instrumentation. The other 12
temporary sites in Italy have been installed and are managed by
Institute of Geophysics and Swiss Seismological Service, ETH Zürich
.
The AlpArray Seismic Network in Italy:
the red inverted triangles are the temporary broad-band sites installed and
managed by INGV, real-time stations have a yellow dot (all but Z3.A316A)
The blue inverted triangles are the permanent Italian sites
that belong to the AASN (in gray the other non broad-band permanent sites).
The red and the blue squares are respectively the temporary and the permanent sites
of the AASN outside Italy.
The complete plan of the AASN is in the inset (from the project site).
The temporary network layout and requirements
All AASN sites have a naming code Z3.AxxxA where Z3 is the FDSN network code,
the first A is reserved, xxx is a three digit code range assigned to each
country and the last A can be turned to B, C, …, etc. in case of major
(>10 m) site changes.
The complete network plan is shown in detail in Fig. . The numeric
range assigned to Italy is 280–329. The 12 sites managed by ETH have a
numeric code
in the range 280–299
and are located mainly in the NW part of Italy. The 20 sites managed by INGV
have
a numeric code ranging from 300 to 329 and
are located in
the NE
area, the Po plain area and Tuscany (see Table ). All
permanent broad-band stations located in Northern and Central Italy (∼150) are contributing to the
AASN
and are managed by several
italian
Institutions (see Table for a complete list).
Scouting, installation and maintenance of INGV AlpArray stations were performed by four
sub-teams, each of them responsible for one geographical sector
(see Table ).
Eventually, after a revision of the original installation plan
and after a cost-benefit evaluation,
we decided to turn
4 out of the 20 sites selected for temporary deployment into permanent installations of the Italian National Seismic Network.
These sites have been registered with the IV network code.
Permanent broad-band stations in Italy contributing to the AASN: 115 sites.
CodeSitesNameManagerIV48Italian National Seismic NetworkIstituto Nazionale di Geofisica e Vulcanologia (INGV)GU28Regional Seismic Network of North Western Italy (RSNI)University of Genova (UniGE)OX20North-East Italy Seismic Network (NEI)Istituto Nazionale di Oceanografia e di GeofisicaSperimentale (INOGS)MN4MedNet Project (MedNet)INGVSI7Province SüdtirolZentralanstalt für Meteorologie und Geodynamik(ZAMG)ST2Trentino Seismic NetworkGeological Survey-Provincia Autonoma di TrentoSite selection
As a rule permanent broad-band sites are
carefully selected
in extremely low noise
remote
areas and the sensor vault is usually designed and built to minimize all
sources of signal pollution like thermal and atmospheric variations.
Such a careful scouting is not applicable to temporary seismic
projects for which site selection, permissions and installations have to be
accomplished in a short time (typically 2–3 months) and with limited
resources. Moreover the AASN site must respect also a geometrical
constraint: installation must be performed
within
3 km from the
theoretical location
(6 km can be acceptable in very difficult situations).
Examples of installations for four representative soil types (rows).
The station position (red triangles) is indicated on distant (first column)
and closer (second column) views extracted from Google Earth. Pictures from
each installation are shown in the third and fourth columns. The stations are:
(a) Z3.A302A, Resana (Treviso, Veneto); (b) Z3.A303A,
Lido di Venezia (Veneto); (c) Z3.A316A, Montecristo Island
(Livorno, Tuscany); (d) Z3.A318A, Donoratico (Livorno, Tuscany).
In our temporary deployment we looked for
a satisfactory
compromise between
site accessibility, safety, low seismic noise level, power supply
availability and possibly also good
mobile connection.
The AASN requires that the noise levels should be 20 dB lower than the New
High Noise Model (NHNM) on all components in the 1–20 Hz
frequency range, 20 and 10 dB lower than the NHNM respectively for the
vertical components and the horizontal components in the 30–200 s period
range. These constrains apply to rock site and for soft soil are relaxed by
10 dB.
Meeting these requirements has proved to be quite difficult since
most of the sites assigned to INGV are on large alluvial plains affected by a
high cultural noise produced by population and industrial activities (i.e.
). In these areas fulfilling both constraints
(geometrical and low noise) has been extremely hard.
Whenever possible we
preferred
the basement of one storey
buildings due to safety and accessibility
In these cases we usually benefit also of power from grid and good UMTS
coverage for data transmission.
Station design
The INGV mobile pool seismic station is based on the REF TEK 130 digitizer
(either model S or 01) storing data on standard 4 GB/8 GB Compact Flash
disks. The station is supplied in a carrying box together with a standard
42Ah battery, GPS antenna, solar regulator, 80 W solar panel and all needed
cables ready for deployment. Several types of broad-band sensors are
available, mainly Nanometrics Trillium Compact 120 s, Trillium 120P and a
few Streckeisen STS2. All the stations installed have been Huddle tested
with a 48 h long data acquisition against a reference station composed by a
REFTEK 130S and a Streckeisen STS2 sensor, now installed on Z3.A303A (Lido,
see Fig. a). Given a certain heterogeneity of the
instrumentation (see Table ) with two types of
datalogger (REFTEK 130-01 and 130S) and 3 types of seismic sensors (1
Streckeisen STS2, 2 Nanometrics Trillium 120P,
13
Nanometrics Trillium Compact 120 s) the Huddle test can guarantee the
accuracy of the sensor response data and the uniformity of the acquisition
parameters. Moreover we have been able to exclude from the experiment some of
the sensors in the pool that
were
showing deviations from the theoretical response.
The key factor in reducing the sensor noise at long periods consist in
minimizing the thermal variations around the sensor
A careful thermal insulation of the sensor is usually enough to meet the
noise specifications (see installation photos in Fig. a and
b). Wherever possible we
used
a standard double wall heavy plastic box reinforced with a supplementary
40 mm thick layer of polystyrene.
In the case of the Nanometrics Trillium Compact this is not always enough to
ensure a low long-period noise level. The sensor is small and lightweight
and these features might contribute
to signal pollution due to the strain of the cable and to the high thermal
capacity of the aluminium case. To minimize these effects the sensor is
wrapped using a standard packaging film so that the sensor cable is kept
firmly to the body of the sensor and reaches the ground with a glazing angle.
This minimizes eventual stresses on the sensor from the cable but also the
thermal contact of the aluminum case with the environment. A custom
polystyrene box has been designed to insulate the sensor leaving the minimum
air around the case. Using standard commercially available 40 mm polystyrene
slabs, a number of 300×300 mm tiles are easily cut. A 128 mm hole
is drilled in the center of each tile that is put on top of the others
aligned along the hole. The two uppermost tiles are not drilled at all. They
just cover the assembly and guarantee that the thinnest part of the shield is
at least 80 mm tick. The initial assembly of this box is visible in
Fig. a. This work is basically easy enough to be performed
also on site with the standard installation equipment.
To test the effectiveness of this solution we have acquired the data from
both an insulated Trillium Compact and the reference STS2. Both sensors were
placed nearby on the plinth in the laboratory and acquired with two different
REFTEK 130S units configured for the final field deployment. Without the
thermal insulation the Trillium Compact appear to have a higher noise level
respect to the bare STS2. With the insulation box the Trillium Compact has a
noise comparable to the STS2 except for the vertical component that shows a
bimodal behavior. PPSD plots computed using 48 h of continuous recording at
100 sps with the ObsPy library () are shown in
Fig. b.
A comparative plot of the mean value of the PPSD is shown in
Fig. c for each component with the New Low/High Noise model,
the yellow band is the -20 dB from NHNM zone required as maximum noise
level for AASN stations on stiff soil. Both the Trillium horizontal
components appear now to have lower noise than the STS2 above 20 s (indeed
the Trillium EAST component does appear almost a shifted copy of the STS2,
this may be due to the asymmetry of the plinth that is rectangular). The
Trillium vertical component is bimodal and has an average noise level much
higher the STS2 one for periods
longer
than 10 s. Only a fraction of PPSD mimic the behaviour of the STS2 vertical
component. This is likely due to the very low weight of the sensor and can be
minimized burying the sensor with fine sand. This very low cost and easy to
manufacture thermal shield has been used in several sites with satisfactory
results. An example is shown in Fig. c.
A custom made thermal insulation box for the Nanometrics Trillium Compact.
The sensor is small and lightweight but these features are prone to signal
pollution due to the strain of the cable and the high thermal capacity of
the aluminium case.
(a) the initial assembly of the 300×300 mm polystyrene
tiles with a central 128 mm hole. The sensor is wrapped using a packaging
film so that the sensor cable is kept firmly to the body of the sensor
and reaches the ground with a glazing angle.
(b) PPSD plots computed on 48 h of continuous recording at 100 sps
with the ObsPy library. On the left the Trillium 120c and on the right
the Streckeisen STS2 (the station codes used are fictitious).
(c) The means of the PPSD shown in (b) are compared for
each component with the New Low/High Noise model, the yellow band is the -20 dB
from NHNM zone. With the insulation box the Trillium Compact has a noise
comparable to the STS2 except for the vertical component that shows a bimodal behavior.
Where possible the sensor has been buried at least 60 cm below the surface
on a 10–15 cm thick concrete pier. The sensor has been also shielded from the ground using buckets or
tubes filled with fine river sand. An example of such installations is shown
in Fig. d.
Another source of thermal instability when installing
inside buildings
is the
floor
itself that is in contact with the air of the room and with sun exposed
walls. To minimize this a granite insulation
base plate
similar to that used in
the Mediterranean Very Broadband Seismographic Network (MedNet, 1988)
stations has been designed (one has been used with the STS2 – see
Fig. b). Unfortunately due to new bureaucratic procedures the
purchase of this base plates has been severely delayed and they were not yet
available at the time of the installation. A few of this
base plates
will be installed at the next maintenance in the stations that appear to
suffer most from
floor
instabilities.
To increase reliability and minimize on-site technical intervention, all the
stations connected to the mains have been equipped with an industrial power
supply and backup batteries (at least 80 Ah) accounting for the power
consumption of the UMTS data transmission system. The two solar powered
stations have been equipped with two independent solar power systems: one for
the seismic station and one for the UMTS router to minimize the probability
of loosing seismic data. Moreover all stations have at least 8 GB of disk to
store data with an autonomy of at least 8 months.
Real-time data transmission and the experiment data management
The benefits of portable real-time seismic networks are several and well
known. During the management of a temporary experiment from the real-time
data flow is possible to detect and quickly fix problems related to power
supply, time synchronization, disk failures and, most important, seismic
signal quality degradation due to unexpected noise sources or sensor
alignment/tampering. This usually minimizes the field maintenance trips and
maximizes both the quantity and the quality of the acquired data.
We routinely use a real-time UMTS based data acquisition system suitable for
temporary experiments. The system developed at INGV is described in detail in
. It consists of a VPN system that allows to connect all the
stations to a data acquisition server using UMTS and/or any other internet
connection.
Unfortunately at the beginning of the experiment only four UMTS devices were
available and the purchase of new devices and SIM cards was severely
delayed. Most of the stations have been connected only during spring 2016.
Figure shows the data availability of the real-time
system. Despite the several small gaps typical of UMTS connections, the
overall completeness of the real-time data is around 97 %,
useful to monitor the data quality and to schedule the station maintenance.
At least 3–5 stations need to be optimized either installing a higher gain
antenna or changing the service provider. This work will be done during the
next scheduled maintenance.
The stations
Z3.A312A and Z3.A313A, together with the 4 sites turned permanent, are also
connected real-time with a similar UMTS based system
used to acquire data of the
regional accelerometric network .
The data management of the experiment has been designed with the goal of
having all data available on the INGV EIDA node as soon as possible.
Figure shows the block diagram of the data flow at the INGV
data center.
All temporary stations store continuous data in the native REF TEK binary
format on Compact Flash support with at least ∼8 months of autonomy.
Every six months the stations are routinely serviced and the data is
collected and processed to obtain a quality checked mini SEED archive that is
exported to the INGV EIDA node. All but Z3.A316A (Montecristo) are equipped
with UMTS routers and the data is also collected in real-time and made
available to the INGV National Seismic Network for monitoring purposes and
finally archived on the EIDA node. The State of Health (SOH) real-time data
is used to diagnose technical problems to the station power supply, GPS
timing system and seismic sensor.
The first path is formed by the real-time data flow. Data is collected via
UMTS and made available to the National Seismic Server on an internal
SeedLink server. From this point on the path is the same as for all the data
collected by the National Network.
Data is fed to the monitoring system and used for the seismic monitoring and
the bulletin. We do not automatically include the AASN data in the routine
surveillance system due to the large number of triggers related to high
anthropic noise at most of the stations. However, Z3 station data can be
manually loaded during location revision if needed. We do use AASN data in
the revised locations of the Bollettino Sismico Italiano
(http://cnt.rm.ingv.it/bsi) published every four months. The real-time
data is directly archived to the INGV EIDA node and made available to
authorized users.
Roughly every six months data is gathered from station disks during the
periodic station maintenance, converted to mini SEED and exported to the EIDA
node filling eventual transmission gaps.
The State of Health (SOH) data is routinely analyzed from the real-time data
flow to spot technical problems.
Final stations configuration and site descriptions
The scouting work began in August 2015 and by the end of the year all
stations but Z3.A304A were installed, 4 of which real-time since the
beginning. The Z3.A304A installation was delayed by bureaucratic problems
with the permits, it was finally installed at the beginning of February,
2016. See Table for station installation dates, sensor
type, housing and equipment. The sensor housing type, as defined by SED
and used for the AlpArray project, is for most stations (17) building, but
there are also 2 free-field installations and 1 urban free-field site.
Moreover, 2 sensors are deployed in buried hand-dug vaults. Only 2 stations
are powered by solar panels, all the rest have power grid connection. In the
following we give a short description for each of the temporary sites and
their preliminary noise characterization.
Z3.A300A has been installed in the basement of a two storey modern
building in the NE part of the Muzzana del Turgnano village (Udine, Friuli).
The installation area is just a few tens of kilometers North of the Laguna di
Marano and is composed of soft alluvial soil. The building, occasionally used
by local Civil Protection, guarantees fair thermal insulation, power grid
connection and the maximum level of security. The sensor has been thermally
shielded with a 10 cm thick polystyrene box. However, the site is not very
far from a state road (SS14), a warehouse and a water tower. Both high
frequency and long period noise are quite close to the NHNM.
Z3.A301A is buried in the garden of a one-storey building in the
Northern suburbs of the Isiata village (Venezia). The sensor is placed on a
10 cm thick concrete pier, about 60 cm below the surface, insulated with a
bucket of sand. A plastic 60×60 cm box protects the sensor from the
soft ground around. The building, which ordinary hosts the packaging
activities of an ONG, guarantees power grid connection and a high level of
security. The site is less than 50 m away from minor roads. The whole area
was reclaimed at the beginning of the last century and is composed of very
soft lacustrine soil. The water level at the time of the installation was at
a depth of only 50 cm, while the average altitude of the whole area is
around -8 m a.s.l. This is a typical case in which long period noise
requirements are substantially met with a -23 dB for vertical and -7 dB
for horizontal components (relative to NHNM), but the high frequency noise is
substantially same order or slightly larger than the NHNM.
Z3.A302A is installed in a relatively recent one-storey industrial
building hosting several rarely used municipality offices and warehouses, a
few kilometers East of the Resana village (Treviso, Veneto). The site is
connected to the power grid and offers a high level of security. The floor is
made of a large thick concrete layer. Eventually the only office with enough
space available for the installation was on the South side, therefore more
prone to thermal variations of the walls and of the concrete floor
(Fig. a). The sensor has been shielded with a custom dual wall
plastic box with an additional 4 cm thick layer of polystyrene. While the
PPSD levels are acceptable (-13.6 dB for vertical and -5 dB for
horizontal components at long periods) the PPSD plot shows a highly bimodal
behavior at both ends of the frequency spectrum, confirming daily thermal and
cultural noise variations. The thermal insulation is going be optimized using
a marble
base plate
like those
used in
the MedNet stations.
Z3.A303A is installed inside a Middle Age fortress located at Lido
di Venezia (one of the islands of the coastal strip of the Venetian Lagoon).
The fortress is inside the Golf Club area, which guarantees power grid
connection and a high level of security (Fig. b). The sensor is
placed at the furthest corner from the entrance, on a marble basement (to
optimize thermal insulation) resting on a concrete floor and has been
shielded with a custom dual wall plastic box with an additional 4 cm thick
layer of polystyrene. The soil is mainly composed of consolidated sand. The
noise level is overall satisfactory and meets the specifications at long
periods, while at high frequencies is slightly above the requirements. At
long periods the North component is roughly 5–7 dB above the Est components
because of some site effect that will be investigated.
Z3.A304A is located inside a small bathroom (2×2 m) of the
local cemetery of Bellombra village (Rovigo, Veneto), with power grid
connection and the highest level of security. The site is located in the
deltaic and alluvial deposits of the Po River. The sensor is placed on a tile
floor covering a 50 cm thick screed floor and is shielded by a plastic box
sealed by foam. The site is 100 m from the nearest road, 300 m from a water
well and 1 km from the village. Only the vertical component matches the
noise level requirements at long periods. Horizontal components at both low and high
frequency closely match the high noise model.
Z3.A305A is located in a small building inside the cemetery of Ca'
Zuliani village (Rovigo, Veneto), with power grid connection. The sensor is
in the furthest corner from the door, placed directly on the concrete floor
that is 1 m thick to reach the ground floor. The sensor has been covered
with a plastic box with an additional 4 cm thick layer of polystyrene,
sealed by foam. The site is 500 m far from the river and a secondary road.
The site is located in the deltaic and alluvial deposits of the Po River.
Being an alluvial plain site the noise level is satisfactory for the vertical
component at long periods, while horizontal components approach the high
noise level. Also high frequency noise is close or greater than the high
noise level due to the high population density.
Z3.A306A is located inside a utility cabin, close to residential
buildings in Località San Pietro (Viano, Reggio Emilia). The location
guarantees power grid connection and the maximum level of security. The
sensor is installed directly on the screed floor at the furthest corner from
the main door and has been shielded by a plastic box with an additional 4 cm
thick layer of polystyrene, sealed by foam. The site is on the top of a hill
characterized by a succession of limestones and sandstones alternated with
clay. The station overall matches the noise requirements, only the horizontal
components enter the -10 dB zone at periods higher than 80 s.
Z3.A307A is located in a log cabin inside the protected natural area
of “La Rizza” (Bentivoglio, Bologna). The site offers power grid connection
and the maximum level of security. The sensor is placed on the concrete floor
at ground level on the alluvial deposits of the Po River, and is shielded by
a plastic box with an additional 4 cm thick layer of polystyrene, sealed by
foam. Potential noise sources are the trees around the log cabin, while
inside the cabin the presence of mice induced us to protect further the
instrumentation with a fibreglass box. The station barely matches the noise
requirements with minor peaks at high frequencies.
Z3.A308A is located inside a building in the local cemetery of
Longastrino village (Argenta, Ferrara). The location guarantees power grid
connection and the highest level of security. The sensor is placed above a
tile floor, in a corner of the room, close to the window but far from the
door, and is covered with a plastic box with an additional 4 cm thick layer
of polystyrene, sealed by foam. The cemetery is close to the main road
(50 m) but 1 km far from the village. The site is located on the alluvial
and deltaic deposits of the Po River. The noise requirements are met only on
average. While the vertical components is satisfactory, both horizontal
components suffer large daily variations due to the tile floor that we were
not allowed to alter.
Z3.A309A is installed in an outdoor safe corner of the local
cemetery of Martorano (Cesena), in the rear of the building in front of the
main entrance. The location offers power grid connection and a high level of
security. The sensor is placed on a concrete floor and it was protected from
temperature variation with sand, foam panels and bricks. The site is located
on the fluvio-continental and deltaic deposits of the Po plain and is 300 m
from the main road. The noise requirements for soft soil are met with small
high frequency exceptions.
Z3.A312A is installed in the basement of a municipal building
(Bozzolo town, Mantova) lying on alluvial deposits (gravel and sand) of the
Po plain. The sensor is placed above a tile floor and is shielded by a
plastic box with an additional 4 cm thick layer of polystyrene. The site is
at 30 m a.s.l. bordered on two sides by the yard of the building, by a
street and a car park on the other two sides. The station has power grid
connection and a buffer battery to compensate for a short time period the
temporary power outage. The noise requirements for soft soil are met with
small high frequency exceptions.
Z3.A313A is installed in a corner of the cemetery of Volta Mantovana
(Mantova), lying on alluvial deposits (sand and clay) of the Po plain. The
site is at 64 m a.s.l., limited by the cemetery wall on one side, by a
street next to a canal on another side and by open fields on two sides. The
sensor is placed on a concrete layer and is covered by a plastic box. The
station has power grid connection and a buffer battery to compensate for a
short time period the temporary power outage. The noise are met by the
vertical components but both horizontal are already order of the high noise
level for periods longer than 30 s.
Z3.A316A is located inside the cellar of the Villa Reale building
house in the Cala Maestra cove at the Monte Cristo Island
(Fig. c). The geologic substratum consists of a monzogranite
with large feldspars phenocrysts. The sensor is surrounded by thick (80 cm)
walls and lies on a concrete base placed on the floor of the cellar, built
directly on the granite. Only few persons live on the island. The station is
powered by electric power line. The noise level is very satisfactory and
suffers mainly of the sea waves influence.
Z3.A317A is located inside the cellar of the main building of the
municipal hall at the Capraia Island. The geologic substratum consists of
lava flows and pyroclastic deposits of an old volcano. Thick walls surround
the sensor, which lies on a concrete base placed on the floor of the cellar,
built directly on the lava flows. The station is powered by electric power
line. The noise level is satisfactory with moderate high frequencies levels.
Z3.A318A is located between vineyards in the Argentiera estate
(Donoratico, Livorno). The geologic substratum consists of sedimentary rocks
(clay, marls and limestone). The sensor is completely buried and secured by
fence and info labels (Fig. d). In the installation site the
rock (marls and clay) crops out just one meter below the surface. In the
estate few seasonal farm activities occur. The station is powered by solar
panels. The noise levels are satisfactory.
Z3.A319A is located in a farm on the hills just north east of the
small town of Santa Luce (Pisa). The geologic substratum consists of
sedimentary rocks (clay, sand and limestone). The sensor is installed
outdoor, locked to a substratum of concrete in order to maximize the coupling
between soil and sensor. A double shelter has been used to maximize the
thermal insulation of the sensor. Few seasonal farm activities occur in the
area. The site is 2 km far from a wind power plant. The station is powered
by solar panels. The noise levels are satisfactory but a bimodal behavior is
evident.
IV.OPPE is installed in the back side of the cemetery of Oppeano
(Verona) lying on alluvial deposits of the Po plain. The sensor is placed on
a concrete plinth and is shielded by a 4 cm thick polystyrene inner casing,
covered by a wooden outside box. The site is at 20 m a.s.l. limited by the
wall of the cemetery on one side and by open fields on the three other sides.
The station has power grid connection and a buffer battery to compensate for
a short time period the temporary power outage. The noise level is
satisfactory for all components at low frequencies, considered that it is an
alluvial plain site. At high frequencies the noise is larger than the high
noise level due to the high population density.
IV.ORZI is installed in the back side of the cemetery of Orzinuovi
(Brescia) lying on alluvial deposits of the Po plain. The sensor lies on a
concrete plinth and is covered by a 4 cm thick polystyrene box. The site is
at 83 m a.s.l. bordered by the wall of the cemetery on one side and for the
three other sides by open fields. The station has power grid connection and a
buffer battery to compensate for a short time period the temporary power
outage. The noise level is satisfactory for all components at low
frequencies, considered that it is an alluvial plain site. At high
frequencies the noise is larger than the high noise level due to the high
population density.
IV.SARZ is installed in the basement of the school of Sarezzano
(Alessandria) lying on sandstones and marls rocks, at the NW limit of the
Northern Apennines. The sensor is placed directly on the ground and is
shielded by a 4 cm thick polystyrene box. The site is at 266 m a.s.l.
limited on two sides by the yard of the school and on the other two sides by
a small square. The station has power grid connection and a buffer battery to
compensate for a short time period the temporary power outage. Considered
that this is a rocky site, the station performance is not fully satisfactory
even if the noise level on average is below the high noise level.
IV.ZONE is installed in the basement of the school of Zone (Brescia)
lying on lacustrine deposits of a valley whose slopes are characterized
mainly by Val Sabbia sandstone and Esino limestone. The station is located at
the Southern limit of the Central Alps, at 691 m a.s.l. surrounded on three
sides by the school yard and bordered by a narrow street on the other side.
The station has power grid connection and a buffer battery to compensate for
a short time period the temporary power outage. The sensor is placed above a
tile floor and is covered by a 4 cm thick polystyrene box. While the
vertical component is satisfactory in the whole frequency range, horizontal
components suffer from site effects at frequencies above 1 Hz and they are
already order of the high noise level for periods longer than 30 s.
Seismic noise level and site effects
All data acquired in the first 40–60 days from the station installation have
been analyzed to asses the noise levels.
Figure shows the mean of all stations PPSD grouped per type of
site soil. We see immediately that all the rock sites meet the noise
requirements with a minor exception in the high frequency band. The stations
on soft soil (Po Plain and Adriatic coast) are instead very close to the
maximum noise allowed for long periods (mainly order of 10 dB lower than the
NHNM), but the high frequency noise is mainly order or higher than the NHNM,
due to the high level of cultural noise in those areas.
Comparison of the mean vertical PPSD of all the AASN stations managed by INGV as function
of the soil type using the data of the first 40–60 days after installation.
Gray lines represent the Peterson NHNM and NLNM, in gray the -20 dB
area below the NHNM.
All rock sites are below or approach the NHNM -20 dB level and satisfy reasonably well
the experiment specification both at longer periods and at higher frequencies.
Most soft soil site (mainly Po plain and Adriatic coast) are below the NHNM -10 dB
for long period but fail to meet the NHNM model for higher frequencies mostly
because of cultural noise typical of those areas.
The worst soft soil sites are only a few dB below the NHNM model at long periods.
Basically the same behavior is observed for the horizontal components that at
longer periods are usually at least 15 dB above the vertical component.
Noise levels measured on horizontal components are on the average 15–20 dB
higher than the vertical component in the long period range.
A more complete analysis will soon be possible using
the first year of
data to asses seasonal variations and eventually fix unsatisfactory stations
during the
next
maintenance. A more detailed analysis of the noise performance of these
stations will be the subject of a future study and is beyond the scope of
this work.
The PPSD plots of all temporary stations can be accessed on
http://coremo.rm.ingv.it/Projects/AlpArray/PPSD. This site has been developed specifically for hosting the AlpArray
Italy project data and will be maintained and improved during the course of
the project.
Nevertheless a first sensitivity analysis shows satisfactory results. The
sensitivity is overall pretty good on all sites even if some site effect is
evident on a few sites. Examples of local and telesismic event records are
shown in Figs. and . A more quantitative analysis
of the sensitivity and of the detection threshold of each station will
soon be performed using the first
year
of data.
Example of waveforms recorded by the INGV AlpArray temporary stations
for the M3.0 local earthquake occurred close to Poggio Rusco (Mantova, Italy)
on 25 February 2016, 20:55:16 (UTC), 10 km focal depth. Waveforms are corrected for instrument response.
Example of waveforms recorded by the INGV AlpArray temporary stations
for the M7.8 teleseismic earthquake occurred in Ecuador on 16 April 2016, 23:58:37 (UTC),
focal depth 19.2 km. Waveforms are corrected for instrument response.
The black trace is from Z3.A303A (Lido station) on shore in the coastal
strip of venetian lagoon and the drift is very likely related to the water tides.
Conclusions
We deployed a total of 20 new broad-band stations in fall 2015 to complete
the AASN in Italy in time for the official AlpArray project start of
operations: 1 January 2016. Only one station (Z3.A304A) was delayed by
municipality permits up to the beginning of February 2016.
While rock sites meet and sometimes exceed the project noise specifications,
the sites on soft or alluvial soil are more challenging. Soft soil areas are
also densely inhabited and present usually a difficult trade-off between
quality of signal and security of housing. Never the less we were able to
meet the project specifications in most sites for the long period
(30–200 s) part. Not so good is the situation with high frequency noise
(1–20 Hz) that is usually order of the NHNM in most soft soil sites. These
most challenging
sites
are located in the Po plain and in the NorthEast Adriatic
coast and very little can be done to improve the signal quality satisfying
the geometric constrains.
The real-time archive so far. Only 4 stations (Z3.A300A–Z3.A303A) were installed already equipped with UMTS routers.
The equipment needed for real-time data transmission has been available for
all stations only in spring 2016 and with all SIM cards from the same
telephone company.
At least 3–5 stations need to be optimized either installing a higher gain antenna or
changing the service provider. This work will be done at the next scheduled maintenance
around fall 2016. The overall completeness of the data archive is around 97 %, very useful for
monitoring the data quality and scheduling the station maintenance.
The first analysis of teleseismic and local events shows anyway that detection
thresholds and sensitivity is acceptable even for very soft soil sites.
All stations but Z3.A316A (Montecristo) have been turned real-time using UMTS
and are currently contributing to the Seismic National Network monitoring
system and to the bulletin . The real-time
acquisition rates are order of 99 % in best sites and a few sites
experiencing transmission problems due to insufficient UMTS signal or
service quality will be optimized soon after the summer.
The AlpArray seismic stations are available on EIDA
(http://www.orfeus-eu.org/eida/). The Z3 network is currently not
publicly accessible by decision of the AlpArray Working Group. All Z3
waveform data and other restricted stations are exclusively available to
registered members of the Core Group of the AlpArray Seismic Network. Visit
http://www.alparray.ethz.ch/seismic_network/backbone/data-access/ for
further information on data access and
http://www.alparray.ethz.ch/seismic_network/backbone/data-policy-and-citation/
for information of data policy. Information on the AlpArray project are
available at www.alparray.ethz.ch. In the AASN, the
permanent stations are contributed via the following networks codes:
BW,
CH , CR,
CZ ,
FR ,
G ,
GE ,
GR,
GU ,
HS,
HU ,
IU ,
IV ,
MN ,
NI ,
OE,
OX ,
PL,
RD,
RF ,
SI,
SK,
SL,
ST ,
SX,
TH.
The complete member list of the AlpArray Working Group can be found
at http://www.alparray.ethz.ch.
A. Govoni and C. Piromallo prepared the manuscript with contributions from
all co-authors. A. Govoni coordinated the station deployment together with C.
Piromallo, S. Mazza and L. Margheriti. L. Bonatto developed procedures for
data–set qualification and analysis. M. Moretti and M. Vallocchia managed
the INGV mobile pool and the supplies. A. Govoni performed the instrument
setup, the Huddle test, setup the real-time UMTS data acquisition system and
developed most of the data management procedures. A. Govoni, L. Margheriti
and A. Nardi participated to scouting, installation and maintenance of
Z3.A300A–Z3.A303A stations. F. Mazzarini, D. Piccinini and M. Capello
participated to scouting, installation and maintenance of Z3.A316A–Z3.A319
stations. S. Salimbeni, A. Cavaliere, S. Pondrelli and S. Danesi participated
to scouting, installation and maintenance of Z3.A304A–Z3.A309 stations. Ezio
D'Alema, S. Lovati, M. Santulin and M. Massa participated to scouting,
installation and maintenance of Z3.A311A-Z3.A312A and IV.SARZ,IV.ORZI,
IV.OPPE, IV.ZONE stations. The AlpArray Working Group worked out the network
layout, the quality guidelines and the standards for the seismic data
exchange.
The authors declare that they have no conflict of
interest.
Acknowledgements
We do thank all people, municipality staff and the State Forestry Corps
(Corpo Forestale dello Stato) for hosting the seismic stations and for their
help during the site scouting, installation and maintenance.
We do acknowledge the incredible organization and planning work done
by the AlpArray project coordinators
Edi Kissling, György Hetenyi, Irene Molinari and John Clinton
at ETH, Zurich.
We also wish to thank Paolo Casale and Augusto Bucci, MEDNET staff, for
valuable suggestions and discussions on very broad band site selection and
installation, and Alfonso Mandiello for his help in data management.
We are grateful to the INGV Earthquake Division for the main financial
support allocated to the project (INGV internal funds). We acknowledge as
well Daniela Pantosti, Alberto Michelini, Paolo Augliera, Gilberto Saccorotti
and Francesca Quareni (Departments Directors) for encouragement.
We thank the INGV National Seismic Network staff for help and support for the
data management procedures.
We are grateful to Florian Fuchs and Irene Molinari for the useful suggestions that greatly improved the paper.
All plots and maps were prepared using the GMT and gnuplot
packages. Most data processing (notably PPSD and waveform
deconvolution) was done using the ObsPy
package. Edited by: D. Pesaresi
Reviewed by: F. Fuchs and one anonymous referee
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