UR2 – INGV Roma. The UR used two different types of equipment according to the array layout (1D versus 2D geometry). For 1D arrays, the data were collected by three Geometrics-GEODE data loggers connected in a LAN configuration. The GEODE is a 24 channels instrument based on a 24 bits AD converter. The data acquisition is driven by a laptop PC that is also used as data storage unit. Data are stored in SEG2 binary format. The maximum number of available channels is 72 with a maximum geophones spacing of 10 meters. The sensors connected to the GEODE’s line are GEOSPACE vertical geophones with a natural frequency of 4.5 Hz. 1D array is used both for active and passive data acquisition. A minigun source is used for active surveys. For 2D arrays we used 12 or 13 stand alone seismic instruments based on Reftek 130 (24 bits) and Xxxxxxxx Xxxxxxxx (21 bits) data loggers. Each unit, synchronized using GPS receivers, operated in continuous recording mode with a sampling rate of 200-250 samples per second. Data were recorded in a native binary format and converted in SEGY or SEG2 format. The recording units were connected to Xxxxxxxx LE3D-5s seismometers with a natural frequency of 0.2 Hz and high sensitivity (400 V/m/s). This choice allows exploring the low resonance frequencies expected for sites located on deep alluvial planes. To reduce the bias introduced by errors in sensors location, the position of the stations is determined by differential GPS measurements obtained performing a real time kinematic survey with a Leica Systems 1200 GNSS instrument. The differential corrections are obtained through a GPRS connection to a network of reference stations (xxx.xxxxxxx.xx). The positioning system consents to operate in real time providing distance and azimuth between any couple of points in the array. This feature allows to draw the array’s geometry in advance and to easily find the position of the recording stations on the field. With this approach the error in positioning can be lowered to few (5-10) centimetres. The 1D array typical acquisition scheme is based on 72 geophones with a spacing distance chosen according to the desired depth of investigation. For shallow seismic bedrock the spacing was set to 1 meter for a total deployment length of 71 meters. Only in one case (Cassino R.AN. station CSS), due to the lack of space, we deployed 54 sensors for a total length of 53 meters. For deeper sites geophones spacing was set to 2 meters for a deployment length of 142 meters. For the...
UR2 – INGV Roma. As an example of the UR activities, we present the work on the R.A.N. site of Rieti (RTI, lat. 43.831 – lon. 12.829). The site is located in the Rieti alluvial basin, a typical intermountain Apennines’ basin crossed by the Velino River that flows in the South – North direction. The area presents a very flat morphology due to the presence of lacustrine Holocene deposits related to an old lake drained starting from 271 a.C., through the opening of an artificial channel in the NW side of the area in correspondence of the of Marmore falls. The basin is surrounded by mountain ranges consisting of outcropping rocks, mainly of Eocene time. Limestone is also outcropping in small hills aligned in the western sector of the basin (Colle San Balduino, Montisola). In the SouthWestern part of the basin an extended plate of Pleistocene Travertine deposits. This unit can reach a thickness of few tenths of meters and represents the outcropping formation in the SouthEastern part of the basin, including the historical centre of Rieti, the main town in the area. Figure 9 shows a shaded relief map of the area and a picture of the field used for the arrays deployment. a) b) Figure 9 – a) shaded relief map of the Rieti basin (left), the red star shows the position of R.A.N. – RTI strong motion station located in the Chiesa Nuova neighborhood. b) picture of the site selected for MASW measurements. Due to lack of space, the area where the R.A.N. station is installed does not allow to perform any array measurement. Consequently we had to move into an open field close by (150-200 meters) to install both 1D and 2D arrays. The 1D array was composed by 72 sensors with a spacing of 2 meters for a total length of 142 meters. The array was energized by minigun shots with offset of 40, 20 and 0 meters at both sides of the deployment. An extra shot point was chosen at the centre of the array. For every offset two shots were performed. Data sampling rate was set at 0.125 ms, recording window duration was of 2 seconds. Figure 10 - Geometry for 1D (red line) and 2D (red dots) array. Minigun position is marked by blue dots. The green marker indicates the R.A.N. (RTI) station. Using the same array configuration 10 seismic noise windows 30 seconds long were recorded for the passive technique. Some extra window was also recorded while a van was moving at the eastern end of the deployment. In this case data sample was set to 2 ms. The 2D array was based on 12 stand alone stations recording continuo...
