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Geophysical investigations for seismic zonation in municipal areas with complex geology: The case study of Celano, Italy
ARTICLE IN PRESS Soil Dynamics and Earthquake Engineering 28 (2008) 950– 963
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Technical Note
Geophysical investigations for seismic zonation in municipal areas with complex geology: The case study of Celano, Italy E. Cardarelli a,, M. Cercato a, R. de Nardis b, G. Di Filippo c, G. Milana d a
` di Roma, D.I.T.S., Via Eudossiana, 18-00184 Rome, Italy ‘‘Sapienza’’ Universita Dipartimento della Protezione Civile, Via Ulpiano, 11-00193 Rome, Italy Via della Lungara 81/C, 00165 Rome, Italy d Istituto Nazionale di Geofisica e Vulcanologia,Via di Vigna Murata 605-00143 Rome, Italy b c
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 2 May 2008
A quantitative prediction of seismic site effects requires the definition of a subsoil parametric model for ground motion numerical modelling. This paper describes an application of integrated geophysical methods to define an earthquake engineering parametric model for the seismic zonation of the municipal area of Celano, Italy. In municipal areas of such extent, particularly in case of complex geology, subsurface characterization is an optimization procedure, where the objective function to be minimized is the uncertainty related to the subsoil features, under the constraint of fixed resources and logistical limitations. In the particular case of Celano, correlation between different geophysical results was very profitable in discriminating different geological scenarios in the historical centre and in areas designed for urban expansion, while defining the elastic properties of the near-surface deposits throughout the municipal area. & 2008 Published by Elsevier Ltd.
Keywords: Geophysical surveys Dynamic soil properties Site effects Seismic microzonation In-situ soil testing
1. Foreword In ground response analysis for seismic zonation, as defined by TC4 ISSMFE [1], the required output is the distribution of ground motion intensity at the surface level for a given area of interest (Fig. 1a). Given a certain seismic-input scenario, a key task to obtain reliable prediction of seismic ground response is to build a parametric model of the subsoil for numerical modelling. The layer geometry, the topography of the land surface and the stiffness properties of the geological units are key parameters for seismic site effects analysis. Seismic response analysis is an open field of research, as assessed by recent large international projects such as the EUROSEIS [2,3], and the IUGS-UNESCO IGCP Project 414 [4], focusing on site characterization and numerical modelling in urban areas. These projects involved the acquisition of large amount of measures, requiring economical resources which are not ordinarily available. In more conventional case studies applied to municipal areas [5], particularly in case of complex geology, subsurface characterization is an optimization procedure, where the objective function
Corresponding author. Tel.: +39 06 44585079; fax: +39 06 44585080.
E-mail address: [email protected] (E. Cardarelli). 0267-7261/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.soildyn.2008.05.003
to be minimized is the uncertainty related to the subsoil features, under the constraint of fixed resources (in terms of funding and time) to complete the study (Fig. 1b). This paper describes an application of integrated geophysical methods to define an earthquake engineering parametric model for the municipal area of Celano, Italy (Fig. 2). The city of Celano was selected as the subject of a multidisciplinary study for seismic zonation (related papers [6–9]), being an high-risk seismic area with a complex geological setting, a high-value historical centre and needs for planning the future urban expansion. The problems to be addressed when defining an earthquake engineering model fall into two principal classes.
Definition of soil layering and subsoil geometry. Characterization of the dynamic properties of the geological units that are the relevant physical parameters for site effects estimation.
With regard to the characterization of soil dynamic properties, the relative merits and drawbacks of in-situ and laboratory testing (see [10] for a review) are well known, and it is a wise practice to consider them as complementary to define the soil behaviour at different strain levels.
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Fig. 1. (a) Seismic zonation flowchart, after [1]. (b) Flowchart for the geophysical investigations described in this paper.
In-situ characterization using seismic prospecting is confined to the low-(elastic or linear) strain range. Several seismic prospecting techniques allow to estimate the shear-wave velocity, which is of direct use for defining the elastic stiffness properties. In the case history presented in this paper, attention will be focused on geophysical techniques, commonly available to the engineering practice. We used seismic S-wave crosshole (CH) [11] and surface wave testing (SWT) [12,13] to infer the S-wave velocity of shallow deposits, and CH tomography, downhole (DH) profiling and seismic refraction for P-wave velocity. Inferring lowstrain damping parameters from SWT [14] is not routinely used so we let this for more specific applications. In geologically complex areas, geophysical surveys should be performed extensively to give a consistent picture of the subsoil. The integrated interpretation of results from different techniques is one of the key issues to lead toward meaningful subsoil models, and a close connection with geology is needed to interpret the geophysical results in terms of geological features. We suggest considering survey planning as recursive, as it should be adjusted according to the results coming in-itinere, allowing to identify which sites need a supplement of study and are better suited for direct testing (borehole location). As far as the depth target of the survey is concerned, we usually suggest not to limit the investigation depth to 30 m, as it is common practice [15]. Particular care should be taken in defining
the bedrock, if any, within the depth targets of the study; as in many situations direct inspection of the bedrock is not feasible due to its depth, the use of geophysical techniques is a low-cost tool to reach deeper targets, very important for bedrock identification.
2. Geophysical measurements in the municipal area of Celano When dealing with seismic zonation at the urban planning scale, it is common practice to merge areas of similar destination of use and common geological scenarios for both planning the field survey and representing the results. For a detailed description of the geological setting refer to the related paper by Bianchi Fasani et al. [7]. With reference to Fig. 2, we distinguish three main areas in the municipal area of Celano. The first area (Marked with CC) is the historical city centre (CC), which is a residential area with historical buildings of great value such as the Piccolomini castle (XII century) which is built on an ancient debris slide deposits [7], which also outcrop forming a small relief (Borgo Monterone) located few hundred meters south-west (Fig. 5 of [7]). The residential expansion (RE) area is the one labelled as RE in Fig. 2. We further characterized a third area represented with the
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Fig. 2. Location of the geophysical surveys in the municipal area of Celano.
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Table 1 Surface geophysical surveys: acquisition parameters and layout Survey name
Area
Max AB/2 (m)
Survey name
Area
Max AB/2 (m)
VES VES VES VES VES VES VES VES VES VES
RE RE IN IN IN RE RE RE IN IN
250 500 320 320 320 320 500 500 320 250
Electrode number
Electrode spacing (m)
Total length (m)
Electrical resistivity tomography (ERT) ERT1 CC Wenner, Dipole Dipole ERT2 IN Wenner, Dipole Dipole ERT3 IN Wenner, Dipole Dipole ERT4 RE Wenner, Dipole Dipole
28 28 28 28
8 8 8 10
216 216 216 270
Survey name
Area
Nbr of shots-Nbr of geoph.
Receiver spacing (m)
Time sampling–Trace length (ms)
Maximum offset (m)
Seismic refraction L1-A L2-A L3-A L1 L2 L3 L4 L5 L6 L7 L8
CC IN IN RE RE IN IN IN RE RE IN
24 12 24 24 24 24 24 24 24 24 24
8 8 8 10 10 10 10 10 10 10 10
Vertical electrical sounding (VES)– Schlumberger array VES 1-A CC 250 VES 2-A IN 100 VES 3-A RE 250 VES 4-A IN 250 VES 5-A RE 100 VES 6-A IN 100 VES 7-A RE 100 VES 8-A CC 100 VES 9-A CC 30 (calibration) VES 10-A CC 20 (calibration) VES 11-A IN 100 VES 12-A IN 100 Survey name
Area
Electrode Array
Receivers–7 Receivers–5 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7
Shots Shots Shots Shots Shots Shots Shots Shots Shots Shots Shots
1 2 3 4 5 6 7 8 9 10
IN label, which is the industrial district located across highway A-24. In the areas RE and IN, the geological scenario is different from the CC: here the outcropping formations are heterogeneous fluvial and alluvial fan deposits. Shallow soil characterization and bedrock identification are the two main targets in a microzonation study at this scale. Logistical limitations for survey execution can be severe when working in urban areas, because of buildings and infrastructures. In this case study, we applied the following geophysical techniques (whose relevant acquisition parameters are reported in Table 1):
2.1. Seismic refraction Standard acquisition layout was typically 24 channels with uniform spacing with seven shots for each line, using a shotgun seismic source. The travel time curves were constructed for each shot and each line was interpreted with the delay time method adapted for shallow targets [16].
2.1.1. Vertical electrical soundings (VES) Seventeen Schlumberger-array VESs were collected within the municipal area; the field curves were inverted for 1D models using the steepest descent technique and the application of digital filtering for the forward problem [17]. The cable spread length was limited in most cases by the presence of infrastructures.
0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6
50 60 50 50 50 50 50 50 50 50 50
2.1.2. Electrical resistivity tomography (ERT) Three ERT sections were performed using dipole–dipole and Wenner arrays and inverted using a Quasi-Newton damped-leastsquares algorithm [18].
2.2. Surface wave testing (SWT) To estimate S-wave velocity profile in selected sites, we performed four surface wave tests using linear arrays [19]. Geophone spacing was selected as 5 m. The experimental dispersion curves were extracted from the power spectrum density (PSD) [20] and inverted using the algorithm proposed by Rix and Lai [21].
2.3. Microtremors (MT) MT measurements are frequently used to detect surface geology characterized by simple horizontal layering and high impedance contrasts [22]. In some cases, MTs analysis allows to point out 2D and 3D effects in the soil response. MT data were collected using long time recording windows [23] in nine sites in correspondence to other geophysical measurements. In some cases the measurements were repeated to assess if data were stable in time. For a detailed elastic characterization of P-wave and S-wave velocity of the near-surface geological formations, we used.
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Fig. 3. Results of the surveys ERT1 and L1 in the (CC area).
2.4. Downhole (DH) and crosshole (CH) seismic prospecting DH P-data were interpreted using the interval velocity (see [24], pp.207–208). In CH surveying, VS velocities have been calculated using a well-known procedure [25] using a DH hammer source able to generate waves with opposite polarization. The CH P-wave tomography data were acquired using the same source with 12 DH hydrophones and several surface geophones for each shot. The picked first arrivals were inverted using the inversion algorithm described in [26].
3. Outline of the results The results of the geophysical investigations will be described with regard to the three main territorial areas: city centre (CC), industrial district (IN ) and residential expansion (RE).
3.1. City centre (CC) As described in [7] the historical centre of Celano is built on an ancient debris slide. The surface investigations were executed to give a general insight about the subsoil structure in the town centre and as a guidance for borehole location. Referring to Fig. 2, four VES, named S8-A, S1-A, S9-A and S10-A were collected in this area. The last two were essentially calibration soundings on the outcropping limestone, respectively, at the top of the Celano hill and on the Borgo Monterone deposits, which showed a resistivity higher than 500 O m. The ERT1 and seismic refraction line L1-A were executed in the only area that allowed cable deploying in the CC. The main purpose of these lines was to verify if the outcropping limestone deposits (rock avalanche debris [7]) were continuous in depth and, if so, to estimate the thickness of the alluvial overburden in the area between the two outcropping limestone hills.
Fig. 4. Field and theoretical curves for the S1-A and S8-A vertical electrical soundings (CC area).
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The results of these surveys are shown in Fig. 3 identifying a 12–14-m-thick alluvial overburden. The seismic refraction P-wave interpretation consists in three layers. The shallower weathering layer (backfilling material) has a P-velocity ranging between 500 m/ s (Receivers 1–12) and 400 m/s (Receivers 13–24). The intermediate layer has a velocity of about 700 m/s dipping from about 15 m depth (Receivers 24) to 22 m depth (Receivers 1). The underlying refractor, with an average velocity of 2400 m/s and high resistivity (more than 500 O m), is identified as the limestone forming the Celano hill and outcropping in the Borgo Monterone area. VESs S1-A and S8-A, although affected by 2D effects (Fig. 4) show an evident drop in resistivity in the last part of the curves. With this experimental evidence, we confirmed that the outcropping limestone is not continuous in depth and thus constitutes a detrital reworked material and can be consequently defined as a limestone breccia. Remarkably, these limestone deposits do not constitute the geological bedrock of the town centre. To define the thickness of this limestone breccia, four boreholes were executed in the CC area. Crosshole CH1-A was superimposed on the previous electric/seismic lines ERT1 and L1-A. The results of the CH1-A (P-wave tomography and S-wave profiling) are shown in Fig. 5.
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The boreholes confirmed the presence of a conductive layer under the limestone breccia, which passes into clay silts and sandy silts deposits at 37.4 m depth. The shallow overburden (approx. 0–12.50 m) has a shear velocity of about 400–500 m/s, while the S-wave velocity of the limestone breccia increases from 700 m/s at the top layer (where the contamination with the overlying alluvial deposits is evident from coring) to an average value of 1000 m/s in the central part of the landslide deposits. As shown by the P-wave tomography (Fig. 5), this limestone breccia is quite heterogeneous, with P-wave velocity values in the range 1800–2400 m/s. Two DH P-wave profiles (DH1 and DH2) were executed to evaluate the heterogeneity of the limestone breccia; the results of these surveys are showed in Fig. 6. The P-wave velocity ranges are in agreement with the results coming both from CH1-A and seismic refraction prospecting L1-A. MT measurements were performed in four sites of the CC area. These sites were located in the Piccolomini Castle (MT–CA), at DH2 downhole (MT–DH), at CH1-A crosshole (MT–ASL) and on the outcropping limestone deposits at Borgo Monterone (MT–BM). Data analysis shows the presence of significant amplification factors for MT–CA and MT–DH site as in Fig. 7a and b. In the first
Fig. 5. Results of the crosshole test CH1-A in the CC. Left: P-wave tomography (RMS residuals: 0.42 ms). Right: S-wave velocity profile. Borehole distance is 5 m.
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The MT–DH data show an amplification function with a sharp peak centred at 5–5.5 Hz. This resonance frequency can be matched by numerical modelling using 1D models derived from DH data (Fig. 7b). For the other sites (MT–ASL and MT–BM), where the limestone breccia is shallow or outcropping, no clear amplification is shown (Fig. 7c and d), since the maximum H/V ratio is between 3 and 3.5 with no sharp peaks. For the ASL site, a resonance frequency giving a moderate peak at 6 Hz can be matched using CH data (Fig. 7c). The interpretation of geophysical measures pointed out important features of the subsoil model in the town centre (CC):
The historical centre hill is formed by ancient debris-slide deposits, partially covered by more recent alluvial deposits.
This debris-slide limestone breccia is continuous between the
two outcropping hills of Borgo Monterone and of the historical CC (Piccolomini Castle hill) and is covered by alluvial deposits. The P- and S-wave velocity of the relevant geological units was defined using borehole seismic prospecting. The interpreted measurements confirmed the absence of a limestone bedrock in the first 100 m.
Because of these results, it seemed straightforward the seismic response numerical analysis of this area should consider 2D effects. 3.2. Area for residential expansion (RE) Five VESs (S1, S2, S6, S7 and S8) were collected in this area (Fig. 2). They were performed using the Schlumberger quadripole array. Maximum spacing AB/2 was equal to 500 m. The main purpose of the VESs was to determine the presence of a resistive bedrock. Only VES S6 showed a resistive layer (900 O m at a depth of 150 m) which is compatible with a limestone bedrock. On the other hand, VES S1 and S7 did not show any evident change of resistivity at depths greater than 60 m, excluding the presence of limestone deposits as far as the maximum depth reached (about 200 m in VES S7). The same results were obtained in the electrical soundings S2 and S8 where the resistivity values in the deeper layers are below 150 O m (Fig. 8). Seismic refraction lines L1-L7, L2 and L6 were interpreted by three-layer models:
the P-wave velocity of the weathering layer varies from 500 to 700 m/s;
the P-wave velocity of the intermediate layer ranges from 900 to 1200 m/s;
the bottom refractor (1800–2400 m/s of P-wave velocity) is detected between 15 and 22 m depth along the lines.
Fig. 6. Results of the DH1 and DH2 P-wave profiling (CC area).
case, the amplification cannot be related to the velocity distribution inferred by the other geophysical surveys. The amplification function is very stable in time and reaches a peak amplitude of 5 at frequency between 3 and 4 Hz. This result is probably related to 2D topographic effect.
The above investigations were used to select the appropriate sites for seismic CH. The results of CH1-S5 are shown in Fig. 9. In CH1-S5 a decrease in S-wave velocity (about 500–300 m/s) is shown at between 14 and 16 m depth; it can be associated with the transition from sandy silt-sand to silt-sand silt clay deposits. This confirms the presence of a conductive layer as detected in VES S1. In the P-wave tomography, velocity changes agree with the ones in seismic line L1 and L7: the velocity ranges from about 600 m/s in the near surface to 1200 m/s in the intermediate zone. In the deepest part, the P-velocity increases and confirms the heterogeneity of the deposits.
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Fig. 7. Results of MT measurements in the CC area. Black solid lines are the transfer function from 1D modelling.
In the RE area three SWT measurements were performed to investigate larger soil volumes. The selected sites were close to the CH1-S5 crosshole (SWT1), the CH2-S3 crosshole (SWT4) and the L2 refraction line (SWT2). For the array SWT1, the dispersion curve is obtained in the 7–25 Hz frequency range (Fig. 10). Phase velocity is always higher than 300 m/s with a clear increase at low frequency, suggesting a shallow velocity contrast in agreement with CH data. Due to the array geometry and resolution it is not possible to detect the velocity inversion (showed by the CH data) at about 15 m depth. SWT4 shows a dispersion curve well defined in the 5–16 Hz range with low surface velocities of about 250–270 m/s (Fig. 11). In Fig. 11 (on the right side), the SWT4 inverted model is compared with the S-wave velocity profile derived from crosshole CH2-S3. The agreement is quite good, even if the SWT does not have enough resolution to detect the velocity gradient shown by CH data below 20 m. Four sites of the RE area were selected for MT measurements. They are named: MT-L1 at the CH1-S5 crosshole site, MT-L2 at the seismic line L2, MT-L4 at the CH2-S3 crosshole, and MT-OUT. The first three sites do not show any significant amplification factor (Fig. 12a–c). This feature suggested the absence of important impedance contrasts. The same conclusions were obtained using SWT and CH data. Also the MT-OUT does not show important amplification factors except for the low-frequency peak centred at 1 Hz (Fig. 12d), probably related to the deepest part of the sedimentary basin.
All the geophysical surveys carried out in this area confirm the absence of deposits with the characteristics of the Celano-hill limestone breccia, showing elastic parameters and stiffness which are typical of alluvial deposits. Resistivity values compatible with a limestone bedrock were measured in the VES S6 but not elsewhere in this area, indicating that the limestone bedrock of Mt. La Serra di Celano [7] is steeply immerging S-SW. The absence of high impedance contrasts at shallow depths was also confirmed by active and passive seismic measurements.
3.3. Industrial area (IN) This area is located south of the CC and, from a geological point of view, is composed of heterogeneous alluvial and detrital deposits. The physical and mechanical parameters collected by geophysical surveys are influenced by the presence of the water table at shallow depths, see for instance Fig. 8 in [7]. In this area, geophysical investigations consisted in nine VESs (S2-A, S4-A, S6-A, S12-A, S3, S4, S5, S9 and S10), six seismic lines (L2-A, L3-A, L3, L4, L5, L9), two ERT sections (ERT2 and ERT3) and the CH3-S2 crosshole. The VESs show a remarkable heterogeneity in the shallower layers; at deeper targets the values of resistivity are below 250 O m.
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Fig. 8. Seismic profile of line L1_7 and VES S1 and S7.
Fig. 9. Results of the crosshole test CH1-S5 (RMS residuals: 0.37 ms). Left: P-wave tomography. Right: S-wave velocity profile. Borehole distance is 5 m.
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Fig. 10. Results of SWT1; Left: dispersion curve. Right: Inverted S-wave model
Fig. 11. Results of SWT4; Left: dispersion curve. Right: Inverted S-wave model (solid line) and crosshole CH2-S3 shear velocity profile (crosses).
The seismic refraction lines L2-A, L3-A and L3 are characterized by a three-layer model with P-wave velocity ranging from 400 to 500 m/s in the shallower layer and from 800 to 1400 m/s in the intermediate layers. The bottom refractor (1800–2050 m/s) is located between 16 and 20 m depth, corresponding to the higher resistivity values detected in this area. The two sites, southern of Borgo Monterone (MT-BM site), where the ERT2 and ERT3 were collected, were selected to evaluate if the limestone breccia (rock avalanche deposits) was present. In Figs. 13 and 14 we report the data collected in these two sites, which show a commendable agreement between the seismic refraction and ERT sections. None of the surveys showed high resistivity and P-wave velocities comparable to those measured for the outcropping limestone breccia. The CH survey CH3-S2 (Fig. 15), located close to the seismic line L3 and VES S3, confirmed that none of the drilled deposits has the mechanical characteristics of the limestone breccia. The geological units between 18 and 30 m depth are gravelly deposits, finely interbedded with softer alluvial deposits. The P-wave tomography shows maximum velocity value of 2100 m/s.
In a similar way, in the S-wave profile the velocity of the bottomhole deposits is smaller than 600 m/s. In the seismic lines L4 and L9 a P-wave velocity of 1500 m/s, probably due to saturated material, is detected under a weathering layer. Close to CH3-S2 crosshole the SWT3 was performed. The dispersion curve is defined in the 5–25 Hz frequency range (Fig. 16). Surface velocity is quite low (about 250 m/s). At high frequency, a possible jump to an higher mode is possible. At frequency lower than 8 Hz, a clear increase of velocity can be explained with a sharp impedance contrast. The fitting obtained inverting the dispersion curve is quite good as well as the agreement with the CH3-S2 S-wave velocities (Fig. 15). In this area, only one measurement of MT was performed at the MT-L3 site near the CH3-S2 crosshole. In this case, an amplification factor of about four is found at frequency between 2 and 3 Hz with secondary peaks at higher frequencies. These results are very stable as far as time is concerned, since measurements repeated few months apart produce the same results (Fig. 17). The fundamental resonance frequency obtained by MT data is in agreement with the peak obtained by 1D numerical modelling from the SWT and CH S-wave profiles (Fig. 17).
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Fig. 12. Results of MT measurements in the residential expansion (RE) area. Black solid lines are the transfer function from 1D modelling.
Fig. 13. Results of the surveys ERT2 and L2-A in the IN area.
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Fig. 14. Results of the surveys ERT3 and L3-A in the IN area.
Fig. 15. Results of the crosshole test CH3-S2. Left: P-wave tomography (RMS residuals: 0.45 ms ). Right: S-wave velocity profile (right). Borehole distance is 5 m.
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Fig. 16. Results of SWT3; Left: dispersion curve. Right: Inverted S-wave model.
Fig. 17. Collected data of MT-L3. The black solid line is the transfer function from 1D modelling.
Table 2 Elastic soil characterization surveys: acquisition parameters and layout Survey name
Area
Borehole depth (m)
Spacing of measures
Time sampling–trace length (ms)
Investigation depth (m)
0.1–102.3 0.1–102.3 0.1–102.3 0.1–102.3
44 28 26 28
Crosshole– S-velocity profiling CH1-A CC CH1-S5 RE CH2-S3 IN CH3-S2 IN
52 30 30 30
Survey name
Borehole depth (m)
Number of receivers
Time sampling–trace length (ms)
Investigation depth (m)
Crosshole– P-velocity tomography CH1-A CC CH1-S5 RE CH2-S3 IN CH3-S2 IN
52 30 30 30
14 16 16 16
0.1–102.3 0.1–102.3 0.1–102.3 0.1–102.3
52 28 28 28
Survey name
Borehole depth (m)
Number of receivers
Time sampling–trace length (ms)
Investigation depth (m)
Downhole– P-velocity profiling DH1-A CC DH2-A CC
52 52
12 12
0.1–102.3 0.1–102.3
50 50
Survey name
Source type (kg)
Nbr and type of geophones
Time sampling–trace length
Maximum offset (m)
35 35 35 35
24 24 24 24
1 ms/2.048 s 2 ms/4.096 s 1 ms/2.048 s 2 ms/4.096 s 1 ms/2.048 s 2 ms/4.096s 1 ms/2.048 s 2 ms/4.096 s
20 20 20 20
Area
Area
Area
Surface wave testing (SWT) L1-SWT RE L2-SWT RE L3-SWT IN L4-SWT RE
2 1 1 1
Impact mass Impact mass Impact mass Imp. mass/minibang
Receivers–7 Hz; 4.5 Hz Receivers–7 Hz Receivers–7 Hz Receivers–7 Hz
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Table 3 Microtremor surveys: acquisition parameters Survey name
Area
Technique
Datalogger/sensors
Number of tests
Recording Time (h)
Microtremors survey MT–ASL MT–BM MT–OUT MT-DH MT–CA MT–L1 MT–L2 MT–L3 MT–L4
CC RE RE CC CC RE RE IN RE
H/V H/V H/V H/V H/V H/V H/V H/V H/V
MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s
2 1 1 1 2 2 2 2 2
3, 24 3 1 3 5, 24 1, 1 1, 1 1, 1 1, 1
Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura
4. Conclusions Geophysical techniques play a fundamental role in seismic zonation studies. Particularly, when the geology of the investigated area of interest is complex, fairly common in seismic areas where surface instabilities, faults, etc. usually complicates a great deal the geologic situation, they should be run extensively to limit the geometrical uncertainty in the representation of the subsoil model (Tables 2 and 3). Application of integrated geophysical surveys to the municipal area of Celano (L’Aquila, Italy), was very profitable in defining the main features of the subsoil below the historical centre and in areas designed for next urban expansion. We demonstrated that the historical centre is built on limestone breccia deposits, partially covered by more recent alluvial deposits. The shear velocity of shallow soils was determined up to 45 m depth, using high-resolution seismic borehole prospecting and SWT. The limestone breccia deposits are not present in the RE and IN areas, where the geological scenario is different. The results of our work have been further used for defining quantitatively the local site effects using seismic modelling analysis [8].
Acknowledgements The authors wish to thank the Abruzzo Region and the Celano Municipality for the financial support of the research. The authors are grateful to C. D’Alessandro who processed some of the microtremor recordings as part of her M.Sc. Thesis (‘‘Sapienza’’ Universita` di Roma). References [1] Technical Committee for Earthquake Geotechnical Engineering, TC4, ISSMFE. Manual for zonation on seismic geotechnical hazards, 1993. [2] Chavez-Garcia FJ, Raptakis D, Makra K, Pitilakis K. Site effects at Euroseis testI. Determination of the valley structure and confrontation of observations with 1D analysis. Soil Dyn Earthquake Eng 2000;19(1):1–22. [3] Chavez-Garcia FJ, Raptakis D, Makra K, Pitilakis K. Site effects at Euroseistest—II. Results from 2D numerical modelling and comparison with observations. Soil Dyn Earthquake Eng 2000;19(1):23–39. [4] Panza GF, Vaccari F, Romanelli F. Realistic modelling of seismic input in Urban Areas: A UNESCO-IUGS-IGCP project. Pure Appl Geophys 2001;158(12): 2389–406.
[5] Yilmaz O, Eser M, Berilgen M. A case study of seismic zonation in municipal areas. The Leading Edge 2006;25(3):319–30. [6] Pace B, Boncio P, Brozzetti F, Lavecchia G, Visini F. From regional seismic hazard to ‘‘scenario earthquakes’’ for seismic microzoning: A new methodological tool for the Celano Project. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2007.11.001. [7] Bianchi Fasani G, Cavinato GP, Petitta M, Scarascia Mugnozza G, Voltaggio M. The geological model of Celano town area for seismic microzonation activities. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/ j.soildyn.2008.05.006. [8] Pergalani F, Compagnoni M, Petrini V. Evaluation of site effects using numerical analyses in Celano (Italy) finalized to seismic risk assessment. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2008.05.004. [9] Martinelli A, Cifani G, Cialone G, Corazza L, Petracca A, Petrucci G. Building vulnerability assessment and damage scenarios in Celano (Italy) using a quick survey data-based methodology. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2008.03.002. [10] Kramer SL. Geotechnical earthquake engineering. Englewood Cliffs, NJ: Prentice-Hall; 1996. [11] ASTM Standard D4428/D4428M-07 Standard test methods for crosshole seismic testing. ASTM Int, (2004). [12] Xia J, Miller RD, Park CB. Estimation of near-surface shear wave velocity by inversion of Rayleigh waves. Geophysics 1999;64(3):691–700. [13] Socco LV, Strobbia C. Surface-wave method for near surface characterization: a tutorial. Near Surf Geophys 2004;2(4):165–85. [14] Xia J, Miller RD, Park CB, Tian G. Determining Q of near-surface materials from Rayleigh waves. J Appl Geophys 2002;51(2–4):121–9. [15] Eurocode 8. Design provisions for earthquake resistance of structures. DD ENV 1998-1-1: 1996. [16] Bernabini M. Alcune considerazioni sui rilievi sismici a piccola profondita`. Bollettino di Geofisica Teorica ed Applicata 1965;7(26):106–18 (in Italian). [17] Koefoed O. Resistivity sounding measurements. Amsterdam: Elsevier; 1979. [18] Loke MH, Baker RD. Rapid least-squares inversion of apparent resistivity pseudosections, by a quasi-Newton method. Geophys Prospecting 1996; 44(1):131–52. [19] Park CB, Miller RD, Xia J. Multichannel analysis of surface waves. Geophysics 1999;64(3):800–8. [20] Milana G, Barba S, Del Pezzo E, Zambonelli E. Site response from ambient noise measurements: new perspectives from an array study in Central Italy. Bull Seismol Soc Am 1996;86(2):320–8. [21] Rix GJ, Lai CG, Foti S. Simultaneous measurement of surface wave dispersion and attenuation curves. Geotech Test J 2001;24(4):350–8. [22] Bard P. Microtremor measurements: a tool for site effect estimation? In: Irikura, Kudo, Okada, Sasatani, editors. The effects of surface geology on seismic motion, 1999. p. 1251–79. [23] Bard PY, Extracting information from ambient seismic noise: The SESAME project (Site EffectS assessment using AMbient Excitations)’’. European Project No. EVG1-CT-2000-00026 SESAME, 2002, /http:// sesame-fp5.obs.ujf-grenoble.fr/index.htmS Review meeting, Brussels, Belgium. [24] Telford WM, Geldart RP, Sheriff RE. Applied geophysics. 2nd ed. Cambridge: Cambridge University Press; 1990. [25] Cardarelli E, Bernabini M, Tommasi P. P and S waves cross-hole surveys, characterisation of the Orvieto hill formations. In: 72nd SEG Annual Meeting. Salt Lake City, 2002. p. 1468–1471. [26] Bernabini M, Cardarelli E. Variable damping factors in travel time tomography. J Appl Geophys 1997;38(2):131–41.
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Technical Note
Geophysical investigations for seismic zonation in municipal areas with complex geology: The case study of Celano, Italy E. Cardarelli a,, M. Cercato a, R. de Nardis b, G. Di Filippo c, G. Milana d a
` di Roma, D.I.T.S., Via Eudossiana, 18-00184 Rome, Italy ‘‘Sapienza’’ Universita Dipartimento della Protezione Civile, Via Ulpiano, 11-00193 Rome, Italy Via della Lungara 81/C, 00165 Rome, Italy d Istituto Nazionale di Geofisica e Vulcanologia,Via di Vigna Murata 605-00143 Rome, Italy b c
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 2 May 2008
A quantitative prediction of seismic site effects requires the definition of a subsoil parametric model for ground motion numerical modelling. This paper describes an application of integrated geophysical methods to define an earthquake engineering parametric model for the seismic zonation of the municipal area of Celano, Italy. In municipal areas of such extent, particularly in case of complex geology, subsurface characterization is an optimization procedure, where the objective function to be minimized is the uncertainty related to the subsoil features, under the constraint of fixed resources and logistical limitations. In the particular case of Celano, correlation between different geophysical results was very profitable in discriminating different geological scenarios in the historical centre and in areas designed for urban expansion, while defining the elastic properties of the near-surface deposits throughout the municipal area. & 2008 Published by Elsevier Ltd.
Keywords: Geophysical surveys Dynamic soil properties Site effects Seismic microzonation In-situ soil testing
1. Foreword In ground response analysis for seismic zonation, as defined by TC4 ISSMFE [1], the required output is the distribution of ground motion intensity at the surface level for a given area of interest (Fig. 1a). Given a certain seismic-input scenario, a key task to obtain reliable prediction of seismic ground response is to build a parametric model of the subsoil for numerical modelling. The layer geometry, the topography of the land surface and the stiffness properties of the geological units are key parameters for seismic site effects analysis. Seismic response analysis is an open field of research, as assessed by recent large international projects such as the EUROSEIS [2,3], and the IUGS-UNESCO IGCP Project 414 [4], focusing on site characterization and numerical modelling in urban areas. These projects involved the acquisition of large amount of measures, requiring economical resources which are not ordinarily available. In more conventional case studies applied to municipal areas [5], particularly in case of complex geology, subsurface characterization is an optimization procedure, where the objective function
Corresponding author. Tel.: +39 06 44585079; fax: +39 06 44585080.
E-mail address: [email protected] (E. Cardarelli). 0267-7261/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.soildyn.2008.05.003
to be minimized is the uncertainty related to the subsoil features, under the constraint of fixed resources (in terms of funding and time) to complete the study (Fig. 1b). This paper describes an application of integrated geophysical methods to define an earthquake engineering parametric model for the municipal area of Celano, Italy (Fig. 2). The city of Celano was selected as the subject of a multidisciplinary study for seismic zonation (related papers [6–9]), being an high-risk seismic area with a complex geological setting, a high-value historical centre and needs for planning the future urban expansion. The problems to be addressed when defining an earthquake engineering model fall into two principal classes.
Definition of soil layering and subsoil geometry. Characterization of the dynamic properties of the geological units that are the relevant physical parameters for site effects estimation.
With regard to the characterization of soil dynamic properties, the relative merits and drawbacks of in-situ and laboratory testing (see [10] for a review) are well known, and it is a wise practice to consider them as complementary to define the soil behaviour at different strain levels.
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Fig. 1. (a) Seismic zonation flowchart, after [1]. (b) Flowchart for the geophysical investigations described in this paper.
In-situ characterization using seismic prospecting is confined to the low-(elastic or linear) strain range. Several seismic prospecting techniques allow to estimate the shear-wave velocity, which is of direct use for defining the elastic stiffness properties. In the case history presented in this paper, attention will be focused on geophysical techniques, commonly available to the engineering practice. We used seismic S-wave crosshole (CH) [11] and surface wave testing (SWT) [12,13] to infer the S-wave velocity of shallow deposits, and CH tomography, downhole (DH) profiling and seismic refraction for P-wave velocity. Inferring lowstrain damping parameters from SWT [14] is not routinely used so we let this for more specific applications. In geologically complex areas, geophysical surveys should be performed extensively to give a consistent picture of the subsoil. The integrated interpretation of results from different techniques is one of the key issues to lead toward meaningful subsoil models, and a close connection with geology is needed to interpret the geophysical results in terms of geological features. We suggest considering survey planning as recursive, as it should be adjusted according to the results coming in-itinere, allowing to identify which sites need a supplement of study and are better suited for direct testing (borehole location). As far as the depth target of the survey is concerned, we usually suggest not to limit the investigation depth to 30 m, as it is common practice [15]. Particular care should be taken in defining
the bedrock, if any, within the depth targets of the study; as in many situations direct inspection of the bedrock is not feasible due to its depth, the use of geophysical techniques is a low-cost tool to reach deeper targets, very important for bedrock identification.
2. Geophysical measurements in the municipal area of Celano When dealing with seismic zonation at the urban planning scale, it is common practice to merge areas of similar destination of use and common geological scenarios for both planning the field survey and representing the results. For a detailed description of the geological setting refer to the related paper by Bianchi Fasani et al. [7]. With reference to Fig. 2, we distinguish three main areas in the municipal area of Celano. The first area (Marked with CC) is the historical city centre (CC), which is a residential area with historical buildings of great value such as the Piccolomini castle (XII century) which is built on an ancient debris slide deposits [7], which also outcrop forming a small relief (Borgo Monterone) located few hundred meters south-west (Fig. 5 of [7]). The residential expansion (RE) area is the one labelled as RE in Fig. 2. We further characterized a third area represented with the
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Fig. 2. Location of the geophysical surveys in the municipal area of Celano.
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Table 1 Surface geophysical surveys: acquisition parameters and layout Survey name
Area
Max AB/2 (m)
Survey name
Area
Max AB/2 (m)
VES VES VES VES VES VES VES VES VES VES
RE RE IN IN IN RE RE RE IN IN
250 500 320 320 320 320 500 500 320 250
Electrode number
Electrode spacing (m)
Total length (m)
Electrical resistivity tomography (ERT) ERT1 CC Wenner, Dipole Dipole ERT2 IN Wenner, Dipole Dipole ERT3 IN Wenner, Dipole Dipole ERT4 RE Wenner, Dipole Dipole
28 28 28 28
8 8 8 10
216 216 216 270
Survey name
Area
Nbr of shots-Nbr of geoph.
Receiver spacing (m)
Time sampling–Trace length (ms)
Maximum offset (m)
Seismic refraction L1-A L2-A L3-A L1 L2 L3 L4 L5 L6 L7 L8
CC IN IN RE RE IN IN IN RE RE IN
24 12 24 24 24 24 24 24 24 24 24
8 8 8 10 10 10 10 10 10 10 10
Vertical electrical sounding (VES)– Schlumberger array VES 1-A CC 250 VES 2-A IN 100 VES 3-A RE 250 VES 4-A IN 250 VES 5-A RE 100 VES 6-A IN 100 VES 7-A RE 100 VES 8-A CC 100 VES 9-A CC 30 (calibration) VES 10-A CC 20 (calibration) VES 11-A IN 100 VES 12-A IN 100 Survey name
Area
Electrode Array
Receivers–7 Receivers–5 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7 Receivers–7
Shots Shots Shots Shots Shots Shots Shots Shots Shots Shots Shots
1 2 3 4 5 6 7 8 9 10
IN label, which is the industrial district located across highway A-24. In the areas RE and IN, the geological scenario is different from the CC: here the outcropping formations are heterogeneous fluvial and alluvial fan deposits. Shallow soil characterization and bedrock identification are the two main targets in a microzonation study at this scale. Logistical limitations for survey execution can be severe when working in urban areas, because of buildings and infrastructures. In this case study, we applied the following geophysical techniques (whose relevant acquisition parameters are reported in Table 1):
2.1. Seismic refraction Standard acquisition layout was typically 24 channels with uniform spacing with seven shots for each line, using a shotgun seismic source. The travel time curves were constructed for each shot and each line was interpreted with the delay time method adapted for shallow targets [16].
2.1.1. Vertical electrical soundings (VES) Seventeen Schlumberger-array VESs were collected within the municipal area; the field curves were inverted for 1D models using the steepest descent technique and the application of digital filtering for the forward problem [17]. The cable spread length was limited in most cases by the presence of infrastructures.
0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6 0.2–204.6
50 60 50 50 50 50 50 50 50 50 50
2.1.2. Electrical resistivity tomography (ERT) Three ERT sections were performed using dipole–dipole and Wenner arrays and inverted using a Quasi-Newton damped-leastsquares algorithm [18].
2.2. Surface wave testing (SWT) To estimate S-wave velocity profile in selected sites, we performed four surface wave tests using linear arrays [19]. Geophone spacing was selected as 5 m. The experimental dispersion curves were extracted from the power spectrum density (PSD) [20] and inverted using the algorithm proposed by Rix and Lai [21].
2.3. Microtremors (MT) MT measurements are frequently used to detect surface geology characterized by simple horizontal layering and high impedance contrasts [22]. In some cases, MTs analysis allows to point out 2D and 3D effects in the soil response. MT data were collected using long time recording windows [23] in nine sites in correspondence to other geophysical measurements. In some cases the measurements were repeated to assess if data were stable in time. For a detailed elastic characterization of P-wave and S-wave velocity of the near-surface geological formations, we used.
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Fig. 3. Results of the surveys ERT1 and L1 in the (CC area).
2.4. Downhole (DH) and crosshole (CH) seismic prospecting DH P-data were interpreted using the interval velocity (see [24], pp.207–208). In CH surveying, VS velocities have been calculated using a well-known procedure [25] using a DH hammer source able to generate waves with opposite polarization. The CH P-wave tomography data were acquired using the same source with 12 DH hydrophones and several surface geophones for each shot. The picked first arrivals were inverted using the inversion algorithm described in [26].
3. Outline of the results The results of the geophysical investigations will be described with regard to the three main territorial areas: city centre (CC), industrial district (IN ) and residential expansion (RE).
3.1. City centre (CC) As described in [7] the historical centre of Celano is built on an ancient debris slide. The surface investigations were executed to give a general insight about the subsoil structure in the town centre and as a guidance for borehole location. Referring to Fig. 2, four VES, named S8-A, S1-A, S9-A and S10-A were collected in this area. The last two were essentially calibration soundings on the outcropping limestone, respectively, at the top of the Celano hill and on the Borgo Monterone deposits, which showed a resistivity higher than 500 O m. The ERT1 and seismic refraction line L1-A were executed in the only area that allowed cable deploying in the CC. The main purpose of these lines was to verify if the outcropping limestone deposits (rock avalanche debris [7]) were continuous in depth and, if so, to estimate the thickness of the alluvial overburden in the area between the two outcropping limestone hills.
Fig. 4. Field and theoretical curves for the S1-A and S8-A vertical electrical soundings (CC area).
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The results of these surveys are shown in Fig. 3 identifying a 12–14-m-thick alluvial overburden. The seismic refraction P-wave interpretation consists in three layers. The shallower weathering layer (backfilling material) has a P-velocity ranging between 500 m/ s (Receivers 1–12) and 400 m/s (Receivers 13–24). The intermediate layer has a velocity of about 700 m/s dipping from about 15 m depth (Receivers 24) to 22 m depth (Receivers 1). The underlying refractor, with an average velocity of 2400 m/s and high resistivity (more than 500 O m), is identified as the limestone forming the Celano hill and outcropping in the Borgo Monterone area. VESs S1-A and S8-A, although affected by 2D effects (Fig. 4) show an evident drop in resistivity in the last part of the curves. With this experimental evidence, we confirmed that the outcropping limestone is not continuous in depth and thus constitutes a detrital reworked material and can be consequently defined as a limestone breccia. Remarkably, these limestone deposits do not constitute the geological bedrock of the town centre. To define the thickness of this limestone breccia, four boreholes were executed in the CC area. Crosshole CH1-A was superimposed on the previous electric/seismic lines ERT1 and L1-A. The results of the CH1-A (P-wave tomography and S-wave profiling) are shown in Fig. 5.
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The boreholes confirmed the presence of a conductive layer under the limestone breccia, which passes into clay silts and sandy silts deposits at 37.4 m depth. The shallow overburden (approx. 0–12.50 m) has a shear velocity of about 400–500 m/s, while the S-wave velocity of the limestone breccia increases from 700 m/s at the top layer (where the contamination with the overlying alluvial deposits is evident from coring) to an average value of 1000 m/s in the central part of the landslide deposits. As shown by the P-wave tomography (Fig. 5), this limestone breccia is quite heterogeneous, with P-wave velocity values in the range 1800–2400 m/s. Two DH P-wave profiles (DH1 and DH2) were executed to evaluate the heterogeneity of the limestone breccia; the results of these surveys are showed in Fig. 6. The P-wave velocity ranges are in agreement with the results coming both from CH1-A and seismic refraction prospecting L1-A. MT measurements were performed in four sites of the CC area. These sites were located in the Piccolomini Castle (MT–CA), at DH2 downhole (MT–DH), at CH1-A crosshole (MT–ASL) and on the outcropping limestone deposits at Borgo Monterone (MT–BM). Data analysis shows the presence of significant amplification factors for MT–CA and MT–DH site as in Fig. 7a and b. In the first
Fig. 5. Results of the crosshole test CH1-A in the CC. Left: P-wave tomography (RMS residuals: 0.42 ms). Right: S-wave velocity profile. Borehole distance is 5 m.
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The MT–DH data show an amplification function with a sharp peak centred at 5–5.5 Hz. This resonance frequency can be matched by numerical modelling using 1D models derived from DH data (Fig. 7b). For the other sites (MT–ASL and MT–BM), where the limestone breccia is shallow or outcropping, no clear amplification is shown (Fig. 7c and d), since the maximum H/V ratio is between 3 and 3.5 with no sharp peaks. For the ASL site, a resonance frequency giving a moderate peak at 6 Hz can be matched using CH data (Fig. 7c). The interpretation of geophysical measures pointed out important features of the subsoil model in the town centre (CC):
The historical centre hill is formed by ancient debris-slide deposits, partially covered by more recent alluvial deposits.
This debris-slide limestone breccia is continuous between the
two outcropping hills of Borgo Monterone and of the historical CC (Piccolomini Castle hill) and is covered by alluvial deposits. The P- and S-wave velocity of the relevant geological units was defined using borehole seismic prospecting. The interpreted measurements confirmed the absence of a limestone bedrock in the first 100 m.
Because of these results, it seemed straightforward the seismic response numerical analysis of this area should consider 2D effects. 3.2. Area for residential expansion (RE) Five VESs (S1, S2, S6, S7 and S8) were collected in this area (Fig. 2). They were performed using the Schlumberger quadripole array. Maximum spacing AB/2 was equal to 500 m. The main purpose of the VESs was to determine the presence of a resistive bedrock. Only VES S6 showed a resistive layer (900 O m at a depth of 150 m) which is compatible with a limestone bedrock. On the other hand, VES S1 and S7 did not show any evident change of resistivity at depths greater than 60 m, excluding the presence of limestone deposits as far as the maximum depth reached (about 200 m in VES S7). The same results were obtained in the electrical soundings S2 and S8 where the resistivity values in the deeper layers are below 150 O m (Fig. 8). Seismic refraction lines L1-L7, L2 and L6 were interpreted by three-layer models:
the P-wave velocity of the weathering layer varies from 500 to 700 m/s;
the P-wave velocity of the intermediate layer ranges from 900 to 1200 m/s;
the bottom refractor (1800–2400 m/s of P-wave velocity) is detected between 15 and 22 m depth along the lines.
Fig. 6. Results of the DH1 and DH2 P-wave profiling (CC area).
case, the amplification cannot be related to the velocity distribution inferred by the other geophysical surveys. The amplification function is very stable in time and reaches a peak amplitude of 5 at frequency between 3 and 4 Hz. This result is probably related to 2D topographic effect.
The above investigations were used to select the appropriate sites for seismic CH. The results of CH1-S5 are shown in Fig. 9. In CH1-S5 a decrease in S-wave velocity (about 500–300 m/s) is shown at between 14 and 16 m depth; it can be associated with the transition from sandy silt-sand to silt-sand silt clay deposits. This confirms the presence of a conductive layer as detected in VES S1. In the P-wave tomography, velocity changes agree with the ones in seismic line L1 and L7: the velocity ranges from about 600 m/s in the near surface to 1200 m/s in the intermediate zone. In the deepest part, the P-velocity increases and confirms the heterogeneity of the deposits.
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Fig. 7. Results of MT measurements in the CC area. Black solid lines are the transfer function from 1D modelling.
In the RE area three SWT measurements were performed to investigate larger soil volumes. The selected sites were close to the CH1-S5 crosshole (SWT1), the CH2-S3 crosshole (SWT4) and the L2 refraction line (SWT2). For the array SWT1, the dispersion curve is obtained in the 7–25 Hz frequency range (Fig. 10). Phase velocity is always higher than 300 m/s with a clear increase at low frequency, suggesting a shallow velocity contrast in agreement with CH data. Due to the array geometry and resolution it is not possible to detect the velocity inversion (showed by the CH data) at about 15 m depth. SWT4 shows a dispersion curve well defined in the 5–16 Hz range with low surface velocities of about 250–270 m/s (Fig. 11). In Fig. 11 (on the right side), the SWT4 inverted model is compared with the S-wave velocity profile derived from crosshole CH2-S3. The agreement is quite good, even if the SWT does not have enough resolution to detect the velocity gradient shown by CH data below 20 m. Four sites of the RE area were selected for MT measurements. They are named: MT-L1 at the CH1-S5 crosshole site, MT-L2 at the seismic line L2, MT-L4 at the CH2-S3 crosshole, and MT-OUT. The first three sites do not show any significant amplification factor (Fig. 12a–c). This feature suggested the absence of important impedance contrasts. The same conclusions were obtained using SWT and CH data. Also the MT-OUT does not show important amplification factors except for the low-frequency peak centred at 1 Hz (Fig. 12d), probably related to the deepest part of the sedimentary basin.
All the geophysical surveys carried out in this area confirm the absence of deposits with the characteristics of the Celano-hill limestone breccia, showing elastic parameters and stiffness which are typical of alluvial deposits. Resistivity values compatible with a limestone bedrock were measured in the VES S6 but not elsewhere in this area, indicating that the limestone bedrock of Mt. La Serra di Celano [7] is steeply immerging S-SW. The absence of high impedance contrasts at shallow depths was also confirmed by active and passive seismic measurements.
3.3. Industrial area (IN) This area is located south of the CC and, from a geological point of view, is composed of heterogeneous alluvial and detrital deposits. The physical and mechanical parameters collected by geophysical surveys are influenced by the presence of the water table at shallow depths, see for instance Fig. 8 in [7]. In this area, geophysical investigations consisted in nine VESs (S2-A, S4-A, S6-A, S12-A, S3, S4, S5, S9 and S10), six seismic lines (L2-A, L3-A, L3, L4, L5, L9), two ERT sections (ERT2 and ERT3) and the CH3-S2 crosshole. The VESs show a remarkable heterogeneity in the shallower layers; at deeper targets the values of resistivity are below 250 O m.
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Fig. 8. Seismic profile of line L1_7 and VES S1 and S7.
Fig. 9. Results of the crosshole test CH1-S5 (RMS residuals: 0.37 ms). Left: P-wave tomography. Right: S-wave velocity profile. Borehole distance is 5 m.
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Fig. 10. Results of SWT1; Left: dispersion curve. Right: Inverted S-wave model
Fig. 11. Results of SWT4; Left: dispersion curve. Right: Inverted S-wave model (solid line) and crosshole CH2-S3 shear velocity profile (crosses).
The seismic refraction lines L2-A, L3-A and L3 are characterized by a three-layer model with P-wave velocity ranging from 400 to 500 m/s in the shallower layer and from 800 to 1400 m/s in the intermediate layers. The bottom refractor (1800–2050 m/s) is located between 16 and 20 m depth, corresponding to the higher resistivity values detected in this area. The two sites, southern of Borgo Monterone (MT-BM site), where the ERT2 and ERT3 were collected, were selected to evaluate if the limestone breccia (rock avalanche deposits) was present. In Figs. 13 and 14 we report the data collected in these two sites, which show a commendable agreement between the seismic refraction and ERT sections. None of the surveys showed high resistivity and P-wave velocities comparable to those measured for the outcropping limestone breccia. The CH survey CH3-S2 (Fig. 15), located close to the seismic line L3 and VES S3, confirmed that none of the drilled deposits has the mechanical characteristics of the limestone breccia. The geological units between 18 and 30 m depth are gravelly deposits, finely interbedded with softer alluvial deposits. The P-wave tomography shows maximum velocity value of 2100 m/s.
In a similar way, in the S-wave profile the velocity of the bottomhole deposits is smaller than 600 m/s. In the seismic lines L4 and L9 a P-wave velocity of 1500 m/s, probably due to saturated material, is detected under a weathering layer. Close to CH3-S2 crosshole the SWT3 was performed. The dispersion curve is defined in the 5–25 Hz frequency range (Fig. 16). Surface velocity is quite low (about 250 m/s). At high frequency, a possible jump to an higher mode is possible. At frequency lower than 8 Hz, a clear increase of velocity can be explained with a sharp impedance contrast. The fitting obtained inverting the dispersion curve is quite good as well as the agreement with the CH3-S2 S-wave velocities (Fig. 15). In this area, only one measurement of MT was performed at the MT-L3 site near the CH3-S2 crosshole. In this case, an amplification factor of about four is found at frequency between 2 and 3 Hz with secondary peaks at higher frequencies. These results are very stable as far as time is concerned, since measurements repeated few months apart produce the same results (Fig. 17). The fundamental resonance frequency obtained by MT data is in agreement with the peak obtained by 1D numerical modelling from the SWT and CH S-wave profiles (Fig. 17).
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Fig. 12. Results of MT measurements in the residential expansion (RE) area. Black solid lines are the transfer function from 1D modelling.
Fig. 13. Results of the surveys ERT2 and L2-A in the IN area.
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Fig. 14. Results of the surveys ERT3 and L3-A in the IN area.
Fig. 15. Results of the crosshole test CH3-S2. Left: P-wave tomography (RMS residuals: 0.45 ms ). Right: S-wave velocity profile (right). Borehole distance is 5 m.
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Fig. 16. Results of SWT3; Left: dispersion curve. Right: Inverted S-wave model.
Fig. 17. Collected data of MT-L3. The black solid line is the transfer function from 1D modelling.
Table 2 Elastic soil characterization surveys: acquisition parameters and layout Survey name
Area
Borehole depth (m)
Spacing of measures
Time sampling–trace length (ms)
Investigation depth (m)
0.1–102.3 0.1–102.3 0.1–102.3 0.1–102.3
44 28 26 28
Crosshole– S-velocity profiling CH1-A CC CH1-S5 RE CH2-S3 IN CH3-S2 IN
52 30 30 30
Survey name
Borehole depth (m)
Number of receivers
Time sampling–trace length (ms)
Investigation depth (m)
Crosshole– P-velocity tomography CH1-A CC CH1-S5 RE CH2-S3 IN CH3-S2 IN
52 30 30 30
14 16 16 16
0.1–102.3 0.1–102.3 0.1–102.3 0.1–102.3
52 28 28 28
Survey name
Borehole depth (m)
Number of receivers
Time sampling–trace length (ms)
Investigation depth (m)
Downhole– P-velocity profiling DH1-A CC DH2-A CC
52 52
12 12
0.1–102.3 0.1–102.3
50 50
Survey name
Source type (kg)
Nbr and type of geophones
Time sampling–trace length
Maximum offset (m)
35 35 35 35
24 24 24 24
1 ms/2.048 s 2 ms/4.096 s 1 ms/2.048 s 2 ms/4.096 s 1 ms/2.048 s 2 ms/4.096s 1 ms/2.048 s 2 ms/4.096 s
20 20 20 20
Area
Area
Area
Surface wave testing (SWT) L1-SWT RE L2-SWT RE L3-SWT IN L4-SWT RE
2 1 1 1
Impact mass Impact mass Impact mass Imp. mass/minibang
Receivers–7 Hz; 4.5 Hz Receivers–7 Hz Receivers–7 Hz Receivers–7 Hz
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Table 3 Microtremor surveys: acquisition parameters Survey name
Area
Technique
Datalogger/sensors
Number of tests
Recording Time (h)
Microtremors survey MT–ASL MT–BM MT–OUT MT-DH MT–CA MT–L1 MT–L2 MT–L3 MT–L4
CC RE RE CC CC RE RE IN RE
H/V H/V H/V H/V H/V H/V H/V H/V H/V
MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s MARSlite/MO/LE-3D/5s
2 1 1 1 2 2 2 2 2
3, 24 3 1 3 5, 24 1, 1 1, 1 1, 1 1, 1
Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura Nakamura
4. Conclusions Geophysical techniques play a fundamental role in seismic zonation studies. Particularly, when the geology of the investigated area of interest is complex, fairly common in seismic areas where surface instabilities, faults, etc. usually complicates a great deal the geologic situation, they should be run extensively to limit the geometrical uncertainty in the representation of the subsoil model (Tables 2 and 3). Application of integrated geophysical surveys to the municipal area of Celano (L’Aquila, Italy), was very profitable in defining the main features of the subsoil below the historical centre and in areas designed for next urban expansion. We demonstrated that the historical centre is built on limestone breccia deposits, partially covered by more recent alluvial deposits. The shear velocity of shallow soils was determined up to 45 m depth, using high-resolution seismic borehole prospecting and SWT. The limestone breccia deposits are not present in the RE and IN areas, where the geological scenario is different. The results of our work have been further used for defining quantitatively the local site effects using seismic modelling analysis [8].
Acknowledgements The authors wish to thank the Abruzzo Region and the Celano Municipality for the financial support of the research. The authors are grateful to C. D’Alessandro who processed some of the microtremor recordings as part of her M.Sc. Thesis (‘‘Sapienza’’ Universita` di Roma). References [1] Technical Committee for Earthquake Geotechnical Engineering, TC4, ISSMFE. Manual for zonation on seismic geotechnical hazards, 1993. [2] Chavez-Garcia FJ, Raptakis D, Makra K, Pitilakis K. Site effects at Euroseis testI. Determination of the valley structure and confrontation of observations with 1D analysis. Soil Dyn Earthquake Eng 2000;19(1):1–22. [3] Chavez-Garcia FJ, Raptakis D, Makra K, Pitilakis K. Site effects at Euroseistest—II. Results from 2D numerical modelling and comparison with observations. Soil Dyn Earthquake Eng 2000;19(1):23–39. [4] Panza GF, Vaccari F, Romanelli F. Realistic modelling of seismic input in Urban Areas: A UNESCO-IUGS-IGCP project. Pure Appl Geophys 2001;158(12): 2389–406.
[5] Yilmaz O, Eser M, Berilgen M. A case study of seismic zonation in municipal areas. The Leading Edge 2006;25(3):319–30. [6] Pace B, Boncio P, Brozzetti F, Lavecchia G, Visini F. From regional seismic hazard to ‘‘scenario earthquakes’’ for seismic microzoning: A new methodological tool for the Celano Project. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2007.11.001. [7] Bianchi Fasani G, Cavinato GP, Petitta M, Scarascia Mugnozza G, Voltaggio M. The geological model of Celano town area for seismic microzonation activities. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/ j.soildyn.2008.05.006. [8] Pergalani F, Compagnoni M, Petrini V. Evaluation of site effects using numerical analyses in Celano (Italy) finalized to seismic risk assessment. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2008.05.004. [9] Martinelli A, Cifani G, Cialone G, Corazza L, Petracca A, Petrucci G. Building vulnerability assessment and damage scenarios in Celano (Italy) using a quick survey data-based methodology. Soil Dyn Earthquake Eng, submitted for publication, doi:10.1016/j.soildyn.2008.03.002. [10] Kramer SL. Geotechnical earthquake engineering. Englewood Cliffs, NJ: Prentice-Hall; 1996. [11] ASTM Standard D4428/D4428M-07 Standard test methods for crosshole seismic testing. ASTM Int, (2004). [12] Xia J, Miller RD, Park CB. Estimation of near-surface shear wave velocity by inversion of Rayleigh waves. Geophysics 1999;64(3):691–700. [13] Socco LV, Strobbia C. Surface-wave method for near surface characterization: a tutorial. Near Surf Geophys 2004;2(4):165–85. [14] Xia J, Miller RD, Park CB, Tian G. Determining Q of near-surface materials from Rayleigh waves. J Appl Geophys 2002;51(2–4):121–9. [15] Eurocode 8. Design provisions for earthquake resistance of structures. DD ENV 1998-1-1: 1996. [16] Bernabini M. Alcune considerazioni sui rilievi sismici a piccola profondita`. Bollettino di Geofisica Teorica ed Applicata 1965;7(26):106–18 (in Italian). [17] Koefoed O. Resistivity sounding measurements. Amsterdam: Elsevier; 1979. [18] Loke MH, Baker RD. Rapid least-squares inversion of apparent resistivity pseudosections, by a quasi-Newton method. Geophys Prospecting 1996; 44(1):131–52. [19] Park CB, Miller RD, Xia J. Multichannel analysis of surface waves. Geophysics 1999;64(3):800–8. [20] Milana G, Barba S, Del Pezzo E, Zambonelli E. Site response from ambient noise measurements: new perspectives from an array study in Central Italy. Bull Seismol Soc Am 1996;86(2):320–8. [21] Rix GJ, Lai CG, Foti S. Simultaneous measurement of surface wave dispersion and attenuation curves. Geotech Test J 2001;24(4):350–8. [22] Bard P. Microtremor measurements: a tool for site effect estimation? In: Irikura, Kudo, Okada, Sasatani, editors. The effects of surface geology on seismic motion, 1999. p. 1251–79. [23] Bard PY, Extracting information from ambient seismic noise: The SESAME project (Site EffectS assessment using AMbient Excitations)’’. European Project No. EVG1-CT-2000-00026 SESAME, 2002, /http:// sesame-fp5.obs.ujf-grenoble.fr/index.htmS Review meeting, Brussels, Belgium. [24] Telford WM, Geldart RP, Sheriff RE. Applied geophysics. 2nd ed. Cambridge: Cambridge University Press; 1990. [25] Cardarelli E, Bernabini M, Tommasi P. P and S waves cross-hole surveys, characterisation of the Orvieto hill formations. In: 72nd SEG Annual Meeting. Salt Lake City, 2002. p. 1468–1471. [26] Bernabini M, Cardarelli E. Variable damping factors in travel time tomography. J Appl Geophys 1997;38(2):131–41.