Ž .Journal of Applied Geophysics 45 2000 19–32www.elsevier.nlrlocaterjappgeo

ž /Mapping soft soils in the Segura river valley SE Spain : a casestudy of microtremors as an exploration tool

J. Delgado a,), C. Lopez Casado b, A. Estevez a, J. Giner a, A. Cuenca c, S. Molina a´ ´a Departamento de Ciencias de la Tierra, UniÕersidad de Alicante, Ap. Correos 99, 03080 Alicante, Spain

b Departamento de Fısica Teorica y del Cosmos, UniÕersidad de Granada, 18003 Granada, Spain´ ´c Laboratorio de Carreteras, COPUT, Generalidad Valenciana, Ctra. Ocana srn, Alicante, Spain˜

Received 1 November 1999; accepted 4 May 2000

Abstract

This study analyses the spatial distribution of soft soils in a valley with a significant amount of recently accumulatedŽ .Late Pleistocene–Holocene sediments, but where the available geotechnical information on the subsurface is inadequate tostudy the entire valley. To analyse this distribution, we have used a new geophysical prospecting technique, which consistsof estimating the thickness of soft soils based on their resonance frequency. This frequency has been determined from theHrV ratios of microtremors measured at 180 sites in the valley. The results indicate the generalised presence of soft soilsalong almost the entire valley, with thicknesses of up to 67 m. Based on the interpolation of the results, we have drawn up amap showing the spatial distribution and thickness of the soft soils in the valley. This map is of great interest both for futureplans regarding the use of the soil in the region as well as for seismic zoning and soil-response studies. q 2000 ElsevierScience B.V. All rights reserved.

Keywords: Soft soils; Microtremors; HrV ratios; Frequency of resonance; Soil thickness; Geophysical exploration

1. Introduction

Soft soils are at the root of numerousgeotechnical problems due to their low density,high compressibility and low strength. Thesecharacteristics make them potentially collapsi-ble, causing huge settlements of foundations,even under low-magnitude loads. In seismic

Žareas, these soils amplify the shake effects site.effects , usually resulting in severe damage to

) Corresponding author. Tel.: q34-965-903-400; fax:q34-965-903-552.

Ž .E-mail address: jose.delgado@ua.es J. Delgado .

buildings sited over them. Therefore, knowledgeof the thickness of these soils, as well as theirspatial distribution within a region is of greatinterest for land-use planning and for civil engi-neering. However, abundant geotechnical infor-mation, frequently unavailable, is essential forsuch knowledge.

Recent studies have applied traditional seis-mology and seismic engineering methods toestimate the thickness of such soils and so

Žpartially solve this lack of information Ibs-von.Seht and Wohlenberg, 1999 . It is well known

that the frequency of resonance of a soil layerŽ . Ž .f is closely related to its thickness Z throughthe relationship fsVsr4Z, where Vs is the

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 00 00016-1

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3220

average shear velocity of the soil. Ibs-von SehtŽ .and Wohlenberg 1999 have studied both pa-

Ž .rameters f and Z and demonstrated that it ispossible to establish a direct functional relation-ship between them, without knowing Vs. Intheir study, they used the method of HrV spec-tral ratios of microtremors to determine the

Ž .frequency of resonance. Delgado et al. 2000have studied the viability of this technique as amethod of geophysical exploration. Their studyreveals that, based on the frequency of reso-nance, the thickness of the soil can be estimatedwith a margin of error of less than 15%. Boththe above studies considered a microtremor asany vibration of the soil which is of low ampli-tude and whose origin is neither seismic norexplosive. The definition thus includes perturba-

Ž .tions due to human activity traffic, motors asŽwell as those due to natural causes sea waves,

.wind, etc. .The aim of this study is to apply these meth-

ods to determine the distribution and thicknessŽof soft soils in the Segura river valley Alicante,

.SE Spain , an area where these types of soilsare known to exist but where the availablegeotechnical information is insufficient for suchdeterminations.

2. Geological framework

The study zone, the Bajo Segura basin, con-stitutes one of the Neogene-Quaternary depres-sions developed in the Betic CordilleraŽ .Montenat, 1977 . The basement of the basin iscomposed of limestones and marls of Triassic toCretaceous age. Present-day outcrops of thesematerials occur on slopes to the north and north-

Ž .west of the Segura valley Fig. 1 .The sedimentary fill of the basin comprises

Žmaterials from the Upper Miocene Upper Tor-.tonian right up to contemporary materials.

These are mainly composed of conglomerates,sandstones and marls. The latter exhibit signifi-cant quantities of gypsums during the MessinianŽ .Upper Miocene . The distribution of these ma-

terials during the Upper Miocene and Plioceneis controlled by the presence of the forelandŽ .emergent terrain to the north and west of thebasin and by the Mediterranean Sea to the southand east. Close to the slopes, alluvial fans havedeveloped, with accumulations of conglomer-ates and sandstones. Towards the south and eastof the basin, the presence of the open sea haspermitted the accumulation of littoral sedimentsŽ .sandstones and marls and deep depositsŽ .marls . During the Lower Pliocene, the BajoSegura fault, situated in the south of the basinŽ .Fig. 2 , underwent significant activity which

Želevated the southern part of the basin Alfaro,.1995 . From this time until the present day,

sedimentation in the basin has been continentalŽ .alluvial and fluvial , except in the extreme eastwhere the transition to the marine environmentoccurs.

Since its formation, the basin has been sub-ject to compression in a NNW-SSE directionŽ .Alfaro, 1995 . As a result, the sedimentary fillhas been deformed creating a broad syncline inwhich progressively younger sediments have

Ž .been deposited Fig. 2 .The youngest Quaternary sediments de-

posited in the basin are related with the sea-levelrise after the last glaciation and the simultane-ous activity of Segura river and of alluvial fans.They are of diverse origin. Thus, alluvial sedi-ments were accumulated by various alluvial fansderived from the surrounding mountains, partic-ularly along the northern border of the valley.

ŽThey are thick, detritic sediments gravels and.sands , with a progressive increase in fine sedi-Ž .ments fine sands, silts and clays in the distal

portions of the fans. These sediments frequentlydevelop carbonate crusts under the arid climaticconditions of the region and consequently showa certain degree of cementation. Towards thevalley interior, fluvial and palustrine sedimentsdominate, related with the dynamics of the Se-gura river. These sediments are finely detriticŽ .fine sands, silts and clays , frequently contain-ing remnants of organic matter. Next to themouth of the river, these sediments coexist with

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 21

Fig.

1.Si

mpl

ifie

dge

olog

ical

map

ofth

est

udy

zone

.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3222

Fig. 2. Cross-section of the Bajo Segura basin. The vertical dimension of the block has been plotted without vertical scale.

fine beach sands, evidencing the progressiveprogradation of the fluvial medium on the lit-

Ž .toral one Soria et al., 1999 .

3. Geotechnical properties of soft soils in thestudy zone

The geotechnical characteristics of these ma-terials are known from geotechnical boreholes

Ž .Delgado, 1997 . This information shows thatthe superficial part of the fluvial–palustrine andlittoral sediments of the valley must be consid-

Ž .ered soft soils Table 1 . Apart from someexceptions, they are characterised by the highcompressibility of the clayey and silty soils and

Žby the low relative density of the sand 40–.45% , which is highly susceptible to liquefac-Ž .tion Delgado et al., 1998 . Since most of the

Ž .available boreholes are shallow 20 m , it is not

Table 1Average mechanical properties of soils and soft rocks in the valley of the Segura river

3Ž . Ž . Ž .r kgrm Void ratio Qu MPa N SPT cc

( )Soft soils fluÕial, palustrine and coastal sedimentsaSand 1940 0.84 – 13 –

Silt 1950 0.77 0.067 10 0.0561Clay 1925 0.87 0.075 8 0.0561

( )Stiff soils alluÕial sedimentsŽ .Coarse gravel and sandy soils – – – refusal –

Ž .Fine fine sand, silt and clay 2080 0.52 0.313 23 –

Soft rocksŽ .Marls Weathered 2020 0.68 0.272 30 0.0541Ž .Marls Un-weathered 2160 0.47 0.642 refusal 0.0282

Sandstone 2440 – )1 refusal –Conglomerates – – )4 refusal –

Symbol explanation: r — bulk density; Qu — unconfined shear strength; c — consolidation coefficient.ca Ž .Density and void ratio estimated from average relative density of sand in the valley 40–45% .

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 23

known how deep these poor mechanical proper-ties persist; the few deep drillings carried outindicate the presence of soft soils down to 40 m.It should also be noted that the geotechnicalinformation available on the transition zones

Ž .between fluvial–palustrine sediments soft soilsŽ .and alluvial ones stiff soils; Table 1 is scarce.

The dynamic properties of soft soils are alsoŽ .known. Cano et al. 1994 measured Vs by

techniques of down-hole logging and dispersionof surface waves. Their results show that Vs is

Ž .low at the surface 85 mrs and increases to anaverage value of 200 mrs at a depth of 20 m.

Ž .CEDEX 1996 measured the degradation of theshear modulus and the increase of damping withincreasing deformation.

ŽOlder sedimentary rocks Tortonian to Pleis-.tocene show a greater degree of cementation

Ž .see Table 1 , which increases their resistanceand rigidity with respect to the younger sedi-

Ž .ments soft rocks and soft soils, respectively . Ina study on the seismic zoning in this valley,

Ž .Delgado 1997 demonstrated that the most su-perficial part of some sedimentary rocks underthe recent sediments exhibits a certain degree ofalteration, degrading their mechanical proper-

Fig. 3. Transfer functions of those sites exhibiting more than one resonance in spectral ratios and where geotechnicalinformation of the subsoil is also available. The solid line is the transfer function resulting from microtremor HrV spectralratios; the dashed line is the 1-D transfer function.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3224

Table 2Sites where geotechnical information is available and microtremors where also measured

Ž . Ž . Ž . Ž .Site name Thickness of soft soils m Frequency Hz Estimated thickness Eq. 4 m

DERRAM 40.6 1.16 45.7CORRO 38.7 1.4 36.1A2 13.7 3.6 11.0S2 42 1.16 45.7S1 40 1.34 38.2P1 41.9 1.46 34.3S3 43.5 1.28 40.4A3 35 1.59 30.8P2 44.7 1.34 38.2P7 4.1 7.57 4.3S4 44 1.34 38.2A4 35.6 1.53 32.3SCDN 35.3 1.53 32.3S5 27 1.83 25.8S6 33 1.46 34.3AG1 29.8 1.46 34.3AG2 40.5 1.34 38.2P4 35 1.34 38.2AG4 41 1.16 45.7AG6 16.3 2.01 22.9SF2 4.2 8.3 3.9SF1 8.2 4.27 8.9AG7 11.6 2.69 15.9ISI2 11.1 3.66 10.8IES-PA 29 1.46 34.3ED-CAL 40.5 1.34 38.2ED-SF 6.2 5.92 5.9

ties. This circumstance is quite frequent inPliocene and Pleistocene marls. According tothe above author, in such cases, it is appropriateto consider this portion of the rock and theoverlying sediments jointly in order to analyseany geotechnical or seismic response problemsin the soil. Finally, as indicated by their ele-

Žvated density and rigidity much higher than the

Table 3Statistics of fit Zs af b

Estimate Standard T value Probabilityerror level

Intercept 4.00938 0.04194 95.5978 0.0000Slope y1.25566 0.04766 y26.3444 0.0000

Standard error of estimation is 0.145; coefficient of corre-Ž 2. Ž .lation r is 0.965; intercept is equal to ‘‘log a ’’.

.rest of the materials in the basin , the basementmaterials of the basin comprise hard rocks.

4. Determination of the frequency of reso-nance of the soil: microtremor measurement

There are various means of measuring thefrequency of resonance of soil. One is the HrVspectral ratios of microtremors. Detailed de-scriptions of this method can be found in Naka-

Ž . Ž .mura 1989 and Finn 1991 . The reliability ofthis method to determine the frequency of reso-nance of a soil column has been studied both

Žnumerically Field and Jacob, 1993; Lermo and.Chavez-Garcıa, 1993; Lachet and Bard, 1994´ ´

Žand experimentally Duval, 1994; Field et al.,

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 25

Ž .Fig. 4. Plot of frequency of resonance Hz vs. soil thick-Ž .ness m .

.1995; Bard et al., 1997; Seo, 1998 . The resultsshow that the latter is more robust and is acheaper method for estimating the frequencyŽ .Bard, 1998 .

The practical application of this method re-quires the existence in the ground surface of alayer of relatively soft material overlying an-

Ž .other, more rigid layer or semi-space . Thiscontrast between the media is essential, since ifit is lacking, there is no resonance. This fact iswhat makes the method of the HrV ratios ofmicrotremors especially appropriate for thestudy of soft soils, as such soils always providea significant mechanical contrast with underly-ing materials and, therefore, a clearly identifi-able resonance. On the other hand, when thiscontrast is lacking, there is no resonance of thesurface layer and the resulting spectral ratios areflat.

Applying the microtremor HrV ratios, thetransfer function of a point located on the sur-face of the soft soils is obtained by dividing thespectrum of the horizontal components of themicrotremors by the spectrum of the verticalcomponent of the microtremors measured at thesame site. This transfer function has a maxi-mum, or amplification peak, for the frequencyof resonance of the soil. Lermo and Chavez-´

Ž .Garcıa 1993 applied this method in Mexico´and found that the frequency of resonance deter-mined from the HrV ratios is comparable tothat resulting from considering the one-dimen-

Ž .sional 1-D propagation of vertically incidentSH waves. Their results also demonstrate thatthis method offers no information on the har-monics of this frequency.

The measurement and analysis of mi-crotremors was performed as follows: a portableIDS-3602 station was used, with a triaxial ve-

Ž .locity Mark L4C 1 Hz sensor digitising thesignal at 250 samplesrs. No instrumental cor-rection is carried out, as the sensor componentsemployed have the same response character-istics in the frequency range of 0.3–20 Hz.Therefore, we focused the analysis of spectralratios on this frequency range only. At eachpoint, eight microtremor samples of 2-min dura-tion were taken at 2-min intervals. From eachsample, we eliminated those portions withstrong, local impulsive sources of large ampli-

Ž .tude traffic . The remaining measurements weredivided into windows of 4096 samples, whichwere corrected for the base line and the spec-trum was calculated. Before determining thespectral ratio, the spectra were smoothed with athree-point sliding window. The group of spec-tral ratios was used to determine the transfer

ŽFig. 5. Spectral ratios at sites on hard rock Triassic. Ž .limestones and on soft soils Holocene fluvial sediments .

See Fig. 1 to locate points in the figure.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3226

function of the site in accordance with the ex-Ž .pression Duval, 1994 :

2 2 2HrVs S qS r 2S 1( Ž .Ž . Ž .NS EW Z

Upon analysing the resulting spectral ratios,we observed that at some points, there was

Ž .more than one resonance peak Fig. 3 , causinguncertainty in the analysis since the resonancecorresponding to the soils investigated must beidentified. To solve this problem, we took thosesites with more than one resonance in the spec-tral ratios, and for which deep geotechnicaldrill-cores were available which cut through allthe recent sediments to reach the underlyingsedimentary rocks. We then calculated the theo-retical transfer function of the soil. The ninecases analysed all had altered marls beneath therecent sediments. Following the criteria of Del-

Ž .gado 1997 in the modelling, we have consid-Žered that both materials recent sediments and

.altered marls are soft soils. As shear velocitiesŽ .Cano et al., 1994 and curves of variation ofshear modulus and damping with strainŽ .CEDEX, 1996 were available for this study,we therefore performed the modelling using the

ŽSHAKE computer program Schnabel et al.,.1972 , which makes use of all these parameters.

Input motion was always chosen to be a pulseof amplitude 0.01 g.

The modelling results show that the theoreti-cal frequency of resonance of the studied soilscoincides with the resonance observed at higher

Ž .frequencies in the spectral ratios Fig. 3 . Incontrast, the resonance at lower frequenciescannot be explained by the modelling. We inter-

Žpret this frequency as due to another layer or.set of materials existing under the soils that we

studied but not considered in our calculations.This figure also clearly shows the aforemen-tioned fact that the HrV spectral ratios do not

offer any information on the harmonics of thefundamental frequency of resonance. Thus, theharmonics in the modelling have no correspond-ing resonance in the spectral ratios of the mi-crotremors.

5. Relationship between the frequency of res-onance of soft soils and their thickness

The calculation of the thickness of a soilbased on its frequency of resonance requiresinitially a series of control points where both the

Ž .thickness Z and the frequency of resonanceŽ .f of the materials to be studied are known.Based on these parameters, we establish a law

Žof correspondence of the type Ibs-von Seht and.Wohlenberg, 1999 :

ZsAfyB 2Ž .Realisation of a fit of the type indicated by

the above equation represents a simplification ofŽ .the problem. Initially, both variables f and Z

Ž .must be related by the shear velocity Vs .Numerous authors have studied Vs and deter-mined empirical relationships between this and

Žgeotechnical parameters of the soil Kramer,.1996 . Regardless of the relationship, all of

them clearly show that Vs increases with theX Žconfining pressure s Dobry and Vucetic,

.1987 . As the confining pressure is proportionalŽ .to the depth or soil thickness , Vs is also

proportional to soil thickness according to anexpression:

VssaZ b 3Ž .By introducing this expression into the relation-

Ž .ship fsVsr4Z, it is easy to reach Eq. 2 orthe relationship ‘‘ f–Z’’. The above equationsassume that the mechanical characteristics of

ŽFig. 6. Profiles showing thickness of soft soils and the spectral ratios obtained at sites on soft rocks Pliocene conglomerates. Ž— site 25 , stiff soils gravels and sand with calcareous crusts at sites 27, 31, 33, 75 and 76; sand and silts with calcareous

. Žcrust at sites 37 and 78 , and soft soils Holocene palustrine clays — sites 81 and 83 — and fluvial silty sediments — site.52 . See Fig. 1 to locate points in the profiles.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 27

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3228

the soil are constant within the studied layer andthat they do not vary in the study zone either.

ŽThis is the main drawback of the method Ibs-von Seht and Wohlenberg, 1999; Delgado et al.,

.2000 . In areas where the geological and me-chanical characteristics of the soil are highlyvariable, this relationship will necessarily be oflow quality.

In the Segura river valley, Delgado et al.Ž .2000 measured the frequency of resonancefrom microtremors at 23 points where Z wasknown. Microtremors have also been measuredin another four new sites where deep boreholeshave recently been drilled which reach the un-derlying sedimentary rocks. With these 27 pointsŽ .Table 2 , we calculated the coefficients of Eq.Ž . Ž2 , resulting in the following relationship Ta-

.ble 3; Fig. 4 :

Zs55.11 fy1.256 4Ž .The value of parameters A and B are slightlydifferent from those previously calculated by

Ž . ŽDelgado et al 2000 for this area As55.64.and Bsy1.269 . The value of B is close to

that determined by Ibs-von Seht and Wohlen-Ž . Ž .berg 1999 for the Rhine basin Bsy1.388 ,

whereas A is clearly lower in our study zoneŽ .55.11 vs. 96 . This similarity in the B valuesmay be an indication that this index is of lowvariability, while A would actually be a charac-teristic parameter for each region.

6. Results

There exists a notable contrast in the mechan-ical properties of the soft soils and hard rock.The method of the HrV spectral ratios of mi-

Žcrotremors is based on the existence of a soil or.rock layer of low rigidity overlying other more

rigid. In the case of sites located on rock, thiscondition is not met since no contrast existsbetween the material at the surface and those atdepth. Consequently, on outcropping hard rocks,the HrV spectral ratios are flat, without mean-ingful peaks. Fig. 5 presents the spectral ratios

Ž .obtained on hard rock Triassic limestones andon soft soils. It clearly shows that spectral ratiosare very different in each case and that themethod allows differentiation of each type ofmaterial.

According to data presented in Table 1, stiffsoils and soft rocks exhibit different properties,although mutual differences are not so sharp asin the previous case. Nevertheless, the spectralratios resulting from soft soil sites stand outclearly from the ones obtained from stiff soils or

Ž .soft rock sites 52 and 83 in Fig. 6 . Fig. 6shows the spectral ratios obtained at sites lo-cated on the three kinds of materials. The spec-tral ratios obtained at sites on soft rock and stiffsoil are quite similar. In the spectra shown,there exists the mentioned amplification peak atlow frequency, showing greater amplitude onstiff soils than on soft rocks. It is also observedthat the frequency of this peak is progressivelyhigher towards the south of the profiles. This isprobably related to the depth of the basin base-ment, which outcrops to the south of both pro-files: the thinner layer giving rise to this reso-nance would explain the higher frequency ofresonance as the basement becomes shallower.

When the spectral ratios in Figs. 5 and 6 arecompared, it is clear that the main differencebetween stiff soilrsoft rock and hard rock is theabsence of resonance at low frequencies at siteson the latter material. From the spectral ratiosobtained, it seems that the HrV method cannotbe used in this zone for distinguishing stiff soilsfrom soft rocks.

The frequency of resonance was determinedat 180 sites. Spatial interpolation of this fre-quency allowed the map in Fig. 7 to be drawn.This map shows that the frequency varies little

Ž .in the centre of the valley 1–1.5 Hz , whilst itincreases rapidly close to the valley edges andalluvial fans that flank the valley. The lowestfrequencies recorded were at the mouth of theriver, where they fell to as low as 0.85 Hz.

The map given in Fig. 7 is of great impor-tance for earthquake engineering and seismiczoning in so far as the resonance frequency is a

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 29

Fig.

7.R

eson

ance

freq

uenc

yof

soft

soils

inth

est

udy

zone

.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3230

key parameter in seismic design. This frequencywas obtained based on an initial excitation of

Ž .very low amplitude microtremors , for whichthe behaviour of the soil is clearly linear. It isfor this reason that the map is useful for seismiczoning for weak soil motion. Nonetheless, giventhat the method is not reliable for determining

Žthe level of amplification of the soil Lachet and.Bard, 1994; Bard, 1998 , the map would need to

be complemented by other data to zone the areafully.

The thickness of soft soils was obtained fromŽ .the previous map and Eq. 4 . A map based on

these data is shown in Fig. 8. To compare thequality of the estimates, we have also includedthe thicknesses of soft soils determined fromboreholes and core samples.

The maximum thickness determined for thesesoils is 67 m, in an area close to the mouth ofthe Segura river. The area of the valley wherethe soft soils are thicker is orientated roughlynorthwest–southeast, with thicknesses of over45 m. This may be due to a greater depth of thevalley before the accumulation of the morerecent sediments. Notwithstanding, the presence

Ž .of altered marls soft soils underneath the re-cent sediments in many parts of the valley doesnot allow a definitive interpretation of the fig-ure, as more core sample data are necessary todiscern what part of the soft soils belong torecent sediments and what part to altered marls.In some cases, since the estimated depths al-ways exceed the known maximum thicknessesof the recent fluvial, palustrine or littoral sedi-

Ž .Fig. 8. Overall distribution of soft soils in the Segura river valley contour level in meters . The thickness of soft soils atthose points where this depth is known from boreholes is also indicated.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–32 31

Ž .ments Late Pleistocene–Holocene , the entirethickness of these sediments must be classed assoft soil.

The transition between the alluvial fans andfluvial–palustrine sediments is generally sharpand devoid of the characteristic resonance ofsoft soils. This can be interpreted as the absenceor scarcity of this kind of soils in the alluvialsediments. In fact, to the north of the valley, inthe alluvial fan zone, the thickness of soft soilsdecreases quickly, corresponding to the substitu-tion of finer sediments by coarser ones, and thedevelopment of significant carbonate crusts ce-menting these detrital materials.

The basement rocks of the basin outcrop atseveral points in the western sector of the val-ley. The abrupt increase in the thickness of thesoft soils around them is an indication that thealluvial fan deposits starting from these base-ment outcrops are scarcely developed.

7. Conclusions

The Segura river valley contains a significantfill of sediments of recent accumulation. Fromthe available geotechnical information, it wasknown that some of these sediments were softsoils, but their exact distribution and thicknessin most of the valley were unknown. The HrVratios of microtremors were used to determinethe distribution of these soils as well as theirthickness. With this aim, we measured mi-crotremors at 153 sites in the valley. These datawere added to those of other 27 points that hadbeen previously used for establishing a quantita-tive relationship between the frequency of reso-

Žnance of the soil and its thickness f–Z rela-.tionship .

The results obtained have demonstrated thegeneralised presence of soft soils throughout thevalley, although their thickness varies irregu-larly. The substantial thicknesses estimated forthese soils in the zones of fluvial, palustrine andlittoral sedimentation imply poor geotechnicalcharacteristics for the whole recent-sediment fill

Ž .Late Pleistocene–Holocene . In the transitionzones between the alluvial fans and thefluvial–palustrine sediments, where there isscarce geotechnical information, we have alsonoted the presence of these soils, although theirthickness is smaller.

The interpolation of these results has allowedus to draw up a map which reflects the spatialdistribution of soft soils in the entire study zone,including their frequency of resonance andthickness. These maps are of great interest as aninitial document for planning future uses for thesoil in this area, as well as for the preliminaryestimation for the foundation depth of publicworks. Moreover, it is useful for seismic zoningbecause it denotes the boundaries of materialsmore susceptible to amplify soil movementsduring an earthquake, and because it offers in-formation on both static and dynamic character-istics, namely the thickness and frequency ofresonance, respectively.

These results demonstrate the utility of themethod of the HrV ratios of microtremors inthe investigation of soft soils, as it rapidly andaccurately determines the frequency of reso-nance of the soil at low cost. Nonetheless, whenstiff soils overlie soft rocks, this method is notsuitable. The validity and practical usefulness ofestablishing an f–Z type relationship should benoted. Its subsequent use together with the fre-quency of resonance obtained from the HrVratios of microtremors allows the thickness ofsoft soils to be quickly and reliably determined.

Acknowledgements

This work has been supported by researchproject PB96-0327. We wish to thank C.

Ž . ŽDomenech ITC SA and A. Corral SOSAIMA´.SL for the geotechnical information that made

this study possible, and L. Macone for his col-laboration during microtremors measurement.We also wish to thank T.L.Youd, M. Ibs-vonSeht and J. Rector for their constructive com-ments and suggestions.

( )J. Delgado et al.rJournal of Applied Geophysics 45 2000 19–3232

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