www.elsevier.nlrlocaterjappgeo
Application of the CMP refraction method to an archaeological
ž
/
study Los Millares, Almerıa, Spain
´
Beatriz Benjumea
a,), Teresa Teixido
´
b, Jose Antonio Pena
´
˜
a,ca
Andalusian Institute of Geophysics, UniÕersity of Granada, Granada, Spain
b
Cartographic Institute of Catalonia, Barcelona, Spain
c
Department of Prehistory and Archaeology, UniÕersity of Granada, Granada, Spain
Received 3 March 2000; accepted 30 October 2000
Abstract
Obtaining information at an archaeological site by means of geophysical methods can reduce the need for intensive excavation. This paper addresses the use of seismic methods to reveal details in a non-destructive manner at the
Ž .
archaeological site of Los Millares Almerıa, Spain . The seismic refraction method provides information on the low´ frequency component of the model for the shallowest layers. In this way, it is possible to fix the thickness of the surface layer, as well as to determine a velocity model. Use of the refraction method in Los Millares has resulted in the determination of the depth of the calcaric surface upon which the foundations were built. The application of a recently
Ž .
developed method, common-midpoint CMP refraction, allows the detection of local heterogeneities in the near subsurface. This method uses the amplitude, phase and frequency information of the first arrivals. The results highlight the location of anomalous zones characterized by early first arrivals. According to a priori geological and archaeological information, these anomalies can be correlated with buried foundations providing the key information for planning future excavations.q2001
Elsevier Science B.V. All rights reserved.
Keywords: Archaeology; Seismic methods; Refraction seismics; Radon transform
1. Introduction
There has been an increased interest in the appli-cation of geophysical methods to archaeology as these non-destructive techniques provide subsurface information that allows selective siting of follow-up
Ž .
excavations Wynn, 1986 . To date, seismic methods have not been widely used in archaeological investi-gations due to a low data acquisition rate compared
)Corresponding author. Fax:q34-95-816-0907.
Ž .
E-mail address: [email protected] B. Benjumea .
to other geophysical methods and a relative lack of
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resolution in the very shallow subsurface 0–10 m . However, some seismic tomographic techniques have
Ž
been used to locate buried structures Witten et al.,
.
1995 or to evaluate the state of preservation of
Ž
ancient monuments Bernabini et al., 1990;
Car-.
darelli, 1995 . The seismic refraction method has also been applied successfully to measure the
thick-Ž
ness of the sediment fill in caves Weinstein-Evron
. Ž
et al., 1991 and to locate tombs inside tumuli Tsokas
.
et al., 1995 . The use of seismic reflection to
archae-Ž
ological studies Stright, 1986; Dobecki and Schoch,
0926-9851r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.
Ž .
. Ž
1992 is mainly focused on deeper targets 20–30 m
.
depth .
This paper presents the application of a recent
Ž .
seismic method, common-midpoint CMP refrac-tion, in combination with a traditional refraction
Ž .
technique delay time method to aid the planning of
Ž
future archaeological studies. The survey area Fig.
. Ž
1 is the archaeological siteALos MillaresB Almerıa,
´
. Ž
Spain , which belongs to the Copper Age III
millen-.
nium BC . It consists of a Necropolis and a city, surrounded by four defence walls. This site was
Ž .
discovered during the 19th century Siret, 1893 and
Ž
it has been partially studied and excavated Arribas
.
et al., 1985 . Nowadays, work is focused on the preservation of the excavated structures, but there is still interest in discovering new sites to extend the understanding of the evolution of the former city. Non-destructive methods are required for performing
Ž .
Fig. 1. Location of the archaeological site of Los Millares Almerıa, Spain . The seimic profile was carried out in the area limited by the´
Ž .
this last aspect in order to avoid extensive excava-tions.
The paper presents the results of a 47.5-m long
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profile located inside the second wall Fig. 1 where
Ž
superficial observations artefacts and
microtopo-.
graphic variations indicate the possible existence of foundations. The objectives of this work are the
Ž .
following: a to determine the usefulness of the traditional seismic refraction and reflection methods to provide information at this archaeological site, and
Ž .b to examine a data analysis technique CMP re-Ž .
fraction to reveal archaeological structures.
2. Geological and archaeological setting
The archaeological site of Los Millares is located on a plateau, formed by two different alluvial fans
Ž .
developed during the Pliocene Fig. 2 . The lowest one is composed of conglomerates with a fine matrix alternating with coarser materials, which form broad and thick paleochannels. The upper layer is charac-terized by deposits forming a large number of pale-ochannels with less extension and thickness than the
Ž .
older formation. A calcareous crust caliche overlies
Fig. 2. Geologic sequence at the archaeological site of Los Millares. The upper part has been enlarged to show the foundation positions above the caliche layer.
these materials. The city of Los Millares was built above this layer. Wooden buildings were constructed on circular foundations made from this caliche. The foundations are all that have been preserved and are usually 0.5 m high and 1 m wide. The caliche and remaining foundations are now buried by 1–2 m of
Ž .
younger materials Fig. 2 .
3. Seismic data
As a first step of the seismic study at Los Mil-lares, a reflection seismic profile was acquired with the purpose of obtaining information about the
geo-Ž .
logical setting 10–30 m . The result does not pre-sent a clear imaging of alluvial fans because the noisy near-surface environment promotes severe scattering, strong surface waves and static problems
Ž .
in the data Benjumea, 1999 . However, the multi-fold data acquired in the AreflectionB survey, were used instead for a different purpose: to obtain infor-mation about local heterogeneities in the near surface using a technique called CMP refraction. In addition, a seismic refraction profile coincident with the re-flection one was carried out to obtain a background velocity and depth model for the first meters of the subsurface.
The multifold seismic profile, covering a total distance of 47.5 m, was acquired with ninety-six 40-Hz geophones. The receiver interval was 0.5 m, with shot locations spaced every 1 m along the entire profile. The recording instrument was a BISON 9000 series seismograph hooked to a roll-along box. Forty-eight geophones were activated for each shot position using a split-spread geometry and 0.75 m source nearest receiver separation. The source was an 8-kg sledgehammer with five shots stacked. Im-pacts were placed in the centre of a plate to provide a good signal-to-noise ratio for frequencies higher
Ž .
uncer-Fig. 3. Off-end shot gather from the refraction profile. Note the
Ž .
low amplitude of the head-wave along the caliche layer line and the anomalous arrivals between 5 and 15 m offset.
tainty of centimeters. The change in elevation along the profile is 2 m.
The seismic refraction profile was carried out at the same location as the multifold one. It was com-posed by two spreads with 48 receiving stations and a total of 22 shot gathers were acquired.
4. Seismic refraction
The seismic refraction method constrains the depth and the seismic velocity of the shallow subsurface.
Ž
The high velocity of the calcareous crust 1.0–1.8
.
kmrs , compared to the surrounding materials, stricted the first arrival information to energy re-fracted along this layer. In this way, it is possible to determine the velocity and layer thickness of near-surface materials, which is a factor of great interest for future excavations.
4.1. Method
The chosen method for interpreting refraction data
Ž .
was the delay time method Palmer, 1986 . This technique yields strictly surface-consistent delay times and produces a good long-wavelength solution
Ž
with a smooth velocity change of the refractor
Dig-.
gins et al., 1988 .
First arrivals show very low amplitude as well as
Ž .
anomalous arrivals Fig. 3 . The first characteristic can be explained by the strong attenuation due to a
high velocity bed embedded in lower speed material
ŽSherwood, 1967 . The anomalous arrivals are exam-.
ined in more detail with the application of the CMP refraction method in the following section.
The travel time curves are displayed in Fig. 4 where two different layers can be seen. Due to the differences in altitude of the ground level, it was necessary to apply a topographic correction as the first step in the application of the method. A constant velocity for the first layer was calculated as the
Ž .
average of slopes in these curves 600 mrs . The velocities for the second layer were changed until the curve of delay times for direct and inverse shots were parallel. For shots located inside the spread, the velocity is obtained as the average of the velocities
Ž
calculated for the direct and inverse branch Lawton,
.
1989 .
4.2. Results and interpretation
Fig. 5 shows the depth model obtained from delay time method. The refractor dips gently to the south between 1 and 26 m, generally following the ground level. A small depression is observed between 35 and 45 m. Different layer 2 velocities are indicated
Ž
by grey triangles. The lower velocities 1000 and
.
1200 mrs are located at the south end of the profile are interpreted to represent a higher degree of weath-ering than along the rest of the profile where veloci-ties range between 1200 and 1800 mrs. This refrac-tor is interpreted as the caliche layer.
5. CMP refraction
The refraction method provides information about the depth of the caliche layer using the travel infor-mation and assuming a layered earth model. How-ever, the technique is not appropriate for imaging local anomalies, which are the objective of an ar-chaeological study. To obtain information on the heterogeneities within the first layer, the CMP
refrac-Ž .
tion method, developed by Gebrande 1986 and
Ž .
Orlowsky et al. 1998 , was applied to take advan-tage of the multifold geometry of the reflection data set. This method has been applied successfully to engineering and environmental targets. The most important aspect of this method is that it uses the amplitude, phase and frequency characteristics of the first arrival wavetrain to get information about the shallowest layers.
5.1. Method
The method was described in detail by Orlowsky
Ž .
et al. 1998 . The procedure starts with sorting the traces into CMP gathers. On each of these gathers, an identification of the refractors is made. The values
Ž .
of velocity Õi for the refractor i and the shot-geo-phone distances for which the first break phases are
Ž .
due to the refractor range from x1 i and x2 i are
Ž CM P .
used to apply a partial Radon transform Ft , p
i to the CMP wavefield f :
x2 i
CM P CMP CMP
F
Ž
t , p.
sÝ
fŽ
t qp x , x.
Dxi i
x1i
whereDx is the distance between traces in the CMP
domain, p is the average slowness corresponding toi Õi and t is the intercept time. This partial t–p transform enhances the signal-to-noise ratio of the
critically refracted wavetrain corresponding to p ini the CMP domain. The result of this application is a stacked trace in the t–p domain where the first
arrival is the intercept time. Proceeding in the same way for each CMP produces an intercept-time sec-tion, imaging the refractor. This image can show the inhomogeneities within the wave paths of the re-fracted waves.
5.2. Results and interpretation
After identifying the different refractors in the CMP domain, the chosen parameters for the layer 2 were an offset range between 6.75 and 9.75 m and a value for the average horizontal slowness of ps6.66 10y4 srm. In this way, the target of the CMP
refraction application is the first refractor identified as the calcareous layer or caliche. Fig. 6 illustrates
Ž .
Fig. 6. a CMP gather and the result of applying a partialt–p transform for the range of ray parameter and offset chosen. This CMP shows
Ž . Ž .
Ž .
Fig. 7. Time-intercept section combined with the refraction model dashed grey line . The main anomalies detected after applying thet–p transform are indicated by arrows.
the application of the data analysis technique to two
Ž .
CMP gathers. The first CMP gather Fig. 6a shows a continuous first arrival train with similar character-istic in amplitude and phase. Fig. 6b is an example of a CMP gather characterized by disturbances in the first arrivals, which are attributed to local hetero-geneities above the refractor.
Fig. 7 shows the intercept-time section that results from the application of the CMP refraction method. Two zones can be distinguished on the basis of differences in amplitude and in the continuity of the refractor. Between 5 and 36 m, the first arrivals show irregularities corresponding to a heterogeneous medium where some zones depict phase changes and anomalous arrivals at earlier times than the main refraction, especially at the positions 6.5–8, 11.5–16 and 30.5–36 m. The northern part of the profile
Ž)37 m shows arrivals with similar characteristics.
both in phase and amplitude, which indicates a con-tinuous refractor.
The vertical axis is intercept time, which has been converted to depth assuming 600 mrs for the upper
Ž .
layer. The depth to the caliche layer 2 obtained by the refraction model is superimposed as a grey dashed line. The depth obtained by refraction model corre-sponds well with the refractor on the intercept-time section, suggesting that both methods can be used to
determine the depth to the top the caliche. As well, the intercept-time section shows zones of first arrival irregularities. Because the partial t–p transform
us-Ž .
ing a fixed ray parameter p for layer 2, the
ob-served anomalies should correspond to near surface zones characterized by velocities higher than the surrounding background values. This suggests that these anomalies could be caused by buried founda-tions, and hence may represent possible archaeologi-cal targets.
6. Conclusions
The application of traditional seismic refraction and CMP refraction methods provides valuable in-formation at the archaeological site of Los Millares
ŽAlmerıa, Spain . The refraction method allows esti-
´
.mation of the thickness of near surface materials, which is of interest for archaeologists. The efficiency of this method for archaeological purposes strongly depends on the geological conditions. In the data presented in this paper, the presence of a
high-veloc-Ž .
het-erogeneities in the shallow underground, based on the differences of the character of the first arrivals. The anomalies correspond to near surface zones with velocities higher than their surroundings, and can be correlated with buried foundations in the survey area. This method has potential use at any archaeological site where targets may be associated with shallow velocity anomalies. The use of seismic methods can provide information in areas where other geophysical methods fail due to the geological and environmental conditions. This is one of the main advantages of developing the seismic techniques focused on ar-chaeological problems, in spite of the time effort that seismic data acquisition and processing require.
Acknowledgements
We thank Marıa Lujan for her help in acquiring
´
´
seismic data and Susan Pullan for her comments. We are grateful to Peter Weidelt and Gregory N. Tsokas for their review and suggestions. This study was financially supported by the AMB 97-1113-C02-02 project. Funding for this work was provided by a grant awarded to B.B. by Ministerio de Educacion y
´
Ž .
Ciencia Spain .
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