Journal of Applied Geophysics 46 Ž2001. 77–84
www.elsevier.nlrlocaterjappgeo
Application of the CMP refraction method to an archaeological
study žLos Millares, Almerıa,
´ Spain/
Beatriz Benjumea a,) , Teresa Teixido´ b, Jose´ Antonio Pena
˜ a,c
a
c
Andalusian Institute of Geophysics, UniÕersity of Granada, Granada, Spain
b
Cartographic Institute of Catalonia, Barcelona, Spain
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
archaeological site of Los Millares ŽAlmerıa,
´ 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
developed method, common-midpoint ŽCMP. 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. q 2001
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 application of geophysical methods to archaeology as
these non-destructive techniques provide subsurface
information that allows selective siting of follow-up
excavations ŽWynn, 1986.. To date, seismic methods
have not been widely used in archaeological investigations due to a low data acquisition rate compared
)
Corresponding author. Fax: q34-95-816-0907.
E-mail address: [email protected] ŽB. Benjumea..
to other geophysical methods and a relative lack of
resolution in the very shallow subsurface Ž0–10 m..
However, some seismic tomographic techniques have
been used to locate buried structures ŽWitten et al.,
1995. or to evaluate the state of preservation of
ancient monuments ŽBernabini et al., 1990; Cardarelli, 1995.. The seismic refraction method has
also been applied successfully to measure the thickness of the sediment fill in caves ŽWeinstein-Evron
et al., 1991. and to locate tombs inside tumuli ŽTsokas
et al., 1995.. The use of seismic reflection to archaeological studies ŽStright, 1986; Dobecki and Schoch,
0926-9851r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 0 0 . 0 0 0 4 1 - 0
78
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
1992. is mainly focused on deeper targets Ž20–30 m
depth..
This paper presents the application of a recent
seismic method, common-midpoint ŽCMP. refraction, in combination with a traditional refraction
technique Ždelay time method. to aid the planning of
future archaeological studies. The survey area ŽFig.
1. is the archaeological site ALos MillaresB ŽAlmerıa,
´
Spain., which belongs to the Copper Age ŽIII millen-
nium BC.. It consists of a Necropolis and a city,
surrounded by four defence walls. This site was
discovered during the 19th century ŽSiret, 1893. and
it has been partially studied and excavated ŽArribas
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
Fig. 1. Location of the archaeological site of Los Millares ŽAlmerıa,
´ Spain.. The seimic profile was carried out in the area limited by the
second wall ŽII. of the city, which is partially excavated.
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
this last aspect in order to avoid extensive excavations.
The paper presents the results of a 47.5-m long
profile located inside the second wall ŽFig. 1. where
superficial observations Žartefacts and microtopographic variations. indicate the possible existence of
foundations. The objectives of this work are the
following: Ža. to determine the usefulness of the
traditional seismic refraction and reflection methods
to provide information at this archaeological site, and
Žb. to examine a data analysis technique ŽCMP refraction. 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
developed during the Pliocene ŽFig. 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 characterized by deposits forming a large number of paleochannels with less extension and thickness than the
older formation. A calcareous crust Žcaliche. 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.
79
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
younger materials ŽFig. 2..
3. Seismic data
As a first step of the seismic study at Los Millares, a reflection seismic profile was acquired with
the purpose of obtaining information about the geological setting Ž10–30 m.. The result does not present a clear imaging of alluvial fans because the
noisy near-surface environment promotes severe
scattering, strong surface waves and static problems
in the data ŽBenjumea, 1999.. However, the multifold data acquired in the AreflectionB survey, were
used instead for a different purpose: to obtain information about local heterogeneities in the near surface
using a technique called CMP refraction. In addition,
a seismic refraction profile coincident with the reflection 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. Impacts were placed in the centre of a plate to provide
a good signal-to-noise ratio for frequencies higher
than 100 Hz ŽKeiswetter and Steeples, 1994.. A
geophone placed closed to the plate provided a reliable time zero. The chosen sample interval was 0.1
ms and the record length was 200 ms. Field filters
limited the recorded information outside the 32–1000
Hz frequency band. Elevation data points were
recorded every 3 m using a theodolite with an uncer-
80
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
The seismic refraction profile was carried out at
the same location as the multifold one. It was composed 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.
The high velocity of the calcareous crust Ž1.0–1.8
kmrs., compared to the surrounding materials, restricted the first arrival information to energy refracted along this layer. In this way, it is possible to
determine the velocity and layer thickness of nearsurface materials, which is a factor of great interest
for future excavations.
4.1. Method
Fig. 3. Off-end shot gather from the refraction profile. Note the
low amplitude of the head-wave along the caliche layer Žline. and
the anomalous arrivals between 5 and 15 m offset.
tainty of centimeters. The change in elevation along
the profile is 2 m.
The chosen method for interpreting refraction data
was the delay time method ŽPalmer, 1986.. This
technique yields strictly surface-consistent delay
times and produces a good long-wavelength solution
with a smooth velocity change of the refractor ŽDiggins et al., 1988..
First arrivals show very low amplitude as well as
anomalous arrivals ŽFig. 3.. The first characteristic
can be explained by the strong attenuation due to a
Fig. 4. Travel time–distance graphs of the first arrivals.
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
high velocity bed embedded in lower speed material
ŽSherwood, 1967.. The anomalous arrivals are examined 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
average of slopes in these curves Ž600 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
calculated for the direct and inverse branch ŽLawton,
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
by grey triangles. The lower velocities Ž1000 and
81
1200 mrs. are located at the south end of the profile
are interpreted to represent a higher degree of weathering than along the rest of the profile where velocities range between 1200 and 1800 mrs. This refractor is interpreted as the caliche layer.
5. CMP refraction
The refraction method provides information about
the depth of the caliche layer using the travel information and assuming a layered earth model. However, the technique is not appropriate for imaging
local anomalies, which are the objective of an archaeological study. To obtain information on the
heterogeneities within the first layer, the CMP refraction method, developed by Gebrande Ž1986. and
Orlowsky et al. Ž1998., was applied to take advantage 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.
Fig. 5. Velocity and depth model obtained applying the delay time method. The different gray colors of the triangles indicate the range of
velocity for the first refractor. The black circles show the ground level. The velocity for the first layer was established as 600 mrs. Zero
depth represents the altitude for the first geophone, used as reference.
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
82
5.1. Method
The method was described in detail by Orlowsky
et al. Ž1998.. The procedure starts with sorting the
traces into CMP gathers. On each of these gathers,
an identification of the refractors is made. The values
of velocity Ž Õi . for the refractor i and the shot-geophone distances for which the first break phases are
due to the refractor Žrange from x 1 i and x 2 i . are
used to apply a partial Radon transform F Žt CM P, pi .
to the CMP wavefield f :
x2i
F Ž t CM P , pi . s Ý f Ž t CMP q piCMP x , x . D x
x 1i
where D x is the distance between traces in the CMP
domain, pi is the average slowness corresponding to
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 pi in
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 section, imaging the refractor. This image can show the
inhomogeneities within the wave paths of the refracted 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 p s 6.66
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
Fig. 6. Ža. CMP gather and the result of applying a partial t –p transform for the range of ray parameter and offset chosen. This CMP shows
similar characteristics in amplitude and phase for the refracted wavetrain. Žb. Same as Ža. for a CMP gather formed by traces characterized
by disturbances and variation in the characteristics of the first train.
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
83
Fig. 7. Time-intercept section combined with the refraction model Ždashed grey line.. The main anomalies detected after applying the t –p
transform are indicated by arrows.
the application of the data analysis technique to two
CMP gathers. The first CMP gather ŽFig. 6a. shows
a continuous first arrival train with similar characteristic 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 heterogeneities 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 continuous refractor.
The vertical axis is intercept time, which has been
converted to depth assuming 600 mrs for the upper
layer. The depth to the caliche Žlayer 2. obtained by
the refraction model is superimposed as a grey dashed
line. The depth obtained by refraction model corresponds 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 using a fixed ray parameter Ž p . for layer 2, the observed 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 foundations, and hence may represent possible archaeological targets.
6. Conclusions
The application of traditional seismic refraction
and CMP refraction methods provides valuable information at the archaeological site of Los Millares
ŽAlmerıa,
´ Spain.. The refraction method allows estimation 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-velocity layer Žcaliche. results in critically refracted waves
travelling only along the near surface. On the other
hand, CMP refraction method highlights local het-
84
B. Benjumea et al.r Journal of Applied Geophysics 46 (2001) 77–84
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 archaeological 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
Ciencia ŽSpain..
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