Archaeological and groundwater investigations

Archaeological and
Groundwater
investigations using
Seismic Methods
By
Zaidoon T. Abdulrazzaq

Seismic methods are the most effective and expensive of all the
geophysical techniques used to investigate layered media. Seismic
investigations utilize the fact that elastic waves (also called seismic waves)
travel with different velocities in different rocks. By generating seismic waves
at a point and observing the times of arrival of these waves at a number of
other points on the surface of the earth, it is possible to determine the velocity
distribution and locate subsurface interfaces where the waves are reflected or
refracted.
The reflection and refraction survey methods are widely applied to
studies of crustal structure, geological correlation of layered sequences and
mapping of structures in the uppermost crust, and to hydrocarbon exploration.
In the same way, but on a smaller scale, these methods are applied to
environmental and geotechnical problems such as location of water table,
water bearing fracture zones, cavities and sinkholes, delineation of faults and
fractures, archaeological prospection and site investigations of foundation
conditions including determination of depth to bedrock.
In this Seminar, the main focus will be on shallow seismic methods and their
applications for archaeological site and groundwater.
Introduction

1. Applications of seismic methods in Archaeological prospection
Archaeological prospection or (Archaeogeophysics) includes the
diverse geophysical methods employed either in planned excavations to
guide the archaeological research in advance and during the excavation
process or in salvage excavations to provide a rapid assessment during
the development of infrastructures in urban or rural environments. In a
more general basis these techniques are also applicable to support
regional archaeological surveys by locating areas of archaeological
interest and contributing in the settlement pattern analysis (Sarris and
Jones, 2000). Unlike the destructive nature of the archaeological
excavations, geophysical prospection techniques are non-invasive,
providing at the same time a rapid reconnaissance of a site without
disturbing the ground or the monuments themselves. The success or the
failure of these techniques strongly depends on the contrasting physical
properties that exist between the archaeological buried targets and the
hosting material.

The early efforts of geophysical prospection methods for mapping
concealed cultural remains, in terms of measuring the subsurface
earth resistance, date back to late 1930s and early 1940s in USA and
Britain respectively (Linford, 2006). After a period of experimentation,
archaeological prospection presents an abrupt increase during the
1960s and 1970s. This period, known as the “Golden Period” of
archaeological prospection, is closely connected to the design and
development of specialized field instrumentation sensitive to the
relatively low-amplitude / short-wavelengths archaeological signals and
the introduction of dedicated algorithms for the data treatment and
image processing (Scollar et al.,1986).

Geophysical methods can have an integrated part within wider geo-
archaeological projects aiming to outline the extent of settlements or
explore the wider limits of the habitation in a region (Sarris et al., 2014).
Seismic refraction and Electric resistivity techniques ERT (Fig. 1), have
been used in modeling the vertical stratigraphy and mapping the bedrock
of an area in an ultimate effort to study possible locations of ancient ports
(Vafidis et al., 2005) or reconstruct geomorphological characteristics and
their significance for human occupation (Siart et al. 2009).
Geo-Archaeological applications

Fig. 1: a) Layout of the
geophysical grids, ERT lines
and seismic refraction
transects that have been
completed during the
geoarchaeological project in
Istron at the east part of
Crete. b) Velocity depth
slices for the depths 10m
and 50m below the ground
surface. The cross-sectional
velocity distribution along
the dotted lines A and B is
also presented (Shahrukh et
al., 2012).

3D seismic acquisition has been employed for more than 20
years in the hydrocarbon industry but has had limited application in
near surface archaeological and engineering surveys. In
archaeological work, the objective is commonly to locate buried
architectural structures and small-scale man-made features at
shallow depths of penetration (typically 5 m). Object detection at
such shallow depths requires ultra-high-resolution seismic
acquisition and processing methods. These include the use of high
frequency, broadband seismic sources and processing routines that
are geared to maximizing seismic resolution. These requirements
and the need for specialized seismic processing hardware have
largely limited the application of 3D seismic methods for many
near-surface applications.
3D seismic acquisition

The recent advent of more powerful personal computers and
PC based seismic processing and interpretation software has made
ultrahigh-resolution 3D seismic surveys feasible for marine
archaeological work. Whilst the resolution requirements of
archaeological and engineering surveys are significantly greater
when compared to hydrocarbon exploration, at the same time the
survey areas are much smaller, and thus the amount of data
collected remains comparable to that typical in industry (i.e. tens or
even hundreds of giga bytes). The advances in computer technology
have thus stimulated the development and adaptation of 3D seismic
acquisition in shallow geophysical studies. It is now possible to
employ 3D seismic imaging for object detection and mapping of
archaeological structures below the sea floor in full three dimensions.

Between 2004 and 2006 an ultra-high-resolution marine 3D seismic acquisition system
was also developed at the Christian Albrechts-University in Kiel for detailed archaeological
site investigation in very shallow water (Fig. 2). The project received the acronym
SEAMAP-3D, which stands for SEISMO Acoustical Marine Archaeological Prospection in
3D. It is funded by the German Ministry of Education and Research (BMBF).
Fig. 2: SEAMAP-3D ultra-high-resolution seismic acquisition prototype on a lake in
Northern Germany. A refitted catamaran is used as the acquisition vessel. It is
towing the boomer seismic source and the rigid frame hydrophone array

The seismic method traditionally is used for deep exploration and has
been less successful at imaging the near surface due to its low frequency and
noise content (Aitken and Stewart, 2004). GPR conversely is considered to be
a viable geophysical tool in the shallow section due to its inherent high
frequency which translates to finer resolution. The disadvantage of GPR is the
inability to see deeper into the subsurface (Daniels, D.J., 2004). Fig. 3 presents
a display of the GPR and seismic micro-surveys both converted to depth using
their appropriate velocities. It serves to illustrate how much better the resolution
and definition of the GPR section is compared to the seismic micro-survey
record from a depth range of 0.7 – 1.75 m. However, the depth of penetration of
the seismic is superior to that of the GPR. The seismic micro-survey display
shows recorded events up to 45 ms, an additional 25 ms below that of the
GPR. This essentially translates into additional data that the GPR was unable
to “see”. A combination of the two would undoubtedly be advantageous to
archaeologists. Continued research at the University of Calgary will attempt to
do just that by focusing on extracting more detail from the 3C-3D seismic micro-
survey through processing, and applying more rigorous acquisition, processing
and modelling techniques to the GPR data (Kaprowski. and Stewart, 2004).
The GPR and the Seismic Micro-Survey

Fig.3: Comparison of the seismic micro-survey (left) and GPR (right) at depths 0.7-1.75 m.

Marine seismic imaging
In marine seismic imaging, an acoustic source and receivers are
towed behind a ship. The source emits an acoustic pulse that travels through the water and
is reflected from the seabed and subsequent layers of the subsoil (fig. 1). The reflected
energy intensity depends on the different densities of the seabed and sub-seafloor layers,
the denser (i.e.) the seabed or layer the stronger the reflected signal. The reflected signal
then travels harder back through the water to the receiver (fig. 4). The received signals are
recorded, and as the ship constantly moves this will result in a vertical cross section through
the seabed. So-called reflectors on the seismic image mark the boundary between two
distinct subsurface layers.
In order to image the shallow subsurface with the highest possible detail (and to
allow the detection of small buried objects) a high vertical resolution is needed. This implies
the use of high frequency acoustic (seismic) sources, such as boomers and echo-sounders.

Boomer sources have a frequency ranging between 2 and 5 kHz, and offer a good
compromise between resolution (20- 50 cm) and penetration (tens of meters up to
hundred m or more). Echo-sounder sources are generally marked by a higher
frequency (4 to 10 kHz), resulting in an increased resolution (10-20 cm) but often also
a decrease in penetration depth (Missiaen 2008).
Fig. 4 Schematic diagram
showing the principle of
marine seismic imaging. A
seismic source emits a
sound pulse that is
reflected against the sea
floor and deeper layers; the
reflected signals are
recorded by a towed
receiver. As the boat moves
continuously, this results in
a vertical image across the
seabed along the ship’s
trajectory (Missiaen 2010).

Case study 1
Seismic imaging of a buried wooden shipwreck, Wadden Sea
In July 2003 a seismic survey was carried out over a buried wooden shipwreck
(‘Scheurrak SOI’) in the Dutch Wadden Sea (Fig. 5), 25 km NE of Den Helder. The wreck is roughly
25 m long and dates presumably from the late 16th century13 (Manders, 2000). From 1989 to 1997
the wreck has been excavated by divers of the RACM (The Netherlands) - at that time the wreck
was largely exposed on the sea floor (Fig.6). Results showed that the wreck had been broken
lengthwise (the starboard side had broken of the hull of the ship) (Rijkswaterstaat, 2003).
Fig. 5 Left Seismic network (thin blue
lines) recorded over the wreck site in
the Wadden Sea. Black lines = wreck
contours (recorded in excavation when
the wreck was still exposed). Right:
Multibeam recording over the buried
wreck. The wreck is now completely
covered by sediments. Black lines =
wreck contours. Red dots = short poles
used by divers in 1990 to use as an
external measuring grid. Thick blue line
= location of the seismic profile shown in
fig. 6. Courtesy RWS (Missiaen, 2010).

Fig. 6: Example of a 2D
seismic profile (top) and
interpreted line-drawing
(bottom) across the
wreck site in the Wadden
Sea (for location of the
profile see fig. 5). Depth
below the water surface
in milliseconds two-way
travel time (2 ms = 1.5
m). The profile crosses
the wreck more or less
obliquely and the wreck
outline is clearly
observed. The large
diffraction at the bottom
is possibly related to the
ship’s rudder (Missiaen,
2010).

Case study 2
Seismic investigations of Raversijde
The Provincial Domain of Walraversijde, between Ostend and Middelkerke, has been the
focus of a large-scale archaeological research project set up by the Flemish Heritage Institute
(VIOE) and the province of West-Flanders. The main archaeological findings include the remnants
of a Late Medieval settlement, both on the beach (see fig. 7: left and more inland in the dune area
(Pieters, 1992). Due to severe coastal erosion the first settlement, dating from the late 13th
century, was lost to the sea and relocated behind a dyke in the early 15th century. In September
2005 part of a buried Roman dyke was discovered in Walraversijde. The dyke is over 11 m wide
and a little over 1 m high, and has a total length of at least 110 m (Pieters et al. 2006). The dyke is
mainly built of stacked clay blocks, on its western side reinforced with peat (fig. 7: right). The dyke
is oriented roughly perpendicular to the present coastline, which suggests that its purpose was
most likely to embank a tidal gully that stretched further inland (Pieters et al. 2006). In the 70’s
traces of the Roman dyke have been found on the beach of Raversijde (Pieters, 2007).

Fig. 7 Left: Ground plan of a late medieval house on the beach of Raversijde (After
Chocqueel, 1950). Right: Excavation of the Roman dyke in Walraversijde. The dyke is made
up of carefully stacked clay blocks. Wedge-like peat sods (dark color) were used to reinforce
the western side of the dyke, in the forefront a horizontal cross-section of the same peat
sods. The latter clearly indicate three successive reinforcement phases of the dyke.

Applications of seismic methods in
Groundwater investigations
Seismic method plays a major role in groundwater exploration.
Refraction seismic method is more useful in groundwater investigations
in comparison with reflection method because it generally aims at
determining shallow subsurface structures and fractured zones, if any,
in the bedrock (Sigmund 1990). An investigation employing refraction
seismic method was carried out to understand the structures favorable
for groundwater potentiality. Deep aquifers of fractured biotite gneiss
within the bedrock below sandstone have been investigated for
possible potential groundwater locations. Survey and a sample
seismogram of traverse are given in figures 8 (a and b).

Fig. 8: (a) Spread geometry of seismic traverses; (b) Sample seismogram of traverse

The kind of hydro-geological information can be obtained
from seismic prospecting
The propagation velocity of P-waves depends strongly on the
porosity and the water saturation of sediments. S-wave velocity or the
shear modulus, respectively, is mainly determined by the stiffness of the
rock matrix. In consequence, both P-and S-wave velocities depend
significantly on the fracture density of rocks. Therefore, seismic
investigations can contribute basically in different regards to
hydrogeological investigations: To find the groundwater table or define
bed rock levels appear self-evident tasks (Fig. 9). In a more general
sense, seismic investigations can serve to determine the structural and
lithological framework of hydrogeological studies and to quantify the
heterogeneity of aquifers and aquitards off boreholes. In addition, porosity
and fracture density can be investigated (Kirsch, et al., 2006)

The advantages and disadvantages of seismic measurements
compared to other methods
Seismic prospecting provides reliable information on the depth of interfaces. In
comparison to electromagnetic induction and DC-geoelectrical measurements, penetration
depth and structural resolution of seismic measurements are usually higher and less
ambiguous. These advantages are paid for with higher costs in acquisition and
interpretation. The relation of vP and vS to rock parameters such as porosity and pore fill
is not unique. Therefore, seismic measurements have to be combined with other types of
geophysical methods if sedimentary parameters are to be determined in situ. In
hydrogeophysics it is most advantageous to combine geoelectrical and seismic methods
to determine. The sequence “drill hole - seismics DC geoelectrics - EM-induction” can be
regarded as a sequence of decreasing structural resolution and investigation costs.
(Kirsch, et al., 2006).

Fig. 9: Seismic structure of the
weathering zone in a hard rock area
based on the application of P-waves.
Top: Tomographic interpretation of
refracted arrivals. The image shows a
cross-section in terms of P-wave
velocity vP where unconsolidated
material is indicated by low vP values.
Bottom: Corresponding seismic
reflection section showing the complex
layering within the vadoze zone.
Velocity contours for vP = 400, 800 and
1200 m/s are indicated by dashed
lines for comparison (by courtesy of
GeoExpert AG, Schwerzenbach,
Switzerland) (Kirsch, et al., 2006)

Some Applications of shallow seismic methods to
groundwater investigations
In the following, various applications of the shallow seismic methods are demonstrated by a
number of examples.
1. Studying aquifer structure
The investigated site is located in the western Mesaoria area of Cyprus, in the vicinity of
the village of Meniko. The upper part of the geological section in the area is composed mainly of
Pliocene to Middle Miocene marls of the Nicosia formation. The formation is subdivided into two
units. Between the two units, a phreatic aquifer is developed in a clastic horizon consisting of
gravels and sands. The depth to the aquifer layer is about 200 m and its thickness varies from 0
to 80 m. The area suffers from an insufficient supply of fresh groundwater, and it is therefore
important to study the inner structure of the aquifer in the area. For this purpose, a high
resolution seismic reflection survey was carried out at the Meniko site (Fig. 10). The survey was
performed in the framework of the INCO-DC program, with the aim of detecting and mapping the
water-bearing layers and delineating the contact between the sediments of the Nicosia formation
and the igneous rocks. Two reflection lines were shot at the site with a 48 channel recorder
using explosives. The receivers were single 10 Hz geophones; the source and receiver spacing
was 10.0 m. Fig. 11 represents two field records from the survey. The records clearly show a
sequence of reflected events down to times of about 500 ms (Shtivelman, 2003).

Fig. 10: seismic time section along the reflection lines in
Meniko area (Shtivelman, 2003).

Fig. 11 shows a seismic time section along one of the
reflection lines (Shtivelman, 2003).

The Dead Sea area is characterized by extensive development of sinkholes of different
dimensions. Along the western coast, more than 700 sinkholes are exposed, with diameters varying
from several meters to tens of meters and with depths reaching 15 m. Over the last years, the
sinkhole development has increased dramatically, posing a severe problem for a normal functioning
and development of the entire region (Shtivelman, 2003).
In order to understand the mechanism of the sinkhole development, an integrated geophysical
study was carried out in the area. The starting point of the study was the assumption that a direct
relationship exists between the rapid lowering of the Dead Sea (approximately 20 m during last 20
years) and the rate of sinkhole development both in time and space. The lowering of the Dead Sea
level is accompanied by a corresponding decrease of the groundwater level in the vicinity of the
shore and by penetration of unsaturated groundwater into the coastal area. If this water encounters
a shallow salt layer, it may cause its dissolution and formation of cavities within the salt. As a result,
the overlaying material (usually poorly consolidated alluvium) collapses into the cavities, forming
sinkholes at the surface. Thus, the working hypothesis of the study suggests the existence of a
shallow salt layer as a necessary factor for sinkhole development. Since seismic velocity in the salt
layer is supposed to be higher than that of the overlaying alluvium, it makes it a good target for the
seismic refraction method. Therefore, in order to verify the existence of such a layer, seismic
refraction surveys were carried out in several sinkhole areas.
2. Detecting a salt layer in sinkhole areas

The following example shows the results of such a survey. A refraction
line was shot in the vicinity of a number of opened sinkholes, one of
which is shown in Fig. 12. The source of energy was Dynasource (a track-
mounted vacuum-accelerated heavy weightdrop); the receiver spacing
was 5 m. The depth section along the line (Fig. 13) shows a presence of a
relatively high-velocity (2930 m/s) layer at the depths of 20 – 25 m which
can be identified with salt. The layer is overlain by two low-velocity layers
which can be related to the dry (the velocity of 630 m/s) and water
saturated (the velocity of 1650 m/s) parts of an alluvium unit. This
subsurface model was further confirmed by a borehole drilled at the site.
The borehole penetrated the salt layer between the depths of 24 – 36 m.
The refraction surveys carried out in other sinkhole areas produced
similar results, thus confirming the existence of a shallow high-velocity
salt-related layer in these areas (Shtivelman, 2003).

Fig. 12: A sinkhole in the Dead Sea area (Shtivelman, 2003)

Fig. 13: A Depth section along the refraction line located in
the vicinity of the sinkhole of Fig.11 (Shtivelman, 2003)

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