Ultra-high-resolution marine 2D–3D seismic investigation of the Liman Tepe/Karantina Island archaeological site (Urla/Turkey)

Abstract

2D and 3D high-resolution seismic investigations were performed on submerged coastal archaeological sites at Iskele and near to Karantina Island in the Bay of Izmir in western Turkey. Tectonic subsidence of the coastline has submerged a number of archaeological features associated with an important Early Bronze Archaic settlement (Liman Tepe) and the classical Ionian city of Clazomenae. Seismic surveys were focused on imaging of an Archaic harbour structure and other submerged Hellenistic and Roman architectural features. Seismic data were acquired with the SEAMAP-3D ultra-high-resolution 3D marine seismic acquisition system developed for detailed archaeological site investigation.

A 2D reconnaissance survey was performed over a 2 km² area around Karantina Island to evaluate the seismic penetrability and to locate sites for further 3D investigation. This survey predominantly revealed marine sediment layers covering the local bedrock, which is characterized by scattering of seismic energy showing its rocky nature.

Two ultra-high-resolution 3D seismic surveys were performed. The first covered a 350 m × 30 m area in the modern harbour targeting a prominent Archaic harbour structure. The second was acquired across a 120 m × 40 m area on the southeast shore of the Karantina Island close to a Roman architectural feature. The 3D surveys were acquired with nominal line spacings of 1 m, using a 8 × 4 pseudo-rigid hydrophone array and a Boomer source firing at 3 Hz shot frequency. Automated processing of the seismic data using a portable Linux cluster provided stacked 3D seismic volumes with 25 cm × 25 cm bin size on-site.

The 3D seismic survey of the harbour clearly imaged the submerged Archaic structure and the underlying sediment sequence. The seismic time slices reveal two seismic anomalies (2–3 m in diameter) in the harbour basin sediments. The 3D surveys southeast of Karantina identified a thicker marine sediment sequence overlying steeply dipping bedrock reflectors. The sediment sequence records the rapid accumulation and progradation of the coastline following the construction the Alexander causeway linking the mainland with the island in 334 B.C.

Introduction

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 specialised seismic processing hardware have largely limited the application of 3D seismic methods for many near-surface applications.

The recent advent of more powerful personal computers and PC-based seismic processing and interpretation software has made ultra-high-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.

In the last 15 years there has been growing interest in adapting high frequency seismic sources and 3D methods for marine seismic acquisition on a dense survey grid (Henriet et al., 1992). In 1998, within the context of the European MAST III project, efforts were taken for the first time to develop a high-resolution 3D marine seismic acquisition system (Marsset et al., 1998).

Other researchers have demonstrated that high- to ultra-high-resolution 3D seismic acquisition is feasible in shallow water (Missiaen et al., 2002, Müller et al., 2002, Müller, 2005, Pulliam et al., 1996, Scheidhauer et al., 2005) and that 3D methods can be downscaled to meet the horizontal and vertical resolutions needed for engineering applications. It is only relatively recently, however, that systems have been developed with a resolution suitable for application in archaeology (Bull et al., 2005, Vardy et al., 2008, Gutowski, 2005). 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. 1). 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).

In this paper we report on the results of SEAMAP-3D seismic surveys performed on marine archaeological sites near Iskele and Karantina Island in western Turkey (Fig. 2). The area includes an important Bronze Age coastal settlement (Liman Tepe) and the architectural remains of the classical-Byzantine-age Ionian port city of Clazomenae. Ultra-high-resolution 3D seismic surveys were performed in 2006 with the objective of imaging submerged harbour structures and buried architectural features in shallow coastal waters surrounding these sites. The survey results demonstrate that ultra-high-resolution can be achieved in water depths of less than 2 m. The paper also provides technical details of the SEAMAP-3D acquisition system and demonstrates how automated processing can be used to produce a stacked seismic data volume on-site, a few hours after the survey.