Shallow subsurface imaging of archaeological sites in agricultural terrains

Research partners: Zvi Ben Avraham, Itzik Makovsky, and Shmuel Marco

Introduction

Since man appeared on Earth, he was in constant interaction with the upper part of the earth crust. First, like any other living creature, as food source and place for living. Later on for buildings, agriculture, burial sites and more. Today, the influence of man on the upper part of the earth, up to few tens of meters, is greater then ever, including building, mining, extensive agriculture, and unfortunately also pollution.

The most obvious evidence for past human activities are archaeological sites, but extensive agriculture covers large parts of many sites.

From a geophysical exploration point of view, archaeological sites can be regarded as disturbance in the upper part of the subsurface, often containing materials that contrast their surroundings. The surface is often heterogeneous because of erosion, biological activity, and the influence of man. Geophysical methods usually detect some sort of contrasting physical properties of the investigated area, such as density, conductivity, magnetization, and others.  When human influence from past and present, such as archeology and agriculture, are combined together, the result is a complex geophysical signal. Understanding of such signal and separating the influence of the different sources is a challenge to any geophysical method.

Israel is one of the richest countries in archaeological sites. The extensive agriculture, especially in the center and north of the country, covers many these sites. The influence of agriculture concerning archaeology is even greater when dealing with tree plantations.

Research areas

In my work I choose to map two sites involving archaeology and agriculture. The first one in the eastern hippodrome of Caesarea in the centeral part of the Mediterranean coast in Israel. The area was covered with banana plantations until about 8 years ago. The archaeology remains we are looking for include granite pillars and marble pillars and remains of the center wall of the hippodrome.

The other site is the Bronze age Minoan palace near Kibbutz Kabri in the western Galilee, Israel. Today most of the site is covered with avocado plantations. Here we are looking for parts of the Minoan palace that were not excavated in order to decide where to continue the excavation.

Methods

The most popular methods in archeology prospecting are ground penetrating radar (GPR), electromagnetic methods (EM), magnetic methods and remote sensing, Except for remote sensing, these are relatively inexpensive and fast methods that can produce good results. Remote sensing is expensive, but can cover large areas and sometimes irreplaceable in locating new sites. Geophysical methods such as seismic and DC are usually awkward for archeology prospecting, involving larger mane power and expensive.

In this research the EM and GPR methods are used. A magnetometer was used in part of the survey area in Kabri but was ineffective. I used EM31-MK2 to gather data in both sites. In both places the influence of the trees is seen in the data. Next step is to use the GPR in Ceasarea site. GPR might not be used in Kabri for reasons of difficulties in operating between trees in very wet soil and shallow water ponds.

Both tools are based on ground conductivity but use different approach and aspects of it. A comparison between them will also teach us about the abilities of the tools comparing to each other.

Today we concentrate on the EM data processing procedure that will enable us to separate the agriculture and archeology influence. This involves using Fourier transforms, derivatives of different types and.

Examples:

Top 3D view of the raw data. The large Anomalies in the upper right part are pipe and concrete water tunnel.

Gradient of the data. Dark blue lines marks possible buried walls parallel to those of the palace.