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Microgravity investigations are widely applied at present for solving various environmental and geological problems. Unfortunately, microgravity survey is comparatively rarely used for searching for hidden ancient targets. It is caused mainly by small geometric size of the desired archaeological objects and various types of noise complicating the observed useful signal. At the same time, development of modern generation of field gravimetric equipment allows to register promptly and digitally microGal (10^{−8} m/s^{2}) anomalies that offer a new challenge in this direction. An advanced methodology of gravity anomalies analysis and modern 3D modeling, intended for ancient targets delineation, is briefly presented. It is supposed to apply in archaeological microgravity the developed original methods for the surrounding terrain relief computing. Calculating second and third derivatives of gravity potential are useful for revealing some closed peculiarities of the different Physical-Archaeological Models (

Development of a new modern gravimetric and variometric (gradientometric) equipment (permitting registering earlier inaccessible small anomalies and improving the observation methodology) and creation of new methodologies for gravity data processing and interpretation have triggered an arising of microgravity methodology application in environmental and economic minerals geophysics.

Microgravity is recognized now as a effective tool for analysis of various geological inhomogeneities in subsurface, monitoring of volcanic activity, and prospecting of useful minerals (e.g., [

Obviously, the history of gravity (microgravity) application at archaeological sites is beginning from the work of Linnington [

Fajklewicz [_{zz}_{zz}

Kerisel [

Slepak [

Pasteka and Zahorec [

A small number of microgravity measurements were performed in the Bedem archaeological locality in Yugoslavia [

Castiello et al. [

It is known that the trivial formulas of quantitative analysis (based on simple relationships between the gravity field semiamplitude and center of the disturbing body) are widely presented in the geophysical literature (e.g., [

Gravity field intensity

For anomalous magnetic field

Let us consider analytical expressions of some typical models employed in magnetic and gravity fields (Table

Comparison of some analytical expressions for magnetic and gravity fields.

Field | Analytical expression | |
---|---|---|

Magnetic | Thin bed (TB) | Point source (rod) |

Gravity | Horizontal Circular Cylinder (HCC) | Sphere |

Here

It is clear that expressions (

Taking into account all above mentioned, we can apply for the gravity field analysis the advanced interpreting methodologies developed in magnetic prospecting for complicated environments [

We can also calculate the “gravity moment,” which could be used for classification and ranging gravity anomalies from various types of targets. The “gravity moment” of HCC may be calculated by the use of corresponding formula for the magnetic TB [

A significant number of archaeological sites occur under conditions of rugged terrain relief. Uneven observation lines are responsible for variations in the distance from the point of measurements to the source that can strongly complicate quantitative analysis of gravity anomalies [

A simple example of interpreting gravity anomaly from a buried cavity on inclined profile is presented in Figure

Quantitative analysis of gravity anomaly on inclined profile from a buried sphere.

Second and third derivatives of gravity potential could be very useful for defining some important peculiarities of archaeological targets location in different physical-archaeological models (

The

Types of geological bodies used in modeling.

Computing derivatives of gravity potential for a horizontal polygonal prism.

The program has the following main advantages (besides abovementioned ones): (1) simultaneous computing of gravity and magnetic fields; (2) description of the terrain relief by irregularly placed characteristic points; (3) computation of the effect of the earth-air boundary by the method of selection directly in the process of interpretation; (4) modeling of the selected profiles flowing over rugged relief or at various arbitrary levels (using characteristic points); (5) simultaneous modeling of several profiles; (6) description of a large number of geological bodies and fragments. The basic algorithm realized in the

Analytical expression for the first vertical derivative of gravity potential of (

Detailed description of analytical expressions for the first and second derivatives of gravity potential of the approximation model of the horizontal polygonal prism and their connection with magnetic field is presented in Khesin et al. [

It is well known that accuracy of microgravity investigations substantially depend on the accuracy of terrain correction (TC) computing. Two approaches presented below were applied for exact TC calculation for the detailed Bouguer gravity observed at ore deposits occurring in the Lesser and Greater Caucasus.

A first method was applied in the Kyzylbulakh gold-pyrite deposit situated in the Mekhmana ore region of the Nagorny Karabakh (Lesser Caucasus) under condition of rugged relief. This deposit is well investigated by mining and drilling operations and therefore was used as a reference field polygon for testing this approach. A special scheme for obtaining the Bouguer anomalies has been employed to suppress the terrain relief effects dampening the anomaly effects from the objects of prospecting. The scheme is based on calculating the difference between the free-air anomaly and the gravity field determined from a 3D model of a uniform medium with a real topography. 3D terrain relief model with an interval of its description of 80 km (the investigated 6 profiles of 800 m length are in the center of this interval) was employed to compute (by the use of ^{3}). With applying such a scheme the Bouguer anomalies were obtained with accuracy in two times higher than that of TC received by the conventional methods. As a result, on the basis of the improved Bouguer gravity with the precise TC data, the geological structure of the deposit was defined [

Second approach was employed at the complex Katekh pyrite-polymetallic deposit, which is located at the southern slope of the Greater Caucasus (northern Azerbaijan). The main peculiarities of this area are very rugged topography of SW-NE trend, complex geology and severe tectonics. Despite the availability of conventional

Analysis of the numerous archaeological and geological publications as well as the author’s investigation (e.g., [

underground ancient cavities and galleries,

walls, remains of temples, churches, and various massive constructions,

pavements and tombs,

roman aqueducts (under favorable physical-geological environments),

areas of ancient primitive metallurgical activity, including furnaces, (under favorable physical-geological environments).

Examining the different archaeological targets in Israel, it was supposed that microgravity method might be effectively applied at least on 20–25% of ancient sites [

A simplified model example of buried pavement delineation is presented in Figure ^{3 }and occurring at a depth of 1.8 m in uniform medium (^{3}) over the pavement. It makes the delineation of the pavement practically impossible in field conditions (registered anomaly is oscillating about 1 microGal) (Figure

Comparison of Bouguer gravity and vertical gradient anomalies. (a) Bouguer gravity, (b) vertical gradient

Calculations of

Computing of horizontal derivatives from models of two closely disposed caves. (a) computed gravity curve, (b) calculated first horizontal derivative of gravity field

The planning microgravity investigations must be accompanied by development of preliminary physical archaeological models (

Physical-geological model of buried prehistoric cave and computed 3D gravity anomalies. (a) location of projected profiles and disposition of buried cave (view over), (b) 3D computed gravity effects along profiles 1–5, (c) geological-archaeological sequence.

Archaeological remains in the Israeli territory are classified by the degree of microgravity method applicability. The described characteristics of the developed software for combined 3D gravity-magnetic modeling under complicated environments indicate that it is a powerful tool for microgravity examination at archaeological sites. Two earlier applied nonconventional schemes for computing of surrounding terrain relief in ore deposits may be successfully adopted for obtaining

The author would like to thank two anonymous reviewers, who thoroughly reviewed the paper, and their critical comments were helpful in preparing this paper. The author thank the U.S. Agency for International Development and the European Community’s FP7 Program under Grant Agreement no. 225663 Joint Call FP7-ICT-SEC-2007-1 for supporting this investigation.