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Microgravity investigations are now recognized as a powerful tool for subsurface imaging and especially for the localization of underground karsts. However numerous natural (geological), technical, and environmental factors interfere with microgravity survey processing and interpretation. One of natural factors that causes the most disturbance in complex geological environments is the influence of regional trends. In the Dead Sea coastal areas the influence of regional trends can exceed residual gravity effects by some tenfold. Many widely applied methods are unable to remove regional trends with sufficient accuracy. We tested number of transformation methods (including computing gravity field derivatives, self-adjusting and adaptive filtering, Fourier series, wavelet, and other procedures) on a 3D model (complicated by randomly distributed noise), and field investigations were carried out in Ghor Al-Haditha (the eastern side of the Dead Sea in Jordan). We show that the most effective methods for regional trend removal (at least for the theoretical and field cases here) are the bilinear saddle and local polynomial regressions. Application of these methods made it possible to detect the anomalous gravity effect from buried targets in the theoretical model and to extract the local gravity anomaly at the Ghor Al-Haditha site. The local anomaly was utilized for 3D gravity modeling to construct a physical-geological model (

The development of new modern gravimetric and variometric (gradientometric) equipment, which makes it possible to record small previously inaccessible anomalies, has enhanced observational methodology as well as new gravity data processing methods and interpretation. These advances have triggered the rapid rise in the number of microgravity methodology applications in environmental and economic minerals geophysics.

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

At the same time different kinds of noise of different origin complicate analysis of microgravity data. For removing (elimination) the noise components numerous procedures and methodologies were developed. We will analyze in this paper a problem of regional trend removing under complex geological-geophysical environments. Such a problem is highly essential by delineation of weak anomalies from buried karst terranes in the Dead Sea Basin where regional horizontal gravity gradients may exceed values of 10 mGal/km.

Colley [

Fajklewicz [

A nonconventional attempt to use microgravity observations for weight determination of stockpiled ore was reported by Sjostrom and Butler [

Crawford [

Types of noise (disturbances) arising in microgravity investigations were studied in detail in Debeglia and Dupont [

The need for additional computation of the surrounding terrain relief by 3D gravity modeling in ore deposits occurring in the very complex topography of the Greater Caucasus was discussed in Eppelbaum and Khesin [

Abad et al. [

Advanced methods in magnetic prospecting can be adapted to quantitative analysis of microgravity anomalies in complex environments [

Deroussi et al. [

Types of noise associated with microgravity studies of shallow karst cavities in areas of developed infrastructure are presented in detail in Leucci and Georgi [

The importance of gravity field observations at different levels as well as the precise calculation of topographic effects in intermediate and distant zones was analyzed in Eppelbaum [

Kaufmann et al. [

Panisova et al. [

A microgravity survey is the geophysical method most affected by corrections and reductions caused by different kinds of noise (disturbances). A chart showing the different types of noise typical to microgravity studies is presented in Figure

Noise affecting microgravity investigations (adapted from [

These types of noise are described in more detail below.

The

These constitute the most important physical-geological disturbances. The application of any geophysical method depends primarily on the existence of physical properties contrast between the objects under study and the surrounding medium. The

Let us consider the last disturbing factor in detail. The correct removal (elimination) of regional trends is not a trivial task (e.g., [

Negative effect of gravitational anomalies from a local anomalous body observed on inclined and horizontal profiles (after [^{3}; anomaly

Sometimes even simple computing of the first and second derivatives of the gravity field

Computation of the horizontal derivatives of the gravity field for two proximal sinkhole models. (a) Computed gravity curve (level of computation: 0.3 m), (b) first horizontal derivative of gravity field

The area under study—Ghor Al-Haditha—is situated in the eastern coastal plain of the Dead Sea (Jordan) in conditions of very complex regional gravity pattern (Figure

Areal map of the investigated site.

To test methods of regional trend elimination, two theoretical

To calculate the 3D gravity field, 12 parallel profiles with a distance between them of 5 m were applied (Figure ^{3} and ^{3}, resp.) ^{3}. The center of the second small sinkhole was located at a depth of −20 m below the earth’s surface in the first layer, with a contrast density of −2000 kg/m^{3}. Profile 6 was selected as the central one, and the left and right ends of sinkhole 1 were defined as −30 and +30 m, and for sinkhole 2 as −12 and +12 m, respectively. For the 3D gravity field modeling of this and the following examples, mainly the GSFC program [

Scheme of gravity field 3D computation for the model example.

Gravity field anomalies along profile 6 from models of sinkholes.

The compiled gravity map for the 12 profiles for the sinkhole

Compiled gravity map for 12 profiles.

The simplified

Simplified density-geological model of the Dead Sea Transform.

Combined gravity field along profile 6 from models of sinkholes and effect of the DST.

Given that the geological medium is usually more complex than presented in the models in Figures

Inserted randomly distributed noise.

Profile number | Mean value | Standard deviation |
---|---|---|

1 | 0.150 | 0.040 |

2 | 0.160 | 0.030 |

3 | 0.140 | 0.035 |

4 | 0.130 | 0.038 |

5 | 0.170 | 0.029 |

6 | 0.120 | 0.033 |

7 | 0.150 | 0.038 |

8 | 0.140 | 0.032 |

9 | 0.110 | 0.024 |

10 | 0.160 | 0.031 |

11 | 0.125 | 0.025 |

12 | 0.15 | 0.028 |

Figure

Compiled gravity map of the random noise for 12 profiles.

Compiled gravity map for 12 profiles with combined effect from: (1) the DST, (2) sinkholes, and (3) random noise.

To remove the regional trends, different algorithms and methods were applied: the first and second derivatives, self-adjusting and adaptive filtering, Fourier series, wavelet decomposition, principal component analysis, inverse probability, and other methods were applied (altogether more than 30 different procedures).

Examples of applications of (1) the entropy parameter using a moving window with self-adapting size, (2) gradient sounding, and (3) power estimation by the Morlet transformation are presented in Figures

Results of three different methodologies: (a) entropy computation using a moving window with self-adapting size, (b) gradient sounding, and (c) power estimation by Morlet transformation.

Regression analysis is now considered one of the most powerful methods for removing trends of different kinds (e.g., [

Residual gravity map after subtracting bilinear saddle regression.

The gravity map after subtracting a local polynomial regression

Residual gravity map after subtracting local polynomial.

The Ghor Al-Haditha area is located south-east of the northern Dead Sea basin (see Figure

The observed gravity map (Figure

Bouguer gravity map of the Ghor Al-Haditha area (Jordan).

Figure

Results of gradient sounding.

A visual comparison of the residual maps (Figures

Residual gravity map of the Ghor Al-Haditha area after subtracting bilinear saddle regression.

Residual gravity map of the Ghor Al-Haditha area after subtracting local polynomial.

The gravity profiles are constructed along the same line (A–B in Figure

Comparison of gravity curves constructed along profile A–B for Figure

3D modeling indicates that such a gravity anomaly may have been produced by a sinkhole (similar to model 2 in Figure

An initial physical-geological model along profile A′–B′ developed on the basis of 3D gravity field modeling.

The different kinds of noise affecting microgravity investigations amply illustrate the need for careful calculation of each of these disturbing factors. In particular, the influence of regional trends often masks the target local microgravity anomalies. The 3D theoretical

The authors would like to thank anonymous reviewers, who thoroughly reviewed this paper, and their critical comments and valuable suggestions were very helpful in preparing this paper. This publication was made possible through support provided by the U.S. Agency for International Development (USAID) and the MERC Program under terms of Award No. M27-050.