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In the framework of the French–Egyptian Imhotep Project, two spring gravimeters have been installed in the area of Lake Nasser (Egypt) with the aim to establish a first reliable gravity tide model for the region. The two tidal gravity stations are located in Aswan, on the northern edge of the lake and in Abu Simbel in the south, respectively. This study was mainly aimed to obtain a reliable model of the crustal response to tidal forces and, consequently, to increase the accuracy of the geodetic observations, to be used in future geophysical studies in this region as well as to investigate the effect of the Lake level variations on the crustal deformation and related gravity changes. Nearly 3 years of gravity records (from May 2018 to April 2021) were collected. Since no scale factor was available for the two gravimeters, the first step of this study was to achieve a reliable calibration for each of the two collected gravity signals. After removing the instrumental drift, spikes, steps and tares, both gravimeters have been calibrated by fitting the output signal against a synthetic reference signal based on the body tidal gravity response due to Wahr-Dehant Earth model and FES2014 ocean tidal loading model. The calibrated signals have been analyzed with ET34-X-V80 software for tidal analyses. This enabled us to retrieve a set of frequency-dependent gravity factors (amplitude and phase) for the main tidal waves, as well as to obtain gravity residuals. It turns out that the accuracy of the amplitude estimates for the main tidal waves is 0.2 ÷ 1% for LCR_ET16 in Aswan and 1 ÷ 10% for the LCR_D-218 in Abu Simbel. To improve the tidal model at Abu Simbel, LCR_ET16 was stopped in Aswan and relocated there. The first 90 days of gravity recordings from ET16 at Abu Simbel provide promising results, with an accuracy of the order of 0.1% for the main tidal waves, even better than the results obtained in Aswan. The residual gravity signal after tidal subtraction at Aswan is in the range of ± 50 µGal. Further analyses of the instrumental contribution are however needed before to be able to interpret this gravity signal in terms of surface loading (i.e. changes in the water level of Lake Nasser) or underground hydrology.

The local hybrid geoid model for the western desert in Egypt is computed by recent data, including the global geopotential models, gravimetric measurements, and a high-resolution digital topographic model. The research points out the importance of accurate local geoid computation not only as a survey datum but also to figure out the crustal structure. Detailed crustal structure patterns could reveal the origin of artesian aquifers and geothermal activities of the selected region. In this research, recent global geopotential models deduced from recent satellite missions have been used. The oases’ area of the western desert in Egypt is characterized by an irregular topographic pattern suggesting a complicated geoid model. Six recent global geopotential models (GGMs) have been evaluated for long and medium wavelengths. GGM accuracy for each model has been computed using GPS leveling data. EIGEN-6C model with spherical harmonic expansion to degree 250 gives the smallest error with a standard deviation equal to 0.019mGal. Also, a high-resolution digital topographic model with 3arcsec is used for terrain correction (Tc). The resulting geoid has been evaluated using GPS leveling in the selected region. Due to the recent geopotential models with high accuracy, the random distribution and low terrestrial gravity data have not become a problem in calculating the local geoid model.

Interpretations of the tectonic setting of the Nile Delta of Egypt and its offshore extension are challenged by the thick sedimentary cover that conceals the underlying structures and by the paucity of deep seismic data and boreholes. A crustal thickness model, constrained by available seismic and geological data, was constructed for the Nile Delta by inversion of satellite gravity data (GOCO06s), and a two-dimensional (2D) forward density model was generated along the Delta’s entire length. Modelling results reveal the following: (1) the Nile Delta is formed of two distinctive crustal units: the Southern Delta Block (SDB) and the Northern Delta Basin (NDB) separated by a hinge zone, a feature widely reported from passive margin settings; (2) the SDB is characterized by an east–west-trending low-gravity (~−40 mGal) anomaly indicative of continental crust characteristics (depth to Moho (DTM): 36–38 km); (3) the NDB and its offshore extension are characterized by high gravity anomalies (hinge zone: ~10 mGal; Delta shore line: >40 mGal; south Herodotus Basin: ~140 mGal) that are here attributed to crustal thinning and stretching and decrease in DTM, which is ~35 km at the hinge zone, 30–32 km at the shoreline, and 22–20 km south of the Herodotus Basin; and (4) an apparent continuation of the east-northeast–west-southwest transitional crust of the Nile Delta towards the north-northeast–south-southwest-trending Levant margin in the east. These observations together with the reported extensional tectonics along the hinge zone, NDB and its offshore, the low to moderate seismic activity, and the absence of volcanic eruptions in the Nile Delta are all consistent with the NDB being a non-volcanic passive margin transition zone between the North African continental crust (SDB) and the Mediterranean oceanic crust (Herodotus Basin), with the NDB representing a westward extension of the Levant margin extensional transition zone.

This paper is devoted to the problem of the ground-satellite comparison of time-variable gravity. We first review the different methods used to validate satellite gravity observations (ground truth experiment) and point out possible issues. We first show results obtained in Europe where a moderate amplitude hydrological signal exists using both satellite (GRACE) and surface gravity measurements from the GGP (Global Geodynamics Project) network of superconducting gravimeters. We show also the nice agreement between ground and GRACE observations in Djougou (West Africa) where hydrological changes due to monsoon are important. We finally present on-going projects for the so-called ‘null test’ which is related to observations in a region with no (or very small) hydrological contribution to gravity. First results for the Sahara (Algeria) are reported and future missions in Egypt and Saudi Arabia briefly introduced.

The Kalabsha region at the northwest side of Lake Aswan has been a subject of seismological and geodetic investigations since 1981, after the 5.5 M earthquake on 14 November. The subsequent seismic activity was related to the effect of water-level oscillations in the Lake. Geodetic measurements provided indications of regional stress and of the local character of vertical movements. As crustal dynamics represents a potential hazard with respect to the size of the lake, other techniques have been applied to the monitoring system. Repeated gravity test measurements were already made in the late 1980s and in 1994, but only in 1997 was a new epoch of long-term investigation initiated. The measured data showed sufficient accuracy of approx. 5 μGal. Already the first difference of the two campaigns of the years 1997 and 2000 showed a few important features. First of all, the temporal non-tidal changes of gravity exceed the level of confidence significantly. The changes were compared with older available geodetic data, and it was concluded, that the gravity changes were not related directly to the vertical movements of the surface. On the contrary, a water loading effect may be expected on the eastern side of the network, especially in the Kalabsha local net. The development of the stress field is considered to be the main source of observed gravity changes. In both the Kalabsha and Seiyal nets the changes differ according to the relative positions of the particular measuring points with respect to the faults.

at Kattamiaa city to delineate the shallow subsurface cavities. By analyzing “Bouguer anomalies” the low density zones were identified. Following this, we apply automated depth estimation methods to the data, the maximum “analytic signal” was used to identify the horizontal extend of the anomalies, and then, the “Euler deconvolution” method was employed to estimate source location and depth from gravity data; finally we apply AN-EUL which Based on the combination of the analytic signal and the Euler deconvolution methods.

Microgravity surveying has developed considerably over the last ten years however, and with the development of modern high-resolution equipment, careful field acquisition techniques. The micro-gravimetery was selected to investigate this testing site (area under study) for one of the vital projects in the Northern part of Egypt. The proposed microgravity study is comprised of the main area of constructions with grid of 100m x 100m and station interval of 10m. The main objective of the microgravity observations in the area is to find out the possible existence of any shallow mass discontinuity, voids or vertical fractures in the limestone cape. The Bouguer anomaly map of the area shows significant trend of gravity variation with a gravity variation of up to 700 nm/s**2. This is most probably a normal faulting trending NW-SE. However, this deep seated faulting is not the main interests of the current project but the shallow voids or weakness zones. Such small gravity anomalies have been masked under the effect of the relative deep seated geologic structure. Thus, removing the regional gravity trend is essential in order to reveal the possible shallow mass discontinuity in the form of voids or cavities. Then the resulted anomalies were quantitatively interpreted.

Generally, near surface geophysical techniques like the high resolution microgravity approved its applicability for detecting the near-surface caves. This paper is aimed at demonstrating the link between geophysical and engineering parameters of the soil and rocks in one package required by engineers at any site before starting constructions. Microgravity measurements were acquired in the Al-Amal area southeast of Cairo, Egypt as a case study in order to detect the near-surface caves. The survey showed there are indications of voids/caves in the uppermost subsurface layer formed by the Eocene limestone. At the same time, rock samples were collected from an exposed section through a small valley in the study area. The samples were subjected for soil tests to study various mechanical and petrophysical properties. The grain size analysis revealed coarse grained classification of the soil. The other geomechanical and petrophysical analyses provided detailed rock and soil characterization, such as compressive strength, porosity, bulk density and permeability. In accordance with microgravity, it was found that the first 2 – 3 m represent low strength and low stability formation for big constructions. This layer should be either removed, or the foundations should penetrate to bigger depth.