Sherif M. Hanafy

Ground-penetrating radar tomography for soil-moisture heterogeneity

Many ground-penetrating radar (GPR) studies incorporate tomographic methods that use straight raypaths for direct model reconstruction, which is unrealistic for media with gradually changing petrophysics. Ray-bending algorithms can sometimes lead to unreliable resolution, especially at interfaces of abrupt dielectric changes. We present an improved GPR tomography technique based on a combination of seismic tomographic methods and a finite-difference solution of the eikonal equation. Our inversion algorithm uses velocity gradient zones and bending rays that represent realistic geology in the subsurface. We tested the technique on theoretical and experimental models with anomalous bodies of varying saturations and velocity and applied it to data from a GPR field experiment that analyzed the root zones of trees. Synthetic results showed that the resolution of our technique is better than that of published methods, especially for local anomalies with sharp velocity contacts. Our laboratory experiments consisted of four objects buried in sand with various water saturations. The GPR tomogram could map the objects and determine their degree of saturation. The velocities are compatible with those of the complex refraction index method; their relationship to the water content fits a previously published empirical equation. Our original field experiment around a poplar tree could map the heterogeneous subsurface and distinguish a central low velocity beneath the tree from the peripheral negative anomaly of a refill. This zone reflects the whole root zone and is caused by its bulk water content of both the organic root network and its surrounding soils.

Interferometric interpolation of sparse marine data

We present the theory and numerical results for interferometrically interpolating 2D and 3D marine surface seismic profiles data. For the interpolation of seismic data we use the combination of a recorded Greens function and a model-based Greens function for a water-layer model. Synthetic (2D and 3D) and field (2D) results show that the seismic data with sparse receiver intervals can be accurately interpolated to smaller intervals using multiples in the data. An up- and downgoing separation of both recorded and model-based Greens functions can help in minimizing artefacts in a virtual shot gather. If the up- and downgoing separation is not possible, noticeable artefacts will be generated in the virtual shot gather. As a partial remedy we iteratively use a non-stationary 1D multi-channel matching filter with the interpolated data. Results suggest that a sparse marine seismic survey can yield more information about reflectors if traces are interpolated by interferometry. Comparing our results to those of f-k interpolation shows that the synthetic example gives comparable results while the field example shows better interpolation quality for the interferometric method.

Using super-stacking and super-resolution properties of time-reversal mirrors to locate trapped miners

Deadly mine collapses have recently occurred in West Virginia (January 2007, two miners killed); Russia (March 2007, 106 miners killed); Utah (August 2007, six miners and another three from the rescue team killed); Colombia (October 2007, 24 miners killed); and many other places. Locating trapped miners as soon as the collapse occurs will save lives and avoid dangerous searches in the wrong places.

Skeletonized inversion of surface wave: Active source versus controlled noise comparison

AbstractWe have developed a skeletonized inversion method that inverts the S-wave velocity distribution from surface-wave dispersion curves. Instead of attempting to fit every wiggle in the surface waves with predicted data, it only inverts the picked dispersion curve, thereby mitigating the problem of getting stuck in a local minimum. We have applied this method to a synthetic model and seismic field data from Qademah fault, located at the western side of Saudi Arabia. For comparison, we have performed dispersion analysis for an active and controlled noise source seismic data that had some receivers in common with the passive array. The active and passive data show good agreement in the dispersive characteristics. Our results demonstrated that skeletonized inversion can obtain reliable 1D and 2D S-wave velocity models for our geologic setting. A limitation is that we need to build layered initial model to calculate the Jacobian matrix, which is time consuming.

An application of multiscale early arrival waveform inversion to shallow seismic data

We estimate the near surface velocity distribution by applying multiscale early arrival waveform inversion (MEWI) to shallow seismic land data. This data set is collected at Wadi Qudaid in western Saudi Arabia with the purpose of characterizing the shallow subsurface for its water storage and reuse potential. To enhance the accuracy of MEWI, we correct for the attenuation effects with an estimated factor Q, and also extract a natural source wavelet from the data. We then applied MEWI to invert the processed data for tomograms on different scales starting from a traveltime tomogram as our initial velocity model. Results suggest that, compared to traveltime tomography, MEWI can generate a more highly resolved velocity tomogram from shallow seismic data by inverting its low-frequency components on coarse grids and its high-frequency components on fine grids. The estimated water table in the MEWI tomogram is generally consistent with, but 9% deeper than, the traveltime tomogram, showing that the water storage in this wadi might be less than expected from the traveltime tomogram. We believe that the more accurate MEWI tomogram will make an economically important difference in assessing the storage potential of this wadi and wadis throughout the world.

Interferometric interpolation of 3D SSP data

We present the theory and numerical results for interferometrically interpolating 3D marine surface seismic profiles (SSP) data. For the interpolation of SSP data we use the combination of a natural Green’s function (SSP shot gathers) and a modelbased Green’s function for the water-layer model. Synthetic results show that the aliased SSP data with sparse receiver intervals can be accurately interpolated to smaller intervals. The virtual shot gather contains some artifacts so a non-stationary 2D multi-channel image matching filter is used after interferometric interpolation to remove these artifacts. Results suggest that a sparse marine SSP survey can yield more information about the reflectors if data are interpolated by interferometry. This assumes that the sources are located both outside and above the recording aperture.

Demonstration of super-resolution and super-stacking properties of time reversal mirrors in locating seismic sources

We demonstrate with synthetic seismic data the superresolution and super-stacking properties of time reversal mirrors (TRM). Tests on synthetic data show that TRM has the potential of exceeding the Rayleigh resolution limit by a factor of more than 9. This property is accompanied by the fact that TRM has a significant resilience to strong noise. Computer tests validate these properties by accurately imaging the source location from passive seismic data with signal-to-noise ratio of about 0.001. Results also validate that TRM enhances signal by a factor pro-

Preliminary seismic hazard assessment, shallow seismic refraction and resistivity sounding studies for future urban planning at the Gebel Umm Baraqa area, Egypt

Gebel Umm Baraqa Fan, west Gulf of Aqaba, Sinai, is one of the most important tourism areas in Egypt. However, it is located on the active Dead Sea-Gulf of Aqaba Levant transform fault system. Geophysical studies, including fresh water aquifer delineation, shallow seismic refraction, soil characterization and preliminary seismic hazard assessment, were conducted to help in future city planning. A total of 11 vertical electrical soundings (1000–3000 m maximum AB/2) and three bore-holes were drilled in the site for the analysis of ground water, total dissolved solids (TDS) and fresh water aquifer properties. The interpretation of the one-dimensional (1D) inversion of the resistivity data delineated the fresh water aquifer and determined its hydro-geologic parameters. Eleven shallow seismic refraction profiles (125 m in length) have been collected and interpreted using the generalized reciprocal method, and the resulting depth–velocity models were verified using an advanced finite difference (FD) technique. Shallow seismic refraction effectively delineates two subsurface layers (VP ~ 450 m s−1 and VP ~ 1000 m s−1). A preliminary seismic hazard assessment in Umm Baraqa has produced an estimate of the probabilistic peak ground acceleration hazard in the study area. A recent and historical earthquake catalog for the time period 2200 BC to 2006 has been compiled for the area. New accurate seismic source zoning is considered because such details affect the degree of hazard in the city. The estimated amount of PGA reveals values ranging from 250 to 260 cm s−2 in the bedrock of the Umm Baraqa area during a 100 year interval (a suitable time window for buildings). Recommendations as to suitable types of buildings, considering the amount of shaking and the aquifer properties given in this study, are expected to be helpful for the Umm Baraqa area.

Target-oriented interferometric tomography for GPR data

An interferometric form of Fermat’s principle and traveltime tomography is used to invert ground-penetrating radar (GPR) data for the subsurface velocity distribution. The input data consist of GPR traveltimes of reflections from two buried interfaces, A (reference) and B (target), where the data are excited and recorded by GPR antennas at the surface. Fermat’s interferometric principle is then used to redatum the surface transmitters and receivers to interface A so the associated reflection traveltimes correspond to localized transit times between interfaces A and B . The overburden velocity model above interface A is not required. The result after tomographic inversion is a high-resolution estimate of the velocity between interfaces A and B that does not depend on the velocity model above interface A . A motivation for introducing interferometric traveltime tomography is that typical layer-stripping approaches will see the slowness error increase with depth as the layers are inverted. This suggests that...

Opportunities and pitfalls in surface-wave interpretation

AbstractMany explorationists think of surface waves as the most damaging noise in land seismic data. Thus, much effort is spent in designing geophone arrays and filtering methods that attenuate these noisy events. It is now becoming apparent that surface waves can be a valuable ally in characterizing the near-surface geology. This review aims to find out how the interpreter can exploit some of the many opportunities available in surface waves recorded in land seismic data. For example, the dispersion curves associated with surface waves can be inverted to give the S-wave velocity tomogram, the common-offset gathers can reveal the presence of near-surface faults or velocity anomalies, and back-scattered surface waves can be migrated to detect the location of near-surface faults. However, the main limitation of surface waves is that they are typically sensitive to S-wave velocity variations no deeper than approximately half to one-third the dominant wavelength. For many exploration surveys, this limits the ...