Jeroen Groenenboom

Data Processing and Imaging in GPR System Dedicated for Landmine Detection

Improvements of GPR technology can be attained by making adjustments specific for the application of landmine detection on three levels: system design, data acquisition and data processing. In this paper we describe data processing algorithms specially developed for a novel video impulse ultra-wide band front end. With this front end, three-dimensional measurements (C-scans) have been carried out over a controlled test site, using a non-metallic scanner. The test site contained surface-laid and shallow buried landmines, both antitank and antipersonnel, made of plastic, wood, and metal. Because of practical limitations, the data have been acquired on an irregular grid. We have designed data preprocessing and imaging algorithms such that they take into account the specific antenna geometry and its elevation above the ground as well as the irregularity of the data acquisition grid. We show that by tuning the data pre-processing and imaging to the newly designed radar front end and to the particular data acquisition strategy, we obtain clear subsurface images. The resulting images show the ability of the GPR system to detect and visualize small surface laid and shallow buried targets.

Scattering by hydraulic fractures: Finite‐difference modeling and laboratory data

Reservoir production can be stimulated by creating hydraulic fractures that effectively facilitate the inflow of hydrocarbons into a well. Considering the effectiveness and safety of the operation, it is desirable to monitor the size and location of the fracture. In this paper we investigate the possibilities of using seismic waves generated by active sources to characterize the fractures. First, we must understand the scattering of seismic waves by hydraulic fractures. For that purpose we use a finite‐difference modeling scheme. We argue that a mechanically open hydraulic fracture can be represented by a thin, fluid‐filled layer. The width or aperture of the fracture is often small compared to the seismic wavelength, which forces us to use a very fine grid spacing to define the fracture. Based on equidistant grids, this results in a large number of grid points and hence computationally expensive problems. We show that this problem can be overcome by allowing for a variation in grid spacing in the finite‐...

Monitoring the width of hydraulic fractures with acoustic waves

During scaled hydraulic fracturing experiments in our laboratory, the fracture growth process is monitored in a time‐lapse experiment with ultrasonic waves. We observe dispersion of compressional waves that have propagated across the hydraulic fracture. This dispersion appears to be related to the width of the hydraulic fracture. This means that we can apply the dispersion measurements to monitor the width of the hydraulic fracture in an indirect manner. For a direct determination of the width, the resolution of the signal is required to distinguish the reflections that are related with two distinct fluid/solid interfaces delimiting the hydraulic fracture from its solid embedding. To make this distinction, the solid/fluid interfaces must be separated at least one eighth of a wavelength and represent sufficient impedance contrast. The applicability of the indirect dispersion measurement method however, extends to a fracture width that is in the order of 1% of the incident wavelength. The time‐lapse ultraso...

Monitoring hydraulic fracture growth: Laboratory experiments

We carry out small‐scale hydraulic fracture experiments to investigate the physics of hydraulic fracturing. The laboratory experiments are combined with time‐lapse ultrasonic measurements with active sources using both compressional and shear‐wave transducers. For the time‐lapse measurements we focus on ultrasonic measurement changes during fracture growth. As a consequence we can detect the hydraulic fracture and characterize its shape and geometry during growth. Hence, this paper deals with fracture characterization using time‐lapse acoustic data. During fracture growth the acoustic waves generate diffractions at the tip of the fracture. The direct compressional and shear diffractions are used to locate the position of the tip of the fracture. More detailed analysis of these diffractions can be used to obtain information on the geometry and configuration of the fracture tip, including the creation of a zone that is not penetrated by fluid. Furthermore, it appears that the acoustic diffraction is generat...

Ultrasonic velocity and shear‐wave splitting behavior of a Colton sandstone under a changing triaxial stress

Ultrasonic experiments on a dry Colton sandstone placed in a triaxial pressure machine show that effective stress changes lead to distinct anisotropic velocity changes in compressional waves and shear waves. The stress imprint can be recognized from the associated velocity pattern by relating the velocities to the three normal stress directions. The ultrasonic velocities indicate that the sensitivity of the different waves to stress predominantly depends on stresses applied in the polarization and propagation directions of the particular wave mode. Also, stress‐induced changes in shear‐wave splitting are observed.

Automated Acquisition and Processing of 3D GPR Data for Object Detection and Characterization

To improve image resolution of GPR data, proper data acquisition is the first key part of the whole imaging sequence. Obtainable down and cross range resolutions, insofar as they are related to acquisition, are determined by the frequency bandwidth of the GPR equipment, the temporal and spatial sampling rates and the accurate knowledge of the antenna positions. The key parameters for three-dimensional imaging algorithms are wave velocity distribution in the domain of interest, radiation (polarization and amplitude) characteristics of the antenna system and emitted waveform. To this end we have designed and manufactured a measurement frame with high horizontal position accuracy of the antennas, by using a servomotor that automatically moves the antennas over the whole area of the measurement frame for a fixed vertical position of the antennas. When acquisition is completed, the data needs to be processed such that object detection and characterization is most easily accomplished. In most cases, object detection can be carried out on the preprocessed data itself, but in some cases and certainly for characterization purposes improved image resolution is required. This is realized using imaging algorithms, preferably those that take into account the polarization and amplitude characteristics of the antennas.

Guided Waves Along Hydraulic Fractures

Hydraulic fracturing as an oil and gas reservoir stimulation technique aims at creating large fractures around a borehole, which improves the inflow of hydrocarbons at production wells, as well as the injectivity of water at injection wells. For hydraulic fracturing experiments it is desirable to find techniques to estimate fracture dimensions to aid the reservoir engineer in the fracture treatment or design. In this abstract we numerically model the propagation of guided waves along hydraulic fractures. These guided waves could be used in wellbore measurements to estimate fracture length or volume. It is well-known that guided waves can propagate along dry or fluid-filled fractures. In the geophysical community fractures are often described by the linear-slip model and the guided waves that are associated with this model are referred to as generalized Rayleigh waves. Previous studies show that both pressure symmetric as well as pressure anti-symmetric generalized Rayleigh waves exist. In this abstract we focus on hydraulic fractures which are mechanically open, i.e. the fractures faces are disconnected and do not show substantial mechanical contact. As a result the fluid in the fracture is connected and we describe the fracture by a thin fluid-filled layer model. Numerically modeled seismograms show that for these kind of fractures not only generalized Rayleigh waves exist, but also a slow and strongly dispersive guided wave. We will refer to this wave as the slow channel wave, because it appears to be the two-dimensional equivalent of the cylindrical tube wave. The slow channel wave is absent in the linear-slip model because this model does not incorporate the explicit boundary condition of zero shear stresses at fluid-solid interfaces. During hydraulic impedance testing (HIT) a pressure pulse is send down into the wellbore by opening the fluid pump during the hydraulic fracturing treatment. Some studies claim the observation of the reflection of a slow wave in the fluid at the tip of the fracture. In the low-frequency limit the channel wave corresponds to the slow fluid wave related to the waves that are exited in the fracture during HIT-experiments. This also resolves a possible ambiguity when both geophysicists and reservoir engineers talk about fracture waves.

Data processing for a land-mine-detection-dedicated GPR

The requirements on GPR technology for the application of humanitarian landmine detection are severe; 99.6% probability of detection and low false alarm rate. Trying to meet these challenging requirements, an impulse radar system has been designed specifically for the application of landmine detection. The radar system contains a dielectric filled TEM horn transmitting antenna and a small loop receiver antenna below the transmitting antenna. With this radar system three- dimensional measurements have been carried out over a test site containing surface-laid and shallowly buried landmines. The test site contains antitank and antipersonnel mines of metal and plastic. In order to show the performance of the new radar system we have to produce images of the subsurface. The imaging algorithms must then be tuned to the specific acquisition parameters. More specific, the refraction of the waves at the surface and the acquisition geometry of the transmitting and receiving antenna influence the arrivaltime of backscattered energy related to subsurface objects. Since imaging algorithms are based on coherent stacking over this energy we must take into account these factors. We produce clear images of landmines and other subsurface objects using adapted imaging algorithms on the data obtained with the new radar system.

Sparse data acquisition and its influence on imaging

For the detection of buried objects using GPR technology, it is often necessary to acquire data over a two-dimensional area. To obtain a three-dimensional image of the subsurface using such a C-scan we have to apply three-dimensional imaging methods. To avoid artifacts in the image we have to sample sufficiently in both inline and crossline direction. Generally this results in a quadratic increase of the number of data compared to acquisition along a line. Consequently, the acquisition time and costs are increased as well. Often a practical compromise is chosen such that inline spacing is kept small, but the crossline spacing is increased for faster acquisition. We have investigated the influence of the spatial acquisition geometry on the final image that is obtained after three-dimensional imaging. For that purpose, we have modeled the back scattering by a point scatterer created by the radiation of a dipole antenna on a half-space. By using a three-dimensional diffraction stack as imaging algorithm for different acquisition geometries we can observe the acquisition footprint in the image. We conclude that the crossline spacing only weakly influences the resolution in the inline direction, especially for lines at zero crossline offset to objects.

Experimental verification of stress-induced anisotropy

RP1.7 Summary During experiments with a triaxial pressure machine in the laboratory a stress history in rock is induced. While loading and unloading the changes in elastodynamic wave behaviour are measured with ultrasonic transducers. These experiments give insight in the influence of a changing stress state on the wave propagation; the rock becomes anisotropic under differential stress, which expresses itself in an angular dependant change in velocities. A related effect of anisotropy is shear-wave splitting, which is made visible by using single pairs of broad-band shear wave transducers. (5)