Crosshole Reflection Research Articles

Crosshole reflection imaging with ground-penetrating radar data: Applications in near-surface sedimentary settings

Crosshole ground-penetrating radar (GPR) is applied in areas that require a very detailed subsurface characterization. Analysis of such data typically relies on tomographic inversion approaches providing an image of subsurface parameters. We have developed an approach for processing the reflected energy in crosshole GPR data and applied it on GPR data acquired in different sedimentary settings. Our approach includes muting of the first arrivals, separating the up- and the downgoing wavefield components, and backpropagating the reflected energy by a generalized Kirchhoff migration scheme. We obtain a reflection image that contains information on the location of electromagnetic property contrasts, thus outlining subsurface architecture in the interborehole plane. In combination with velocity models derived from different tomographic approaches, these images allow for a more detailed interpretation of subsurface structures without the need to acquire additional field data. In particular, a combined interpretation of the reflection image and the tomographic velocity model improves the ability to locate layer boundaries and to distinguish different subsurface units. To support our interpretations of our field data examples, we compare our crosshole reflection results with independent information, including borehole logs and surface GPR data.

Cross-hole reflection seismic to delineate a relatively thin volcanogenic massive sulphide deposit in shale hosted environment

SUMMARY The seismic reflection method is a high resolution technique that can be used in many exploration environments including mineral exploration. However, mountainous terrain, depth of burial and the steepness of ore bearing structures pose a challenge to the application of surface seismic in mineral exploration. The cross-hole seismic method may present an alternative approach under such conditions. Presented here is a synthetic study examining the capability of the cross-hole seismic method to delineate a volcanogenic massive sulphide ore body in a shale hosted environment. A simple model typical for volcanogenic massive deposits in Tasmania has been considered. There, an elongated steeply dipping volcanogenic massive sulphide deposit with an average thickness of 10 m is seated within a shale rock. The primary aim of the modelling is to test the capability of the technique to delineate relatively medium sized, steeply dipping volcanogenic massive sulphide lens in shale hosted environment. A second objective is to use the technique to prospect for extensions to mineralization along steeply dipping reflectors. Synthetic cross-hole seismic records were generated using a 120 Hz energy source. Kirchhoff VSP migration was applied to wavefield separated shot records and Pre- stacked Depth Migrated images created. The resulting migrated images correlate well with the position and dip of the ore body demonstrating the potential of the cross- hole reflection technique to delineate steeply dipping ore structures in challenging environments.

The crosshole seismic reflection method in opencast coal exploration

In opencast coal exploration in the UK, a dense grid of boreholes is drilled to provide the necessary information on the quantity and quality of coal reserves and on the geological structure. Typically, borehole spacing is 40-60 m, but it may be reduced around faults or where there are old room-and-pillar mineworkings. Even with this density of boreholes, it is not possible to detect faults with throws less than 2-3 m, and these can pose a hazard to stability during excavation which may be particularly critical at site boundaries. Also, estimates of the quantity of coal reserves could be improved if old mineworkings were located more accurately. The potential of borehole seismic surveys in this application was discussed in more detail in an earlier paper (Kragh el al. 1991). Here we report on the development of the crosshole reflection method, making use of opencast exploration boreholes, to provide seismic cross-sections between boreholes. Small explosive charges are used as sources with hydrophones as receivers. The use of downhole sources and receivers leads to excellent resolution compared with surface seismic and VSP surveys; signal bandwidth is typically 200-500 Hz. Readers interested in the application of cross-well surveys to the definition of hydrocarbon reservoirs will note that the environmental conditions for our surveys were relatively undemanding: low temperatures and hydrostatic pressures in uncased boreholes at relatively close separations. Nevertheless, the results indicate the potential of the method for deeper, larger scale surveys in sedimentary rocks, where even higher frequencies can be successfully transmitted (e.g. Harris 1988).

Seismic characterization of fracturing at the Äspö Hard Rock Laboratory, Sweden, from the kilometer scale to the meter scale

Abstract In 1986, the Swedish Nuclear Fuel and Waste Management Co. (SKB) decided to build an underground research laboratory, the Aspo Hard Rock Laboratory (HRL). The Aspo HRL offers the possibility to test and demonstrate a full site investigation sequence. As part of this work, various seismic techniques were tested and demonstrated, from the kilometer to the meter scale. The preliminary investigation stage comprised surface reflection profiling and VSP. The VSP measurements, though limited to one borehole, played an important role in defining the site model. Shallow reflection surveys in crystalline rock have been judged as basically feasible, but further development is required to reduce costs. During the construction of the laboratory, the objective of the seismic investigations was to check and refine the site model. Within the zone of excavation disturbance experiment project, seismic tomographic analyses were made for P- and S- wave velocity and attenuation, at 5–15 m scale. Crosshole reflection imaging was also performed. The tracer retention understanding experiments project also included transmission and reflection tomography but on a 100–300 m scale. For the experiments where both transmission and reflection measurements were processed, the seismic surveys also produced information regarding the intrinsic mechanical properties of the rock besides inferring a geometrical model of the rock structures.

A new method for crosswell reflector imaging

Imaging seismic reflectors with crosshole and VSP tomographic data is only occasionally carried out using Kirchhoff-style migration schemes. We introduce a new kinematic method here for tomographically imaging reflectors. The scheme first picks up traveltimes of each reflected event from common-shot gather tomographic data. It then uses the picked times to image the reflector interfaces with a very important principle: that two reflection points from the same interface that produce reflection pulses on two neighbouring receivers have the same tangent line.The same scheme can be used for surface or crosshole reflection data, but the latter requires some special processing to separate upgoing and downgoing waves. This paper will show how to apply the scheme in tomographic surveys. Traveltimes of each reflection event are initially read from a common-shot gather tomographic seismic section. Then each pair of traveltimes from neighbouring traces is processed in sequence. The two isochronal curves are calculated from the two traveltimes. A common tangent to the two isochronal curves is then found and a reflection segment is defined. In this way, all traveltimes in pairs are processed and a straight or curved interface, corresponding to the locus of tangents, is reconstructed to map the reflection interface.The processing difference between surface seismic reflection data and tomographic (crosshole) seismic reflection data is that the crosshole data contains downgoing waves as well upgoing waves. So the program has to decide if a reflection event comes from above or below the source. Then the program processes the data to obtain the reflector orientation.This imaging method can handle multiple layers and curved interfaces for an arbitrary 2D velocity distribution.

Borehole radar applied to characterization of fracture zones

A new short-pulse radar system (RAMAC) developed by ABEM AB has now been in operation for three years during which more than 100 km of borehole logging has been performed. The bulk of the surveys have been in granites and gneisses.The RAMAC system operates at centre frequencies in the interval 20 to 60 MHz. At those frequencies single-hole reflection ranges of 50 to 150 m are normally obtained in gneissic and granitic rock. Cross-hole ranges have in some cases exceeded 300 m. The large probing range in combination with resolution of the order of a few metres makes borehole radar a unique technique for investigation of fracture zones in crystalline rock.Case histories illustrate application of the RAMAC system in three different configurations (single-hole reflection, cross-hole reflection, and cross-hole tomography) and demonstrate how combination of these three can yield consistent 3D models of fracture zones and other structures.