slowly than the compressional waves that travel through the subsurface rock formations; these body waves are the desired signals for seismic surveys of the subsurface. The surface Rayleigh waves also attenuate more slowly than the body waves. This lack of attenuation exacerbates noise caused by scattering from surface features, an effect geophysicists try to mitigate by proper planning.


> Baseplate breakthrough. The sabkha had insufficient strength to support the vibrating baseplate, which broke the surface.


Moving the vibrators and personnel on site is only part of the planning required. The topogra - phy also impacts the quality of the coupling between source or receiver and ground and may affect signal propagation in the near surface. A rough or rocky surface may cause point loading of the baseplate, which highly distorts the trans - mitted signal. Good source coupling can be achieved in soft sediments, as long as the ground supports the baseplate load. However, if the baseplate breaks through a hard ground surface, the result is again poor coupling, a distorted signal and possibly a cutoff of the high-frequency component of the generated signal (above). The most significant contribution to surface- related noise in seismic acquisition is ground roll, which is a surface wave, or more precisely a Rayleigh wave, that travels at the ground/air interface (below). Rayleigh waves travel more


In wet environments such as swamps, marshes and some sabkhas, the surface wave couples with the liquid and is termed a mud wave.7


A mud wave


is often much slower than a Rayleigh wave because of the weak particle coupling in the water-saturated solid near the surface. Variations in ground elevation require static corrections to the measured seismic signals. Determining the corrections may be particularly difficult in near-surface, weathered soils. Signals in the surface materials may have radically slower velocities than those in the hard rock beneath. If the weathered layer has significant localized variations in thickness, this may require static corrections that change rapidly, both vertically and laterally, within a small area. Sand dunes, sabkhas and marshes pose this problem for seismic acquisition.


In addition to the static correction problem, in sand dunes, body waves may reflect from the bottom of the dune, becoming trapped within the dune itself. In wadis, the top of the water table affects the first breaks in the seismic signal, so the water level is important to discern. Soft materials, such as unconsolidated sand, sabkhas and dry glacial till, also cause high attenuation of the body-wave signal within the surface layer.


Baseplate


Boundaries often scatter the seismic energy, creating noise. These may be topographic changes, such as escarpments, or lithological or mineralization boundaries. The risk of noise from scattering is higher in hard ground such as carbonates and basalt. Resonance of seismic waves occurs in areas that are enclosed by materials of greater acoustic impedance. For example, once a surface wave from hard rock enters a softer claypan, it may become trapped, reflecting back at another boundary with the hard rock. Similar observations are often made in swamps.


Mapping risks to a land seismic party before its deployment is one way to assess potential problems for personnel and equipment. A satel - lite survey that discriminates surface features in detail gives this option. For example, a DEM is particularly useful for identifying structure at a scale of 10 m [33 ft] and larger. It can locate escarpments and highlight other features that have a common elevation signature, either flat (such as claypans, sabkhas, floodplains, swamps and marshes) or varied (such as wadis, sand dunes and glacial moraines). At smaller scales of centimeters to decimeters, radar imagery illumi - nates surface microstructure and texture information by distinguishing diffuse and specular reflections. This provides information about rock structure, fractures and ripples. In addition, minerals have different responses in the infrared range, so those bands are included in studies of lithology.


In most cases, remote-sensing analysis


> Surface modes in seismic acquisition. A vibrator truck directs seismic energy to deep formations as body waves (black). However, significant energy is scattered from this wave or trapped near the surface. Some is refracted at formation boundaries (light blue). Rayleigh waves (purple) travel along the surface and may scatter from escarpments, as shown here, or at changes in lithology (not shown). Other seismic energy may be trapped in soft sediments between harder layers (orange) or reflected at interfaces (red).


incorpo rates information from one or more satellites, from ground observations and maps including infrastructure, and when available, from subsurface geology. Integration of data using a geographic information system (GIS) is critical. A GIS is a tool for storing, visualizing and processing data in a common geographical workspace to help model the world as accurately as possible. The system allows a user to interactively query and analyze data and create maps. Within a GIS, for example, an image from a radar satellite overlain with a combination of visible and infrared bands can be mapped in a common space with the traverse and observa - tions of a ground survey. The GIS software also allows the viewer to see the combined data from any angle or to “fly” through the space. By combining the remotely sensed data with physical models, such as wave propagation and source and receiver coupling to various surface materials, and using logical rules, such as safe slope angle for vehicles, the GIS system displays


44


Oilfield Review


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