Engelkemeir and Khan reported in 2007 that over 300 active gravitydriven faults intersect the earth's surface in the Houston Metropolitan area on the northern edge of the Gulf of Mexico basin. These surface faults have caused damage to roads, pipelines and buildings. LIDAR has proved to be an effective
tool for mapping fault scarps and topographic changes related to potential faults. Nearsurface techniques such as seismic and GPR have proven effective as an aid in determining whether topographic features that resemble scarps are faults or related to some other natural or anthropogenic surface features.
The nearsurface data provide indications of faulting at the two known fault locations and suggest that the scarp at the candidate location studied does not appear to be fault related. It may correspond to an erosional feature such as a fluvial terrace riser. GPR data provided an approximate depth penetration of 75 to150 ft (2040 m).
Near surface characterization of active surface faults can aid in determining whether a particular feature is a fault before fault movements result in damage to structures such as houses or roads. In some cases through creative rezoning the fault affected areas could be used as a park or recreation area. Open space has deliberately been left along a surface fault in several places in the Houston area.
Future seismic work should include a more robust seismic source, such as an assisted weight drop or other high frequency source. This would provide for better signal to noise ratio, deeper penetration and possibly a higher frequency content. More geophones allow a higher stack fold while providing greater offsets to aid in NMO determination.
The authors used geophone spread allowed close sampling of the shallow subsurface, but this is also the region dominated by the direct wave, ground roll and refractions and therefore reflection events could not be confidently identified within these coherent noise arrivals. Longer lines will allow for better
mapping of displaced horizons.
Increasing the depth of penetration by using 50 MHz data may be a focus of future GPR surveys. Using variable offsets and following a reflection seismic processing workflow should provide for better signal to noise ratio, velocity determination and increased resolution. The cause of the noise encountered in the lower half of the section needs to be understood and eliminated. Currently the most likely cause is a weak input signal. Dielectric constants could be measured at selected locations, ideally sampling the near surface by cores to aid in depth conversion. Longer lines, as with the seismic, will provide for better characterization of horizons.
The LIDAR allowed us to accurately map scarps. In the future better nearsurface characterization that would avoid the need to drill will be an excellent complement to the LIDAR mapping. This complementary, multitool imaging approach will aid fault hazard detection and may shed further insight into Houston area tectonics.
[For a more detailed study on the subject area, see: http://www.i2mconsultants.com/downloads/corp/JGG-1-015.pdf ]
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