ISSN:
1365-2478
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
,
Physics
Notes:
Seismic work near Paris first began in 1954. Two major difficulties characteristic of this region rapidly appeared:〈list xml:id="l1" style="custom"〉1) The existence of silicified limestone beds in the plateaus: these high-velocity beds, hard to drill, make it difficult to obtain surface corrections and carry strong “ground-roll” noise waves.2) The feeble structural variations call for a very accurate control of time measurements.Drilling difficulties, and the need to eliminate parasite waves, have led to the adoption of pattern shooting from hammer-drilled holes.The most effective elimination of ground roll has been obtained by shooting with a lateral offset of 600 m. This method has made it possible to work in regions where originally no reflections could be obtained.The difficulty of obtaining surface corrections has induced us to shoot up-hole velocity surveys in deep holes at particularly important points. This work is done by a special crew. At other shot points, surface corrections are calculated from structural maps of surface strata and from velocity-depth curves. The accuracy of these corrections, checked by drilling and velocity surveys, appears sufficient.The determination of lateral velocity variations by surface velocity shooting (X2/T2) is not feasible in this part of the Paris basin, as the accuracy of this method is insufficient.The control of reflection times must take into account the deformation of the signal as a function of the type of recording equipment and filters used, and also the modifications of the character of the reflection due to lateral stratigraphic variations. These variations can be observed by comparing the different velocity logs.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-2478.1961.tb01117.x
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