. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. REFLEXION OF ELASTIC WAVES 151 Since a relatively steep dip results in only a very small change in the time interval being measured, it is necessary that the correction for surface effects be either negligible or very carefully determined. To realise the importance of this I quote an actual example from the Gulf Coast of Texas. In measuring dips on a surface at about 6,000 feet in depth and using a seismometer spread of 600 feet, a dip of 5 degrees will produce a variation from the normal running ti
. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. REFLEXION OF ELASTIC WAVES 151 Since a relatively steep dip results in only a very small change in the time interval being measured, it is necessary that the correction for surface effects be either negligible or very carefully determined. To realise the importance of this I quote an actual example from the Gulf Coast of Texas. In measuring dips on a surface at about 6,000 feet in depth and using a seismometer spread of 600 feet, a dip of 5 degrees will produce a variation from the normal running time of second. It is obvious that a similar error in determining the difference between corrections for surface layers at the extremities of the seis- mometer spread will produce an error of like amount in the computed dip. It is not the purpose of this paper to discuss possible errors in Seismometer Spread «» vt. vrKtnt I it running time 1o firit Mumometer and t' - • • t»t V is j-erjjt velcoKj to depfh d Fig. 11.—Graphical method of computing dip. great detail, but a warning that many insidious possibilities of error exist must be given. The problem is strictly a three-dimensional one, and yet we usually derive our interpretation assuming only two di- mensions. Propagation paths are curved, and yet our limited knowl- edge usually constrains us to assume straight line propagation. Nevertheless, this method also has achieved brilliant results. Figure 12 shows a profile obtained by this method across a deeply buried salt dome in southern Louisiana. This salt dome gives no surface indi- cation of its presence, and yet by this method we can define the prob- able location of the crest of the associated uplift in the sediments to within limits of error of 1,500 feet. POSSIBLE APPLICATIONS The ideal conditions under which to apply reflexion shooting exist when a limestone or series of limestones are present in contact with shale or sand at depths of from 2,000 feet to 7,0
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