In modern times, refraction seismology has been used to examine the near surface in petroleum exploration and engineering applications. In refraction seismology, the relevant seismic arrivals are the direct wave and the “head wave” (critically refracted arrival). For ease of interpretation, the head wave is identified most easily when it is the first event on a seismogram.
An understanding of the direct arrival and the head wave can be obtained by examining Figure 1. Detailed explanations of refraction seismology are included in many texts, including Grant and West (1965) and Kearey and Brooks (1991).
In homogeneous media such as the one in Figure 1, we can think of seismic energy emanating from a point source as a spherical wavefront. In the 2D model of the figure, these become circles. For P-waves, rays constructed perpendicular to wavefronts will describe the direction of wave motion. Consider the wave motion in the two-layer model of Figure 1 for the case in which the velocity of the second layer is greater than that of the first layer. Let the top layer have a thickness h and a velocity v1, and let the velocity of the second layer be v2.
In homogeneous media such as the one in Figure 1, we can think of seismic energy emanating from a point source as a spherical wavefront. In the 2D model of the figure, these become circles. For P-waves, rays constructed perpendicular to wavefronts will
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Fundamentals of Geophysical Interpretation
Fundamentals of Geophysical Interpretation, SEG Geophysical Monograph Series No. 13, is a practical handbook for the petroleum geophysicist. Fundamental concepts are explained using heuristic descriptions of seismic modeling, deconvolution, depth migration, and tomography. Pitfalls in processing and contouring are described briefly. Applications include petroleum exploration of carbonate reefs, salt intrusions, and overthrust faults. The book includes past, present, and possible future developments in time-lapse seismology, borehole geophysics, multicomponent seismology, and integrated reservoir characterization.