Seismic Refraction Versus Reflection

Introduction


The difference between seismic refraction and seismic reflection is never obvious to the non geophysicist, and rarely explained in simple terms by geophysicists. Due to the similarity of the names, many non geophysicists assume that the terms are interchangeable, or are unaware that there are critical differences between the two techniques that may make one vastly preferred or the other completely unusable givenExample Seismic Record site specific conditions or project goals.


General Seismic Principles


Seismic techniques generally involve measuring the travel time of certain types of seismic energy from surficial shots (i.e. an explosion or weight drop) through the subsurface to arrays of ground motion sensors or geophones.  In the subsurface, seismic energy travels in waves that spread out as hemispherical wavefronts (i.e. the three dimensional version of the ring of ripples from a pebble dropped into a pond).  The energy arriving at a geophone is described as having traveled a ray path perpendicular to the wavefront (i.e. a line drawn from the spot where the pebble was dropped to a point on the ripple).  In the subsurface, seismic energy is refracted (i.e. bent) and/or reflected at interfaces between materials with different seismic velocities (i.e. different densities).  The refraction and reflection of seismic energy at density contrasts follows exactly the same laws that govern the refraction and reflection of light through prisms.  Note that for each seismic ray that strikes a density contrast a portion of the energy is refracted into the underlying layer, and the remainder is reflected at the angle of incidence. The reflection and refraction of seismic energy at each subsurface density contrast, and the generation of surface waves (or ground roll), and the sound (i.e. the air coupled wave or air blast) at the ground surface all combine to produce a long and complicated sequence of ground motion at geophones near a shot point. The ground motion produced by a shot is typically recorded as a wiggle trace for each geophone (see Example Seismic Record at right).


Seismic Refraction


Seismic refraction involves measuring the travel time of the component of seismic energy which travels down to the top of rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface as a head wave along a wave front similar to the bow wake of a ship (see Seismic Refraction Geometry below). The shock waves which return from the top of rock are refracted waves, and for geophones at a distance from the shot point, always represent the first arrival of seismic energySeismic Refraction Geometry.


Seismic refraction is generally applicable only where the seismic velocities of layers increase with depth. Therefore, where higher velocity (e.g. clay) layers may overlie lower velocity (e.g. sand or gravel) layers, seismic refraction may yield incorrect results. In addition, since seismic refraction requires geophone arrays with lengths of approximately 4 to 5 times the depth to the density contrast of interest (e.g. the top of bedrock), seismic refraction is commonly limited (as a matter of practicality) to mapping layers only where they occur at depths less than 100 feet. Greater depths are possible, but the required array lengths may exceed site dimensions, and the shot energy required to transmit seismic arrivals for the required distances may necessitate the use of very large explosive charges. In addition, the lateral resolution of seismic refraction data degrades with increasing array length since the path that a seismic first arrival travels may migrate laterally (i.e. in three dimensions) off of the trace of the desired (two dimensional) seismic profile.


Recent advances in inversion of seismic refraction data have made it possible to image relatively small, non-stratigraphic targets such as foundation elements, and to perform refraction profiling in the presence of localized low velocity zones such as incipient sinkholes.


Seismic Reflection


Seismic reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts (see Seismic Refraction Geometry below). Reflected seismic energy is never a first arrival, and therefore must be identified in a generally complex set of overlapping seismic arrivals – generally by collecting and filtering multi-fold or highly redundant data from numerous shot points per geophone placement.  Therefore, the field and processing time for a given lineal footage of seismic reflection survey are much greater than for seismic refraction.  However, seismic reflection can be performed in the presence of low velocity zones or velocity inversions, generally has lateral resolution vastly superior to seismic refraction, and can delineate very deep density contrasts with much less shot energy and shorter line lengths than would be required for a comparable refraction survey depth.


The main limitations to seismic reflection are its higher cost than refraction (for sites where either technique could be applied), and its practical limitation to depths generally greater than approximately 50 feet.  At depths less than approximately 50 feet, reflections from subsurface density contrasts arrive at geophones at nearly the same time as the much higher amplitude ground roll (surface waves) and air blast (i.e. the sound of the shot). Reflections from greater depths arrive at geophones after the ground roll and air blast have passed, making these deeper targets easier to detect and delineate.


Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) where the inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land.


Comparison


The differences between seismic refraction and reflection are summarized in the table below.


Seismic Method Comparison

Refraction

Reflection

Typical Targets

Near-horizontal density contrasts at depths less than ~100 feet

Horizontal to dipping density contrasts, and laterally restricted targets such as cavities or tunnels at depths greater than ~50 feet

Required Site Conditions

Accessible dimensions greater than ~5x the depth of interest; unpaved greatly preferred

None

Vertical Resolution

10 to 20 percent of depth

5 to 10 percent of depth

Lateral Resolution

~1/2 the geophone spacing

~1/2 the geophone spacing

Effective Practical Survey Depth

1/5 to 1/4 the maximum shot-geophone separation

>50 feet

Relative Cost

$N

$3xN to $5xN

Note that in situations where both could be applied, seismic reflection generally has better resolution, but is considerably more expensive. In those situations, the choice between seismic reflection and refraction becomes an economic decision.  In other cases (e.g. very deep/small targets) only reflection can be expected to work.  In still other cases, where boreholes or wells are accessible, neither refraction, nor reflection may be recommended in favor of seismic tomography.


See Also:


  • Bedrock Depth Mapping

  • Karst Geophysics

  • Crosshole And Uphole Tomography

  • Fractures
     
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