Engineering geology

Information can be obtained from seismic prospecting

Seismic prospecting

The propagation velocity of P-waves depends strongly on the porosity and the water saturation of sediments. S-wave velocity or the shear modulus, respectively, is mainly determined by the stiffness of the rock matrix. In consequence, both P-and S-wave velocities depend significantly on the fracture density of rocks. Therefore seismic investigations can contribute basically in different regards to hydrogeological investigations: To find the groundwater table or define bed rock levels appear self-evident tasks. In a more general sense, seismic investigations can serve to determine the structural and lithological framework of hydrogeological studies and to quantify the heterogeneity of aquifers and aquitards off boreholes. Also porosity and fracture density can be investigated.

Seismic structure of the weathering zone in a hard rock area based on the
application of P-waves. Top: Tomographic interpretation of refracted arrivals The
image shows a cross-section in terms of P-wave velocity vP where unconsolidated
material is indicated by low vP values. Bottom: Corresponding seismic reflection
section showing the complex layering within the vadoze zone. Velocity contours
for vP = 400, 800 and 1200 m/s are indicated by dashed lines for comparison (by
courtesy of GeoExpert AG, Schwerzenbach, Switzerland)

What are the advantages and disadvantages of seismic measurements compared to other methods? How do seismics and other geophysical measurements complement each other?

Seismic prospecting provides reliable information on the depth of interfaces. In comparison to electromagnetic induction and DC-geoelectrical measurements, penetration depth and structural resolution of seismic measurements are usually higher and less ambiguous. These advantages are paid for with higher costs in acquisition and interpretation. The relation of vP and vS to rock parameters such as porosity and pore fill is not unique. Therefore, seismic measurements have to be combined with other types of geophysical methods if sedimentary parameters are to be determined in situ.
In hydrogeophysics it is most advantageous to combine geoelectrical andseismic methods to determine, for example, the porosity of sediments or the quality and depth distribution of groundwater horizons. In particular, seismic methods are useful to embed geoelectrical measurements in the framework of geological structure. The sequence “drill hole – seismics – DC geoelectrics – EM-induction” can be regarded as a sequence of decreasing structural resolution and investigation costs. Seismics is a reliable means to extrapolate structural information gathered at boreholes into the area off the borehole.
Reinhard Kirsch

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