The technique of compressional (P) wave refraction is<br>routinely used in geotechnical and hydrogeological surveys.<br>Developments over the last 10 years in field procedures and<br>methods of data analysis have greatly increased the utility of<br>this technique in mapping targets in the shallow environment.<br>Nonetheless, two problems associated with hidden layers are often<br>cited as serious shortcomings with the technique, namely mapping<br>the top of the hidden layer and mapping boundaries below the<br>hidden layer. If a hidden layer is not identified in the data,<br>the errors in depth calculations can be significant. Lankston<br>(1989) has demonstrated the use of the general reciprocal method<br>(GM) for recognizing a hidden layer and mapping of boundaries<br>below the layer. Once recognized, the errors in depth<br>calculations are reduced but are still greater than. for problems<br>where a hidden layer does not exist. This reason is often used<br>to justify more costly P-wave reflection surveys.<br>While not routinely applied in shallow investigations,<br>horizontally polarized shear waves can yield additional<br>information. Shear (S) wave velocity is largely dependent on the<br>shear modulus or stiffness of a material and is virtually<br>unaffected by degree of saturation. As a result of the different<br>nature of S-wave propagation, S-waves may often show boundaries<br>not seen in P-wave surveys and equally the P-wave may show<br>boundaries S-waves do not.<br>The combined use of both P and S-wave seismics are described<br>for two site surveys. Lithology logs were available for both<br>sites and the seismics investigation formed part of a more<br>comprehensive geophysical feasibility study.<br>The hidden layer problem is often encountered in shallow<br>unconsolidated sediment/bedrock surveys where "velocity<br>inversionsl' and "layers too thin" are common. Boundaries can be<br>mapped below hidden layers with GRM. Also, by using both P and<br>S-waves, additional boundaries can be mapped that would not have<br>'been seen using one technique in isolation.


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