Some Limitations of Seismic Refraction Methods in Geohydrological Surveys of Deep Alluvial Basinsa

Abstract

ABSTRACTSeismic methods, combined with available well and geologic data, were used to define the subsurface hydrologic and geologic conditions of the Walnut Gulch Experimental Watershed, a deep, alluvial basin near Tombstone, Arizona. Surface geology of the valley indicates an alluvium‐filled area between igneous intrusives and sedimentary rocks that support the Tombstone Hills on che southwest and the Dragoon Mountains on the northeast. Seismic determinations revealed depths to the water table ranging from near zero at the confluence of Walnut Gulch and the San Pedro River to 475 feet in the central portion of the watershed. The accuracy of predicting the depth to either ground water or basement was ± 6 percent, while that for ground water alone was ± 10 percent.Seismic refraction methods failed to produce satisfactory results when certain conditions or combinations of conditions existed. The great depths of alluvium created the problem of estimating the seismic traverse length necessary to accurately determine the alluvium‐basement interface. Increased compaction of alluvium with depth causes additional interpretation difficulties in some areas. Also, water‐table elevation may not always be accurately determined if it is located above, and close to, a highervelocity layer.A minimum of gravity or well log information greatly increases the reliability of seismic information in these problem cases. Other methods may still be necessary to substantiate the seismic data obtained under these conditions.