4. HIGH RESOLUTION SEISMIC REFLECTION SURVEY IN OXNARD, CA.
Sea water intrusion has been a problem along the coast of Southern California for many decades. In the vicinity of the Oxnard plain this problem was first observed in the 1930s and escalated to a serious problem in the mid-1950s. Subsurface inflow of brine has been credited as the source of high chlorides in wells located behind the seawater front (Izbicki, 1991; Stamos and others, 1992). There have been conflicting interpretations of geophysical logs and well cuttings obtained by others in previous studies regarding the structural orientation of Plio-Pleistocene sediments that include critical aquifer systems. Of particular concern is the existence of a suspected structural high. This uncertainty has hindered the design and construction of a barrier intended to impede the encroachment of the seawater front.
In 2010 NORCAL conducted a high resolution seismic reflection survey in the area surrounding the suspected structural high. We used a 480-channel distributed array seismic system to record un-correlated vibroseis data from triple 28 Hz geophone groupings evenly spaced at 325 per km.
A total of around 10 km of full-fold data on four lines were recorded along roads and trails within and around a suspected structural high. One of the lines was conducted along the bottom of a canal, as shown in the photo at right. A sample shot gather is shown below.
Many of the signal characteristics in the study area are evident on the sample shot gather shown above. The dominant frequency through most of the section is over 100 Hz and, in the interval between 50 and 400 ms, useable frequencies over 150 Hz were recorded. Vertical resolution is around 2 m.
The processed seismic sections provide high resolution images of the subsurface to depths in excess of 3,000-ft as indicated by the sample section shown above. By correlating the seismic sections with existing well data we derived an interpretation of the seismic reflection data that delineates the depth, thickness and configuration of important hydrogeologic units as shown below. The colored zones represent significant aquifers, the white zones represent non-permeable strata and the dashed line correlates with a regional non-conformity.