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Description
The purpose of this investigation was to define the depth and nature of the Santa Margarita River basin in San Diego County California. The de resistivity technique of geophysical investigation is well-suited to characterizing the depth and hydrogeologic nature of the saturated alluvial deposits because water is a good electrical conductor. Three de resistivity soundings using the Schlumberger array were carried out along the west flank of the river. Survey #1 was conducted 30 m from the river; the results indicate a surface layer about 5 m thick with a resistivity of 45 ohm-m and a second layer about 24 m thick with a resistivity of 64 ohm-m. These two layers represent a saturated medium grained sand with varying silt and organic content. These conductive layers overlay a highly conductive clay-rich layer with a resistivity of 11 ohm-m. Survey #2 was conducted 70 m west of the river and parallel to survey # 1. The results indicate a surface layer of dry sand with a resistivity of 24 70 ohm-m to a depth of 1. 3 m. Under this surface layer was a conductive zone about 22 m thick with a resistivity of 41 ohm-m representing a saturated silty sand layer. Underlying the conductive layer was a basement rock with a resistivity of 7600 ohm-m, representing_a fractured granite. Survey #3 was conducted 100 m west of the river and parallel to the canyon wall. The results of the survey indicate a dry sandy surface layer about 1.4 m thick with a resistivity of 2370 ohm-m. The second layer was composed of sandy gravel about 15 m thick with a resistivity of 118 ohm-m overlying a resistive basement of fractured granite having a resistivity of 6000 ohm-m.
