Many of the oil-bearing structures around the Gulf Coast and in Southwest Texas are found along a distinctive type of low-angle normal faulting. Reflection sesimograph shooting across the faults gives normal information on the downthrown side but produces two types of distorted and misleading data below the fault plane on the upthrown side. One type of distortion gives a time lag up to 100 milliseconds to reflections plotted vertically below the upper limit of the fault plane. If this time lag is not recognized, the fault may be located as far as a mile from its true position.The second type of distortion below the fault plane is an apparent increase in dip toward the fault plane in the deeper beds. The magnitude may be great enough to cause some reflecting horizons on the upthrown side of the fault to be plotted actually deeper than those on the downthrown side. This could readily cause error in locating the fault and determining the magnitude and direction of throw.The first 'drop' type of distortion is explained by a refraction pattern downward along a portion of the fault plane before the energy is reflected back from the recognized interface. The second 'increased dip' type of distortion may be either a reflection time lag through the gouge zone of the fault or a refraction across the fault plane and a reflection from the known interface.

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