FINDING SURFACE FAULTS USING AERIAL PHOTOGRAPHS AND GEOPHYSICALLY-LOGGED BOREHOLES

No single tool can be relied upon to locate surface faults in the Gulf Coast region. However, probably the two most important tools are aerial images and geophysically logged boreholes.

Faults appear on aerial images largely because the coastal plain is very flat thus allowing rainwater to accumulate and remain for several days on the downthrown side of an active fault. Normally a fault will not appear on photographs taken a few weeks or more after a rainfall event. As a consequence, one must search through photos taken at various times in order to find a useable one. Generally, faults will not appear on aerial images of densely wooded areas and developed areas.

Geophysically-logged boreholes, properly interpreted, provide the most certain indication of the presence of a fault. Holes are normally drilled quickly to a depth of 300 feet and logged with tools that measure physical properties of the strata encountered in the hole. These properties include natural gamma ray emission (counts per second), electrical self-potential (millivolts), electrical resistivity (ohm-meters) and electrical resistance (ohms). Boreholes are drilled on a line about perpendicular to the strike of a suspected fault, and spaced from a few feet to several hundred feet apart. If a borehole penetrates a fault, strata will be missing at the penetration point thus enabling the interpreter to establish the depth to the fault at that point. If the fault is not penetrated, its presence will be revealed by a vertical offset (throw) of a few to several feet of the strata encountered in two adjacent boreholes. Because nearly all, and perhaps all, Gulf Coast faults are growth faults, their throw increases with depth. Throw that is too small to recognize in a 100-foot deep hole may be recognizable in a 300-foot hole.