COMPARING GROUND PENETRATING RADAR AND ELECTRICAL RESISTIVITY IMAGING METHODS TO EXAMINE SAND BLOW DEPOSITS IN THE NEW MADRID SEISMIC ZONE

For this study, we explore the relative advantages of ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for examining the relationship of earthquake-induced liquefaction deposits to surrounding host sediments. Each technique affords different advantages, but when combined, enhances the reliability of interpretations. The study site is located in the New Madrid seismic zone near Blytheville, Arkansas, where multiple sand blows are visible on satellite imagery. Five GPR surveys were conducted with a 400 MHz bistatic antenna capable of producing interpretable imagery of up to 4 meters depth; this method produces high resolution images of horizontally lying reflectors and the continuity of the sand blows. Four of these surveys were conducted perpendicular to the strike of the sand blow deposits for a combined length of approximately 360 meters, and one survey was conducted on a bias, crossing the two northernmost sand blow deposits. ERT surveys were conducted along profiles oriented parallel with GPR lines. The ERT data were collected with a 2-meter electrode spacing using a dipole-dipole array to yield good resolution of the near-surface sediments. Although ERT produces a lower resolution image of the subsurface than GPR, the depth of penetration for interpretable data is greater. The GPR data show a strong reflector representing the contact between the base of the sand blow and the underlying silty or clayey soils as well as a deepening of the contact and thickening of the sand blow towards the central vent and dike. A stretch of disrupted reflections identifies the main dike. The ERT results provide a detailed distribution of resistivity associated with the underlying and adjacent sedimentary deposits. Low resistivity areas along the profiles denote fine-grained deposits, such as clays and silts, whereas high-resistivity areas indicate coarser-grained deposits. Because resistivities are related to porosity and permeability, the data contribute to understanding the context for the occurrence of liquefaction at the site. The knowledge gained through GPR and ERT about sand blows, related dikes and their relationship to host sediments contribute to the understanding of sedimentary architecture and the role it plays in the formation of liquefaction features.