ALBERT

Description: Electromagnetic induction (EMI) techniques are becoming increasingly popular for near-surface coastal geophysical applications. However, few studies have explored the capabilities and limitations of portable multifrequency EMI profilers for mapping large-scale (101–102 km) barrier island hydrogeology. The purpose of this study is to investigate the influence of groundwater dynamics on apparent conductivity σa to separate the effects of hydrology and geology from the σa signal. Shore-normal and alongshore surveys were performed within a highly conductive barrier island/wind-tidal flat system at Padre Island National Seashore, Texas, USA. Assessments of instrument calibration and signal drift suggest that σa measurements are stable, but vary with height and location across the beach. Repeatability tests confirm σa values using different boom orientations collected during the same day are reproducible. Measurements over a 12 h tidal cycle suggest that there is a tide-dependent step response in σa, complicating data processing and interpretation. Shore-normal surveys across the barrier/wind-tidal flats show that σa is roughly negatively correlated with topography and these relationships can be used for characterizing different coastal habitats. For all surveys, σa increases with decreasing frequency. Alongshore surveys performed during different seasons and beach states reveal a high degree of variability in σa. Here, it is argued that surveys collected during dry conditions characterize the underlying framework geology, whereas these features are somewhat masked during wet conditions. Differences in EMI signals should be viewed in a relative sense rather than as absolute magnitudes. Small-scale heterogeneities are related to changing hydrology, whereas low-frequency signals at the broadest scales reveal variations in framework geology. Multiple surveys should be done at different times of the year and tidal states before geologic interpretations can confidently be made from EMI surveys in coastal environments. This strategy enables the geophysicist to separate the effects of hydrology and geology from the σa signal.