Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

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. Shorenormal 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 sa, 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 sa. 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. © 2016 Society of Exploration Geophysicists. All rights reserved

Deconstructing a polygenetic landscape using LiDAR and multi-resolution analysis

It is difficult to deconstruct a complex polygenetic landscape into distinct process-form regimes using digital elevation models (DEMs) and fundamental land-surface parameters. This study describes a multi-resolution analysis approach for extracting geomorphological information from a LiDAR-derived DEM over a stabilized aeolian landscape in south Texas that exhibits distinct process-form regimes associated with different stages in landscape evolution. Multi-resolution analysis was used to generate average altitudes using a Gaussian filter with a maximum radius of 1 km at 20 m intervals, resulting in 50 generated DEMs. This multi-resolution dataset was analyzed using Principal Components Analysis (PCA) to identify the dominant variance structure in the dataset. The first 4 principal components (PC) account for 99.9% of the variation, and classification of the variance structure reveals distinct multi-scale topographic variation associated with different process-form regimes and evolutionary stages. Our results suggest that this approach can be used to generate quantitatively rigorous morphometric maps to guide field-based sedimentological and geophysical investigations, which tend to use purposive sampling techniques resulting in bias and error. © 2016 Elsevier B.V

Characterizing coastal dune blowouts using GPR in combination with LiDAR and air photos

In the present study, GPR surveys are used to interpret the evolution of representative trough-blowouts within Padre Island National Seashore, based on the assumption that the bedding structures formed during blowout evolution cause reflections that can be recognized in radar transects (Everett 2013). These features do not definitively categorize environments however it is possible to assign specific stratigraphic features with phases of blowout evolution. In order to understand the surficial expression and extent associated with each phase aerial photographs from 1969 to 2010 and LiDAR data from 2005 and 2011 were used to help interpret the structures observed in the GPR surveys and create a three dimensional understanding of blowouts within Padre Island National Seashor