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

Abstract

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 (10 1 -10 2 km) barrier island hydrogeology. The purpose of this study is to investigate the influence of groundwater dynamics on apparent conductivity σ a to separate the effects of hydrology and geology from the σ a 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 σ a measurements are stable, but vary with height and location across the beach. Repeatability tests confirm σ a 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 σ a , complicating data processing and interpretation. Shore-normal surveys across the barrier/wind-tidal flats show that σ a is roughly negatively correlated with topography and these relationships can be used for characterizing different coastal habitats. For all surveys, σ a increases with decreasing frequency. Alongshore surveys performed during different seasons and beach states reveal a high degree of variability in σ a . 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 σ a signal.

Directional dependency and coastal framework geology: implications for barrier island resilience

“…Previous research demonstrates that island morphology and framework geology can be spatially variable at multiple scales alongshore (Hapke et al, 2016;Lentz and Hapke, 2011;Schwab et al, 2013;Wernette et al, 2018;Weymer, 2012Weymer et al, 2015a); however, previous approaches utilized models unable to identify spatial lags that may occur given alongshore sediment transport gradients. Since the goal of this paper is to evaluate SRD and LRD of island morphology and framework geology and to test whether there is directional dependence in island morphology, the current study requires a statistical model capable of accounting for SRD and LRD.…”

“…Padre Island National Seashore (PAIS) on North Padre Island, Texas, represents an ideal location to test this hypothesis because previous studies have documented significant variability in the subsurface framework geology ( Fig. 1; Anderson et al, 2016;Fisk, 1959;Wernette et al, 2018;Weymer et al, 2018;Weymer, 2012, and there is substantial alongshore variation in beach and dune morphology. Given that the dominant current along the central Texas coast flowed from north to south during the Holocene (Sionneau et al, 2008), it follows that the dominant alongshore sediment transport gradient during that time also flowed from north to south.…”

Statistical modeling of the long-range dependent structure of barrier island framework geology and surface geomorphology

“…Thus, we assume the tidal influence on the EMI 336 signal can be neglected over the spatial scales of interest in the present study. Nevertheless, the 337 duration and approximate tidal states of each survey was documented in order to compare with 338 the EMI signal (see Weymer et al, 2016). Tidal data were accessed from NOAA's Tides and 339…”

“…For all surveys, the EMI profiler was used in a vertical dipole orientation with TX and 343 RX coils aligned in the (P-mode) direction parallel to the profile line (Weymer et al, 2016). 344…”

Long-range dependence in coastal framework geology: Asymmetries and implications for barrier island resiliency

“…Previous studies of PAIS have utilized geophysical surveys and sediment cores to document variation in the depth to a buried Pleistocene paleo-surface (Fisk, 1959;Anderson et al, 2016;Weymer et al, 2016;Wernette et al, 2018). Weymer et al (2016) confirmed paleo-channels in the buried Pleistocene paleo-surface using a 100 km alongshore electromagnetic induction (EMI) 30 survey, where areas of lower apparent conductivity are indicative of a deeper buried surface based on the difference in conductivity between overlying Holocene sand and the buried silty clay Pleistocene paleo-surface. Areas where the subsurface apparent conductivity decreased alongshore coincided with paleo-channels which had been previously mapped.…”