A Multi‐Hydrogeophysical Study of a Watershed at Kaiwi Coast (Oʻahu, Hawaiʻi), using Seismic Ambient Noise Surface Wave Tomography and Self‐Potential Data

Abstract

As a Pacific Island, Hawaiʻi's hydrological processes are influenced by strong spatio-temporal gradients in, and close interactions between, rainfall, land-use, near-surface geological conditions, and complicated subsurface hydrogeology. Due to their volcanic nature, the hydrogeology of Hawaiʻi predominantly consists of basaltic bedrock formations that vary in terms of, for example, the type of lava flow deposit ('A'ā vs. Pāhoehoe), age, and thickness of individual flow units. Several factors, such as the age of the islands and climatic variability, control regolith development, and its characteristics (Ryu et al., 2014;P. Vitousek, 2004).Hawaiian hydrogeology possesses strong spatio-temporal gradients and heterogeneity at a variety of spatial scales (Figure 1). For example, basaltic formations are dominated by both small-and intermediate-scale fractures and large-scale faults that strongly affect chemical fate and transport processes (Deng & Spycher, 2019). Mechanical weathering and chemical alteration create saprolite layers with low hydraulic conductivity, forming barriers to flow (Figure 1) and/or causing groundwater to be trapped in perched formations (Hunt, 1996;Mink & Lau, 1980;Oki, 2005). On the island of Oʻahu, as well as on other Pacific islands, valley-ridge complexes are known to generate highly complicated groundwater systems (Oki, 2005). Older alluvium deposits, created during periods of extensive erosion that carved deep valleys in the original volcanoes, are of hydrological importance due to their relatively low hydraulic conductivity, estimated to be in the range of 3.96 × 10 −3 m/day (0.013 ft/day) to 0.33 m/day (1.08 ft/day) (Wentworth, 1938). The low permeability is related to chemical alteration, weathering, and compaction (Oki, 2005;Wentworth, 1951).

Monitoring of Groundwater in a Limestone Island Aquifer Using Ambient Seismic Noise

“…The CCF corresponds to the Green's function of the medium between the sensors [42][43][44][45][46][47][48]. If one of the sensors is considered to be an impulsive source, the retrieval of Green's function between two sensors yields the impulse response of the Earth recorded at the other sensor [49]. Cross-correlation of ambient seismic noise is a powerful tool for imaging the Earth's interior on a local, regional, or global scale (e.g., [38,46,[50][51][52][53][54][55][56][57]).…”

Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

“…Current geophysical imaging techniques are still unable to directly estimate fluxes, as they are based on the contrast in water properties induced through tracers. Nevertheless, very promising results were obtained through the combination of ambient noise surface wave tomography (ANSWT), and self-potential (SP) measurements to image the complex hydrogeology structure and associated groundwater flow paths at a coastal site (Grobbe et al, 2021). The joint interpretation of the SP and seismic data permits to show that groundwater flow occurs in the identified paleo-channels at the erosional surface of the basaltic bedrock (Grobbe et al, 2021).…”

“…Nevertheless, very promising results were obtained through the combination of ambient noise surface wave tomography (ANSWT), and self-potential (SP) measurements to image the complex hydrogeology structure and associated groundwater flow paths at a coastal site (Grobbe et al, 2021). The joint interpretation of the SP and seismic data permits to show that groundwater flow occurs in the identified paleo-channels at the erosional surface of the basaltic bedrock (Grobbe et al, 2021). Despite these improvements, monitoring the dynamics of fluxes remains resource expensive.…”

Protecting the Vital Groundwater Sources of Oceanic Islands

“…Potential techniques include active or passive seismic methods, which primarily provide information about the three‐dimensional subsurface geological structure. These data can also be integrated with self‐potential data to identify and interpret preferential groundwater or contamination plumes as controlled by the geological subsurface structure (Grobbe et al 2021). Seismic noise can also be used for monitoring purposes, such as water table variations (Garambois et al 2019).…”