Hydrogeophysical Investigations at Hidden Dam, Raymond, California

Abstract

Self-potential and direct current resistivity surveys are carried out at the Hidden Dam site in Raymond, California to assess present-day seepage patterns and better understand the hydrogeologic mechanisms that likely influence seepage. Numerical modeling is utilized in conjunction with the geophysical measurements to predict variably-saturated flow through typical two-dimensional dam cross-sections as a function of reservoir elevation. Several different flow scenarios are investigated based on the known hydrogeology, as well as information about typical subsurface structures gained from the resistivity survey. The flow models are also used to simulate the bulk electrical resistivity in the subsurface under varying saturation conditions, as well as the self-potential response using petrophysical relationships and electrokinetic coupling equations.The self-potential survey consists of 512 measurements on the downstream area of the dam, and corroborates known seepage areas on the northwest side of the dam. Two directcurrent resistivity profiles, each approximately 2,500 ft (762 m) long, indicate a broad sediment channel under the northwest side of the dam, which may be a significant seepage pathway through the foundation. A focusing of seepage in low-topography areas downstream of the dam is confirmed from the numerical flow simulations, which is also consistent with past observations. Little evidence of seepage is identified from the self-potential data on the southeast side of the dam, also consistent with historical records, though one possible area of focused seepage is identified near the outlet works. Integration of the geophysical surveys, numerical modeling, and observation well data provides a framework for better understanding seepage at the site through a combined hydrogeophysical approach.

Reservoir assessment using non-invasive geophysical techniques

“…Others, like Mydlikovski et al (2007) used high-frequency electromagnetic methods: The depth of penetration of electromagnetic waves was small due to the fact that, in this medium, electromagnetic waves are strongly attenuated, and the structural weaknesses of levee caused by high level of ground water is characterised by high level of electrical permittivity and low resistivity. For fast detection of seepage inside embankments, the self-potential (SP) technique has been the preferred one (Rittgers et al 2015;Bolève et al 2011;Minsley et al 2011), and salt tracers injection have also been used with SP to enhance the detection of preferential flow path in heterogeneous media (Ikard et al 2012).…”

Developing integrated methods to address complex resource and environmental issues

“…The work of Malinverno (2002) and Sambridge and others (2006) has been adapted to perform model assessment FDEM geophysical datasets using a trans-dimensional Bayesian MCMC algorithm, the details of which can be found in Minsley (2011). This is a one-dimensional algorithm that produces an ensemble of many (tens to hundreds of thousands) layered-earth models of electrical resistivity as a function of depth that are consistent with the measured data.…”

“…Making proposals that efficiently explore the space of plausible models is extremely important and is discussed in detail by Minsley (2011). The algorithm runs for a prespecified number of samples are drawn or until some convergence criterion is met.…”

SP2DINV: A 2D forward and inverse code for streaming potential problems

“…In karstic areas, the SP data have been used to locate the preferential flow pathways due to the presence of sinkholes and crypto-sinkholes (Jardani et al, 2006a). Minsley et al (2011) demonstrated its efficiency to detect leakages in earth dams. In geothermal systems, the self potential anomalies can amount hundreds of millivolts and can be used to reconstruct the large-scale pattern of ground water flow (Ishido, 1989;Revil and Pezard, 1998).…”