Audiomagnetotelluric investigation of seawater intrusion using 2-D inversion of invariant impedances

Violeta Luján

Description

An audio-magnetotelluric survey was conducted to estimate the extension of the seawater intrusion in a coastal aquifer, in Ensenada, Baja California, México. The survey consisted of 134 closely spaced sites along three profiles 5 to 6 km long. The four elements of the impedance tensor were measured at every observation site and used to estimate the series and parallel (s-p) invariant impedances. 2-D resistivity models were obtained along each profile using a Gauss-Newton regularized inversion process. The seawater intrusion is clearly shown in the resulting models as highly conductive anomalies. In some places, the resistivity models show the bedrock and the basement faults in good agreement with structures interpreted from gravity and magnetic modeling. Based on the information provided by the resistivity models and using the available hydrogeologic information, we used Archie’s law in an attempt to gain insights about porosity and TDS distributions in the aquifer. Porosity values between 15 and 25% near the surface reasonably predict the TDS values observed in several shallow wells in the area. TDS values range from 40 g/l typical for seawater to 1 g/l in the eastern edge of the basin, 4 km away from the coastline. The best conditions were found in Profile 2, were the resistivity model predicts TDS values close to 1 g/l below 100 m depth at sites located further than 1 km from the coast. The above results show that s-p invariant impedances produced geologically plausible resistivity models. Hence, they might be a convenient set of magnetotelluric responses to be used for routine 2-D inversion of AMT data.

Informations

• Publié par: erevistas
• Publié le: 01 janvier 2010
• Nombre de lectures: 5
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Geologica Acta, Vol.8, Nº 1, March 2010, 51-66
DOI: 10.1344/105.000001515
Available online at www.geologica-acta.com
Audiomagnetotelluric investigation of seawater intrusion using
2-D inversion of invariant impedances
*
VIOLETA LUJÁN and JOSÉ MANUEL ROMO
División de Ciencias de la Tierra, Centro de Investigación Científca y de Educación Superior de Ensenada, (CICESE)
Carretera Ensenada-Tijuana 3918, frac. Zona Playitas. Ensenada, B.C., 22860, México
* corresponding author E-mail: [e-mail]
ABSTRACT
An audio-magnetotelluric survey was conducted to estimate the extension of the seawater intrusion in a coastal
aquifer, in Ensenada, Baja California, México. The survey consisted of 134 closely spaced sites along three
profles 5 to 6 km long. The four elements of the impedance tensor were measured at every observation site and
used to estimate the series and parallel (s-p) invariant impedances. 2-D resistivity models were obtained along
each profle using a Gauss-Newton regularized inversion process. The seawater intrusion is clearly shown in the
resulting models as highly conductive anomalies. In some places, the resistivity models show the bedrock and
the basement faults in good agreement with structures interpreted from gravity and magnetic modeling. Based on
the information provided by the resistivity models and using the available hydrogeologic information, we used
Archie’s law in an attempt to gain insights about porosity and TDS distributions in the aquifer. Porosity values
between 15 and 25% near the surface reasonably predict the TDS values observed in several shallow wells in
the area. TDS values range from 40 g/l typical for seawater to 1 g/l in the eastern edge of the basin, 4 km away
from the coastline. The best conditions were found in Profle 2, were the resistivity model predicts TDS values
close to 1 g/l below 100 m depth at sites located further than 1 km from the coast. The above results show that s-p
invariant impedances produced geologically plausible resistivity models. Hence, they might be a convenient set of
magnetotelluric responses to be used for routine 2-D inversion of AMT data.
KEYWORDS Seawater intrusion. Magnetotellurics. Baja California. México.
INTRODUCTION alluvial deposits that fll the basin has caused the intrusion
of seawater and the subsequent degradation of the fresh-
Maneadero Valley is a small geological basin located water quality. There is a need to study the geological and
south of the city of Ensenada, between latitudes 31º 41’ hydrogeological situation in the basin as it represents the
and 31º 45’ N, and 116º 34’ to 116 º 38’ W longitude, in source of water supply for extensive agricultural activity
Baja California, México (Fig. 1). The extensive extraction and for human use in Ensenada. Depths to bedrock and
of groundwater from an aquifer contained within the information about the geometry and structure of the basin
51V. LUJAN and JOSÉ M. ROMO. Seawater intrusion study using AMT invariant impedances
The basement rocks consist of a volcanic and
volcanoclastic sequence, largely intruded by granodioritic
rocks of the Peninsular Ranges Batholith (PRB), which
was formed during a regional magmatic accretion event
that occurred along the Baja California Peninsula in
Cretaceous time. Basement rocks outcrop in the hills to the
east of the valley, forming the eastern edge of the basin. The
basement deepens westward to a depth of 900 m close to
the coast and 1650 m offshore in Todos Santos Bay (Pérez-
Flores et al., 2004). At the bottom of the basin, rocks of
the Group Rosario overlay Alisitos basement discordantly.
Group Rosario consists of poorly consolidated marine
sediments (Abbott et al., 1993; Suárez-Vidal, 2006),
these rocks outcrop south of Maneadero valley, in Punta
Banda peninsula and north of Ensenada city. Cruz-Castillo
and Delgado-Argote (2000) describe the intermediate
member of Group Rosario as a medium to fne grained
sandstone inter-stratifed with shale, mudstone, sand
FIGURE 1 Study area in northern Baja California, México, showing the and clay, as well as with conglomerates, overlaid by an
location of three AMT profles in Maneadero Valley, south of Ensenada.
upper member consisting of conglomerate with pebbles
between 2 and 20 cm in diameter, with a size of 10 cm
prevailing. Overlaying Group Rosario discordantly, there
have been derived from gravity and magnetic geophysical is a sequence of Quaternary alluvial sediments consisting
surveys (Cruz-Falcón, 1986; Fabriol et al., 1982; Pérez- of granular material with good permeability represented
Flores et al., 2004; Vázquez, 1980). However, there is by bodies of conglomerates, medium to fne grained
insuffcient information about the physical properties of sands and clays which were deposited at the bottom
the aquifer (Sarmiento-López, 1996; Vega-Aguilar, 1989). of the streams as well as in the low lands surrounding
A particularly useful property is electrical resistivity, as it topographic elevations.
depends on porosity, fuid content and pore-fuid resistivity
which is directly related to the salt content of the water. The currently exploited aquifer is contained in these
Hence, we conducted an electromagnetic survey using alluvial deposits; it is an unconfned aquifer, hydraulically
audiomagnetotellurics (AMT) to investigate the subsurface connected with the Pacifc Ocean. The volume of water
6 3resistivity distribution. Besides contributing to advancing extraction has been estimated to be 25.7 x 10 m /year,
6 3the knowledge of the physical properties of the Maneadero while the aquifer recharge is about 20.8 x 10 m /year
aquifer, the work also illustrates the performance of a (Daessle et al., 2004). This negative balance, sustained
recently proposed set of magnetotelluric response functions for several years, provoked a severe sea-water intrusion
(Romo et al., 2005). As discussed below, these new response that is causing a gradual deterioration of the water quality
functions have some useful advantages that make them in the aquifer. Daessle et al. (2004) analyzed TDS (total
suitable for a regular use in AMT interpretation practice. dissolved solids) in water from 25 wells distributed within
the valley. They concluded that sea-water invasion has
progressed particularly in the central part and in the coastal
GEOLOGICAL SETTING area where values as high as 9.6 g/l occur. The geophysical
information provided by this work helps to estimate the
The valley of Maneadero is located in a small basin extension and magnitude of the intrusion in places where
flled with sediments produced by the erosion of volcanic well information is not available.
and granitic rocks outcropping in the surroundings. The
origin of this structure is associated with a period of
intense erosion and uplift at the end of the Cretaceous, METHODOLOGY
when a rapid uplift of the peninsular block provoked the
erosion of highlands and the formation of deep creeks Geophysical methods designed to investigate the
which transported considerable amounts of sediment to electrical resistivity of the subsurface are widely used
the Pacifc coast (Gastil et al., 1975). The basin is a semi- in groundwater investigations because conduction of
graben structure limited to the south by Agua Blanca fault, electricity in the subsurface rocks depends mainly on their
a seismically active regional fault (Pérez-Flores et al., water content, water salinity and permeability. There are
2004; Suárez-Vidal et al., 1991). several geophysical methods sensitive to the subsurface
52Geologica Acta, 8(1), 51-66 (2010)
DOI: 10.1344/105.000001515V. LUJAN and JOSÉ M. ROMO. Seawater intrusion study using AMT invariant impedances
electrical resistivity. Some of them inject direct current 2-D case, the electrical current fow can be decoupled in two
in the ground and measure the voltages produced at the orthogonal systems: along strike and across strike. These
surface (Kirsch, 2006; Wilson et al., 2006) and some directions defne the so-called TE (transverse electric)
others induce electrical felds that spread underground and TM (transverse magnetic) polarization modes. If the
and produce magnetic felds detectable at the surface coordinate axes are conveniently oriented along the 2-D
(Giroux et al., 1997; Meju et al., 1999; Ritz et al., 1997). strike, the diagonal elements of Z reduce to zero and the
In this work, we used audio-magnetotellurics (AMT): off-diagonal terms are the TE and TM impedances. Thus,
a technique that uses natural electromagnetic felds equation (1) decouples in a pair of independent scalar
produced mainly by electric storms around the world equations.
which propagate as electromagnetic plane-waves in the
ground (McNeill, 1990). The resistivity distribution in the A widely used approach to reduce the measured full
ground is estimated from the horizontal components of tensor to an off-diagonal form assumes that Z is composed
the electric and magnetic felds measured at the surface. of a regional, off-diagonal, 2-D tensor distorted by local 3-D
The temporal variation of the EM felds is registered effects (Bahr, 1988, 1991; Groom and Bailey, 1989, 1991).
for a frequency band between 10 Hz to 100 kHz. At the