TEM surveys in the area of Manzanar, Chile, for exploring a hot water aquifer

Report of the project:
GEOTHERM Programme – Promoting the Use of Geothermal Energy
GEOTHERM - Programm zur Förderung der Nutzung geothermischer Energie

Introduction

In the frame of the BGR project GEOTHERM, a project for promoting geothermal exploration in developing countries, the area NW of the Sierra Nevada, 9^th region of Chile, was surveyed geophysically during a field campaign in 2006.

Aerial photo of the survey area east of the town of Temuco, 9th region. Thick blue dots are MT stations, small red squares TEM stations. Hot water wells are marked with their names in yellow and the light green polygon in the corner down to the right is the border of the national park. Source: BGR

Transient-electromagnetic (TEM) measurements were performed in order to support the interpretation of magnetotelluric (MT) data, which had been collected simultaneously, and possibly to discover anomalous zones within the first hundreds of meters of the subsurface which might be caused by hot water reservoirs.

One result of this survey was the clear indication of high conducting layers at depths ≥ 100 m at a few points close to the very northern and western borders of the area covered. The immediate interpretation was that we are just seeing the edge of a hot water aquifer which is used at the hotel complex of Manzanar, a few km farther to the north. As this aquifer is little known, in particular its extension, and as it might be of considerable economic interest to the local community we decided to realize a second field campaign in order to gather more information on this anomalous zone. The 38 points from 2006, just touching the margin of Manzanar aquifer, could eventually be complimented with an additional 57 new points in 2007.

Field examples

The survey has clearly proven the existence of a low resistivity (ρ < 10-20 Ω*m) structure in the vicinity of the Manzanar well. Two sounding examples of points over the anomalous zone are shown in the figures below. The strong decrease of ρ_a with increasing decay times indicate high conducting layers at depth. At sounding A (left) the decrease starts at much earlier times, i.e. the depth is less: ~ 120 m below surface; at sounding B (to the right) the depth of the high conducting layer is ~ 250 m. The reason is the higher altitude of the second point by ~ 130 m.

Two TEM soundings over the Manzanar low resistivity zone Source: BGR

In the left part of the graphs ρ_a is represented as function of the decay time double-logarithmi­cally: discrete points are the measured values and the continuous curves are the responses for the models represented to the right beside it. The layer resistivities in Ω*m are thereby logarithmically plotted along the x axis and the depth of the layers in meters linearly downward.

Vertical resistivity sections

The two soundings shown are lying on the profile 2 (see the aerial photo above). The models found together with the results of all points along this profile were used to produce a vertical resistivity section.

Vertical resistivity section along profile 2 Source: BGR

Vertical resistivity section along a perpendicular profile Source: BGR

The selected colour scale emphasizes ranges of low resistivities by red or violet colouring. The Manzanar anomaly immediately attracts attention. It starts ~ 2 km west of Manzanar and extends an additional ~ 5 km farther to the ESE. Its depth is ~ 80 to ~250 m below surface, depending on the altitude of the point, and it has a thickness of at least 100 m.

Profile 3 runs perpendicularly to the longitudinal profile 2. (The Manzanar well is close to S106.) From this a width of the anomaly of ~ 2 km can be inferred.

Geological map

Geological map of the survey area. The anomalous zone is hatched in red. Source: BGR

The geological map of the survey area shows essentially volcanic rocks of Tertiary to Quaternary ages and very young sedimentary overburden in the river valleys. The Manzanar anomalous zone, hatched in red, lies below lavas which came from the momentarily not active strato-volcano Sierra Nevada some 20 km to the SE (white patch in the lower right corner). In the geological map the units are called the volcanic set of the Cordillera Principal (quaternary) and the formation Malleco (pliocene-pleistocene). In its central part the anomaly is overlain by young sediments of the rivers Cautín (E-W) and Ñanco (NW- SE).

3-dimensional visualization of the resistivity models found

In order to better visualize the spatial distribution of the low resistivity structures a 3-dimensional presentation was attempted. The subsurface is portioned into so-called voxels, homogeneous prism shaped volume units each with an electrical resistivity calculated by interpolation from the interpreted TEM models. The voxel size used for the presentations shown in here is 200 x 200 x 10 m. The view is from above (30°) and from the SW (S35°W); the vertical exaggeration is five fold.

The material south of a certain vertical EW plane has been removed, in order to give insight into vertical sections. Click to start the animation Source: BGR

Areas of resistivities below a certain value are shown. The number top left, here 25, is the resistivity in Ω*m. Click to start the animation Source: BGR

The one-dimensional models found at each point measured are represented by small circular columns and the hot springs of Manzanar and Malalcahuello have been labelled for a better orientation.

There is little doubt that the low resistivity zone of Manzanar is due to mineralized hot waters. Resistivities < 10 Ω*m can only be explained in this case with higher temperatures and/or elevated contents of ions. The survey has proven the existence of a hot water reservoir with an extension of several km below volcanic rocks with thicknesses of at least 100 m.

The next step in exploring the Manzanar aquifer is drilling holes for verifying temperatures and pump tests.