Differential magnetic soundings in the travale geothermal area

0375- 6505/85 $3.00 + 0.00 Pergamon Press Ltd. © 1985 CNR.

Geothermics, Vol. 14, No. 5/6, pp. 749- 754, 1985.

Printed in Great Britain.

DIFFERENTIAL

MAGNETIC SOUNDINGS GEOTHERMAL AREA

IN THE

TRAVALE

J. M O S N I E R and F. P L A N S O N Laboratoire de G~ophysique Appliqu~e du C.N.R.S. 3D, A venue de la Recherche Scientifique, 45045, Orleans, France

1980 and 1982 the Laboratory of Applied Geophysics of the C.N.R.S. attempted to establish, by magnetic differential sounding, a possible conductivityanomalylinked with the geothermal field of Travale, Tuscany. Some 25 sites were occupied along two profiles, one between Siena and Populonia, near Piombino, the other between Siena and Cecina. An important anomaly of the transient magnetic field (some 15% of the normal field) was discovered in 1980between Gerfalco (in the SW) and Frosini (in the NE). It exactly coversthe geothermal area of Travale. The direction of the telluric currents causing the anomaly is parallel to the magnetic meridian and their maximum depth is of some 2000 m. The 1982 campaign showed that in the north of Travale, anomalous currents move in a NW-SE direction or even completely EW (SW of Volterra) and meet in the sea near Livorno. One possible interpretation of these phenomena as a whole is to assume the presence of veryconductivelayers between Larderello and Travale. The currents which circulate parallel to the coast are channelled locally by this structure, which could be closely linked with the geothermal field. Abstract--In

The Laboratory of Applied Geophysics of the C.N.R.S. in Orleans adopts the "differential magnetic sounding" method which enables them to detect channelling of the natural telluric currents by conductive electric structures. It consists of measuring simultaneously the variations of the horizontal magnetic field at a normal So station (where the density of the teUuric currents is uniform) and at other Si stations spread over the anomalous zone. The AHi differences between the homologous components of the field, recorded at a given time at the So reference station and at the Si station, allow measurement of the excess (or lack) o f density of the telluric currents at the Si station with regard to So, from which information may be obtained about the electric resistivity beneath the Si station. As geothermal reservoirs are often electrically conductive, the objective is to detect indirectly the presence of geothermal sources by means of positive conduction anomalies. A first test, carried out in the Rhine Graben in 1977, actually revealed the existence of such an anomaly between Mulhouse and Landau, covering the main geothermal anomalies of the district. The experiment was carried out in 1980 and 1982 on the Travale geothermal field (contract EG-A2-014-F), designated by the EEC as a test site, for comparison of the various geophysical methods used in the prospecting of geothermal targets. During the first part of the experiment in 1980 23 stations were set up along two profiles: - - the main profile extended from Populonia (near Piombino) in the SW to Cavriglia (near Siena) in the NE (Fig. 1b) an auxiliary profile crossed the Siena graben, between Populonia and Lucignano. The second part of the survey, in 1982, aimed at defining precisely the distribution of the currents inside the quadrilateral area: P o p u l o n i a - S i e n a - C o l l e Val d ' E l s a - C e c i n a . For this purpose four stations of the previous campaign were occupied again: Volente, La Befa, Lagoni (ENEL house) and Populonia. A further six were set up along a Colle Val d ' E l s a - C e c i n a profile. The topographical setting of the various stations is shown in Fig. l(b). The measurements carried out between Populonia and Lucignano did not provide useful information about the telluric circulation in the Siena graben. Indeed, in each of the stations located east of La Befa, the same transient field can be observed, almost identical to that observed concurrently in the stations of Volente, north of Siena. 749

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Differential Magnetic Soundings in Travale

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On the contrary, significant differences appear between the stations of the Populonia - Siena profile. Figure 2 shows an example of differential recording obtained between Populonia and Siena on one hand, and Travale and Siena on the other, together with the normal field in Siena. The figure clearly reveals that the n o r t h - s o u t h component of the anomalous field, AH, is nearly non-existent. Therefore, the currents which are responsible for the anomaly o f the transient field are circulating in the magnetic n o r t h - south direction (N6°W geographic). The amplitude of the anomalous field is a r a n d o m function of time, which can be standardized by using a second Sj reference station. A coefficient, K = ~](AH~x + AD~)/,,](A/-/~) + AD)2) can be introduced, not depending on time, which measures the ratio between the current densities at the Si station (variable) and Sj (permanent). Figure 3 shows the variation of K coefficient between Populonia and Siena. The telluric currents circulating in the Populonia area, near the sea, can reasonably be considered m o r e intense than those circulating in the Siena area. It is assumed that their density around Siena is low or non-existent. Volente can be taken as the first So reference station and Populonia as the second Sj reference station. Thus K is included between 0 and 1. One can clearly see, in Fig. 3, the existence of a conducting zone, between the station of Santa Croce (4) in the SW and Frosini (9) in the NE (Fig. l b) (slightly more resistant between Lagoni and Gerfalco) and a resistant area around Montebamboli. In La Befa and Monticiano, some telluric current can still be found circulating in a N10°E direction. Further to the east, the current density becomes very low again. It is obvious that a channelled current flow circulates between the Mediterranean sea and a conductive structure located beneath the geothermal zone of Travale. The 1982 campaign led to the determination of the direction followed by this current beyond Travale. To ensure comparison with the measurements carried out in 1980, two stations of the P o p u l o n i a - S i e n a profile were kept as reference stations: Volente for So and Lagoni for Sj, rather than Populonia (which is too noisy). Figure 4 shows an example of the anomalous field observed in Campiglia and in Ponteginori (east and south-west of Volterra, respectively) as well as at the Sj reference station (Lagoni). The anomalous field, running E - W in Lagoni, obviously changes to N E - S W in Campiglia and to N - S in Ponteginori. The current responsible for the anomaly of the transient field turns 90 ° between Travale and Ponteginori. The Sassa station, situated further to the east worked correctly only for a short while. However, it seems that the direction of the currents is again NW - SE. In Fig. l(b) the distribution of the anomalous currents is represented by arrows, whose direction is that of the current and length proportional to the intensity of the anomalous field. Thus one obtains a global picture of the telluric circulation in this part of Tuscany. It is obvious that the currents going through the Travale graben come f r o m the Mediterranean sea (undoubtedly from the Livorno area), and return to the sea between Follonica and Grosseto. Fig. i. (a) Simplified map of western Tuscany and location of the geothermal area between Travale - Radicondoli and Volterra. (b) Location of the Differential Magnetic Soundings (D.M.S.) stations during the field campaigns in 1980 (white dots) and 1982 (black dots). Geographic distribution of the stations: P,: (0) Populonia, (1) Montioni, (2) Montebamboli, (3) Progetti, (4) Santa Croce, (5) Gerfalco, (6) Travale, (7) Lagoni, (8) Montalcinelio, (9) Frosini, (10) Cotorniano, (11) Ancaiano, (12) Volente, (13) Castellina, (14) Castelvecchi, 05) Cavriglia. P2: (0) Populonia, (1) La Befa, (2) Bibbiano, (3) S. Nazzario, (4) Chiusure, (5) Trequanda, (6) Casabianca, (7) Farnetella, (8) Lucignano. P3: (1) Volente, (2) Campiglia~, (3) Casole d'Elsa, (4) Mazzolla, (5) Montegemoli, (6) Ponteginori, (7) Sassa. The reference stations are Volente (12) and Populonia (0) for profile P,, Volente (12) and Lagoni (7) for prof'de P3. In each station, the arrow indicates the direction of the anomalous telluric flow and its length is proportional to the local current density (0 in a "normal" station). A, B, C and D are old stations, occupied in 1977.

752

J. Mosnier and F. Planson

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Fig. 2. Anomalous transient field recorded at Populonia (upper curves) and at Travale (middle). The lower two curves are a record of the " n o r m a l field" at Volente on l I October 1980.

This behaviour can be interpreted as follows. There exists, in the Mediterranean sea, an intense telluric flow of a roughly E - W orientation, joining the Atlantic ocean through the south of France. In the area covered by this study, this current has to run alongside the Italian coast, noticeably parallel to the N - S (magnetic) direction. One part of the current overlaps the coastal plain, creating the Populonia anomaly. Another part penetrates the conductive formations (marine evaporites) north of Livorno before proceeding to Volterra (anomalies of Campiglia and Mazzolla), where it is joined by a secondary E - W flow which follows the Cecina valley (Ponteginori anomaly). The current then passes through a strongly conductive area which coincides with the geothermal field of Travale and directly meets the sea south of Follonica (Fig. lb).

Differential Magnetic Soundings in Travale

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Fig. 4. A n o m a l o u s transient field recorded in three stations on 26 May 1982. Note the polarization of the field and the rotation of the telluric flow from Ponteginori (E - W) to Lagoni ( N - S) (Fig. I b).

754

J. Mosnier and F. Planson

Of course it is not possible to state for certain that the conductivity of the rocks near Travale is linked to the presence of steam. It could even be assumed that this field, being of a " v a p o u r dominated" type, should be electrically resistant. This is probably the case in the narrow region SW of Montalcinello, where steam is present at the altitude + 200 m. This resistant zone also appears in the curve of Fig. 3. But, further north, where the geothermal reservoir goes deeper than 1000 m, the rocks should normally be good conductors. This is confirmed by the numerous electric and magnetotelluric studies achieved in the area. It does seem plausible that the conductivity anomaly, responsible for the channelling of telluric currents through the Travale graben, is directly linked to the high temperature of the deep layers. Again differential magnetic sounding proves to be a wide scale prospecting method. It does not allow a precise determination of the distribution of resistivity at depth, but depicts the telluric circulation in the area as a whole, and reveals conducting anomalies, including possible thermal energy sources. For this specific application, it remains a very powerful prospecting technique.