Methodological and experimental approaches to evaluate the volcanic activity by high precision microgravity measurements

Transcription

1 Methodological and experimental approaches to evaluate the volcanic activity by high precision microgravity measurements International Doctoral Research Course for Ingegneria Elettronica, Automatica e del Controllo di Sistemi Complessi Antonio Pistorio XXII Brain Storming Day Catania, 20 May 2011 Coordinator: L. Fortuna Tutor: L. Fortuna, C. Del Negro

2 Summary Discrete gravity measurements: - Hybrid microgravity method - A comparison with traditional methods - Uncertainty estimation - A new arrangement for hybrid measurements Continuous gravity measurements: - An investigation on the lava fountain of the 10 May A problem of Pattern Recognition: explosive or effusion activity? (in progress )

3 Methodological approaches Among all techniques used in geophysics, the gravimetric method is the only one who determines information related to the mass change of structures or bodies buried in a completely non-invasive manner. Microgravity monitoring is used at numerous active volcanoes in order to identify density changes that occur on diverse timescales accompanying volcanic eruptions. Since 1986, discrete microgravity measurements have been intensively performed at Mt Etna and different time gravity changes attributed to subsurface mass redistributions driven by magmatic processes were detected.

4 Hybrid microgravity method The hybrid method allow to obtain gravity information having an accuracy comparable to absolute gravity information by combined use of absolute and relative gravimeters. Hybrid Schema Using an absolute gravimeter are not obligated to reach stable reference stations that are far from the active areas. We use the absolute points located around the volcano edifice as reference for relative measurements acquired in different subarray of the relative microgravity network: - minimization of the instrumental drift; - careful control of the gravimeter used. The sensitivity and complexity of the absolute gravimeter has made it impossible or very difficult to employ a gravimeter in any location. Scintrex CG-3M Relative Gravimeter Micro-g LaCoste FG5 Absolute Gravimeter

5 A comparison with traditional methods no stable reference stations that are far from the active areas ADR 1 - minimization of the instrumental drift - careful control of the gravimeter used

6 Uncertainty estimation where For the traditional relative measurements P n ( gi g j km gij dm Tij ) = ε ij Observation Equation, Marson (1999) P n = weight of observation g ij = difference of gravity between the i-th and j-th station g i = unknown gravity value in the i-th station T ij = time interval between the i-th and j-th station g j = unknown gravity value in the j-th station d m = drift factor k m = unknown scale factor of m-th link ε ij = residual In matrix notation: AX + L = V X = R R = A T 1 A A T L where δ 2 g = σ 0 r ij σ 0 V T V = n r Apart from the systematic errors, the accidental error will increase with the number n of individual differences in subsequent measurements, according to the rule that, after the n-th difference, the measurement error is multiplied for n. Therefore, there should always start and only a reference benchmark, but should be establish a series of benchmarks. where For the hybrid method δ + δ + δ g 2 = δ g 2 FAG 2 abs δ g = Error in the Link δ FAG = Error in the Free Air Gradient estimation δ abs = Error in the g Absolute measure With the hybrid method, the errors are confined at single link

7 * absolute station Uncertainty estimation Traditional relative measurements Hybrid method Pistorio et al., AG, submitted The hybrid approach yields the following advantages: Gravimetric survey carried out using only relative gravimeters (for a) allowing traditional to optimize measurements); traditional techniques all and measures strategies were of the linked microgravity to a single measurements reference station Etna s (Adrano). network, To thus enabling obtain a the drastic gravity reduction value in in the the time required network to accomplish stations, with discrete error, data surveys and were ensuring compensated improvement with in the the least quality of the squares data; method by solving a linear system of observation equations. b) obtaining gravity information with an accuracy comparable to absolute gravity information on an extensive gravity network, hence reducing the typical ambiguity Gravimetric inherent survey in the carried singlecomponent using relative method; and absolute out gravimeters (hybrid method). To c) allowing obtain to the also gravity investigate absolute low value amplitude in microgravity the network variations stations, in with the error, order of a few each µgal relative that were measurements never considered was till now linked at Mt. with Etna, the which absolute could values provide of useful stations information dislocated to better along understanding the the network. geophysical processes preceding and accompanying volcanic phenomena.

8 A new arrangement for hybrid measurements Once discrete gravity survey were carried out along profiles and then referred to a single reference station far away. Actually, taking into account that the measurement error is proportional to the number of n consecutive differences, we have created so many closed loops with a small number of benchmarks in which each loop has an absolute reference station.

9 Continuous gravity measurements High Spatial Resolution Low Temporal Resolution High Temporal Resolution Low Spatial Resolution Discrete Gravimetry Continuous Gravimetry The use of discrete and continuous gravity measurements allow the detection of phenomena with a wide range of evolution rates (periods ranging from minutes to years).

10 An investigation on the lava fountain of the 10 May 2008 Bonaccorso et al., G-Cubed, submitted Gravity sequences acquired with LaCoste & Romberg spring gravimeters (with sampling rate of 1 data point per minute) at BVD and SLN stations between 00:00 (GMT time) of 8 May and 00:00 of 12 May 2010, after removing the effect of Earth tide and instrumental drift. A Chebyshev low pass filter with cut off frequency corresponding to a period of 4 hours was also applied to the gravity signals. BVD: about 250 µgal (down-up variation); SLN: about 15 µgal (up-down variation).

11 An investigation on the lava fountain of the 10 May 2008 volcanic tremor location (0.5-5 Hz) x = = = i yi g = Gρ µ ijk zk arctan xi log( Rijk + yi ) y j log( Rijk + xi ) i 1 j 1 k 1 zk Rijk - density contrast between the host rock and the gas-magma accumulated at the top of the pressuring source: 2.0 g/cm 3 ; - density of the foam: 0.7 g/cm 3 (assuming a 75% vesicularity; Polacci et al., 2006). Explosive Phase - density of the dispersed flow in the conduit (Sigurdsson, 2000), calculated by lowering the density of the gas-magma foam at the top of the pressuring source by 20%: 0.5 g/cm 3 ; - radius of the conduit: 10 m; - increases of the foam volume: ~ m 3. Bonaccorso et al., G-Cubed, submitted

12 An investigation on the lava fountain of the 10 May 2008 a) a rich-gas magma could be trapped in a shallow storage zone and hence generate a lava fountain when an overpressure threshold is exceeded; b) an uprising of a deep magma that intrudes the shallow storage zone which could favour powerful explosive activity; c), d) an attempt of magma intrusion at east of the summit area on 10 May that, in turn, fracturing and weakening the surrounding rocks, could have favoured the subsequent magma uprising on 13 May. In other words, magma intruded on 10 May did not erupt, but prepared the path for the following eruption which occurred three days later. Therefore, after the lava fountain occurrence, the volcano system was in a critically unstable state, and it could be prone to eruption triggering. Bonaccorso et al., G-Cubed, submitted

13 A problem of Pattern Recognition: explosive or effusion activity? Gravity signals are proved able to detect lava fountains which are interest not only in scientific research but also for hazard mitigation 200 eruption Belvedere (BVD) 200 fire fountain Belvedere (BVD) Dg [ugal] Dg [ugal] :00/12may 06:00/13may 18:00/13may 06:00/14may 18:00/14may time [days] :00/9 may 00:00/10may 12:00/10may 00:00/11may 12:00/11may time [days] rise time (tr), fall time (tf), Shape Up/Down (square, sawtooth, ramp, ND), lenght, amplitude, dominant frequency, amplitude ratio between high frequency component during and before the event. DBSCAN clustering Ester et al., 1996

14 A problem of Pattern Recognition: explosive or effusion activity? A SVM is a binary classifier which learns the boundary between objects belonging to two different classes. It works by projecting the objects in a multidimensional space and search for a separating hyperplane in this space. The separating hyperplane maximizes the distance (the "margin") from the nearest training objects. General properties of SVM: unlikely overfitting; ability to manage data with many descriptive characteristics; compacting the information contained in the input data Classification with a Support Virtual Machine (SVM) Kernel Function: linear_kernel 0 (training) 0 (classified) 1 (training) 1 (classified) Support Vectors

15 A problem of Pattern Recognition: explosive or effusion activity? Features Space SVM projecti the objects in a multidimensional space and search for a separating hyperplane in this space.

16 100 0 A problem of Pattern Recognition: explosive or effusion activity? f10mag08eslb Wavelet Spectrum f10mag08bvdb Wavelet Power Wavelet Power Period [minutes] Period [minutes] time [minutes] Cross Wavelet time [minutes] Period time [minutes] Common components between minutes

17 List of pubblications Congressi: [1] Greco, F., Currenti, G., Del Negro, C., Di Stefano, A., Napoli, R., Pistorio, A., Scandura, D., Budetta, G., Fedi, M. (2009). Wavelet multiresolution analysis for the local separation of microgravity anomalies at Etna volcano. Physcon, Facoltà di Ingegneria dell Università degli Studi di Catania, 1-4 Settembre [2] Greco F., Currenti G., D Agostino G., Del Negro C., Di Stefano A., Germak A., Napoli R., Origlia C., Pistorio A., Scandura D., Sicali A. (2010). Absolute and relative gravity measurements at Etna volcano. IAG Symposium on Terrestrial Gravimetry: static and mobile measurements (TG-SMM2010) June 2010 Russia, Saint Petersburg (Oral presentation). [3] Currenti, G., Napoli, R., Greco, F., Pistorio, A., Scandura, D., Del Negro, C., Imaging magma intrusion during the 2008 Etna eruption using a 3D integrated geophysical modeling, 29 Convegno GNGTS, Prato, ottobre Riviste: [1] Greco F., Currenti G., Del Negro C., Di Stefano A., Napoli R., Pistorio A., Scandura D., Budetta G., Fedi M. (2010). Wavelet multi-resolution analysis for the local separation of microgravity anomalies at Etna volcano. In From Physics to Control Through an Emergent View, Fortuna, Fradkov and Frasca (Eds), World Scientific Series on Nonlinear Science, Series B - VOL. 15, pp , ISBN [2] De Angelis M., Greco F., Pistorio A., Poli N., Prevedelli M., Saccorotti G., Sorrentino F., Tino G. M. Measurement of absolute gravity acceleration in Firenze, Solid Earth, Submitted. [3] Bonaccorso A., Cannata A., Corsaro R. A., Di Grazia G., Gambino S., Greco F., Miraglia L., Pistorio A. Multi-disciplinary investigation on a lava fountain preceding a flank eruption: the 10 May 2008 Etna case. Geochemistry, Geophysics Geosystems, Submitted. [4] Napoli R., Pistorio A., Scandura D., Currenti G., Greco F., Del Negro C. Vectorial magnetometers for noise reduction in volcanomagnetic monitoring at Mt Etna. Geomatics, Natural Hazards and Risk, Accepted. [5] Pistorio A., Greco F., CurrentiG., Napoli R., Sicali A., Del Negro C., Fortuna L. High precision microgravity measurements using absolute and relative gravimeters at Mt Etna (Italy). Annals of Geophysics, Submitted.