STT13-P01

Room:Convention Hall

Time:May 27 18:15-19:30

Subsurface imaging with EM migration of magnetic fields
HYODO, Daisuke1 ; GOTO, Tada-nori1 ; MIKADA, Hitoshi1 ; TAKEKAWA, Junichi1∗
1

Graduate school of Engineering, Kyoto University

The underground impedance data is estimated by observing and analyzing both electric and magnetic components in themagnetotelluric (MT) method. Although this method is often used in fault surveys or geothermal prospecting, it has the twoproblems caused by the observation of electric components. One is that the static shift effect bias the apparent resistivity at thesurface, and the other is that it is difficult to secure broad places for the observation of electric fields. Furthermore the analysis
of electromagnetic data is also challenging because of its high computational costs, especially in three-dimensional cases. This
causes barriers to application to large amounts of field data. So, a new technology which can estimate the subsurface information
simpler and faster is needed.
In this study, we suggest the new method to use magnetic components at the surface in order to overcome these problems. Thestatic shift effect can be controlled and the great mobility of magnetometers makes it possible to obtain data at a lot of sites, ifmagnetic fields are used as analysis data. Magnetic components are applied to not inversion method but migration method, whichis generally the visualization method in seismic data processing to save computational cost. We adopted the PSPI (Phase-Shift
Plus Interpolation) method (Gazdag and Sguazzero, 1984) and applied this method to the magnetic fields at the surface. In thecase of applying the migration technique to the electromagnetic data, it is very important to select the correct frequency band toprevent the multiple reflection and attenuation of the electromagnetic waves.
Through the numerical calculation analyses and the analysis of field data, it was confirmed that applying migration method tomagnetic components is the effective. In this migration method, the large value of reflection intensity is estimated around thearea if the proper resistivity structure is assigned to the migration. This means that the resistivity structures in the subsurface
have an effect on the migration results. In the analysis of the real field data around an active fault, large reflection intensity wasestimated beneath the active fault. We discussed the results of the estimated apparent resistivity cross section and the migration
results with the geological structure of the area and a previous 2D MT inversion result.
In conclusion, our method can estimate the underground resistivity structure simpler and faster. The combination of thiselectromagnetic migration method and inversion has possibility to reveal more detailed subsurface resistivity structure.

Keywords: electromagnetic migration, magnetic fields

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STT13-P02Room:Convention Hall

Time:May 27 18:15-19:30

Pareto-optimal Joint Inversion Modelling and Data Set Compatibility Analysis
SCHNAIDT, Sebastian1 ; KRIEGER, Lars1∗ ; HEINSON, Graham1
1

Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Adelaide (AUS)

In the process of modelling geophysical properties, jointly inverting different data sets can greatly improve the results. In order
to conduct a joint inversion modelling, a relationship between the different data sets has to be established, which can be either beof analytical or structural nature. Classically, the joint inversion problem is then expressed as a scalar objective function, which
combines the misfit functions of all involved data sets combined with a joint term accounting for the assumed connection between
the data sets. This method bears two major disadvantages. Firstly, a weighting of the data sets is enforced by aggregating all
misfit terms. Secondly, the data sets cannot be assessed with respect to their compatibility. Data sets are compatible if they aresensitive to similar physical features, and the joint interpretation of incompatible data sets can lead to incorrect resulting models.
In order to generate unbiased joint inversion models, it is important to mitigate the influence of weighting and to analyse thecompatibility of data sets. Therefore, we have developed a new approach to the joint inversion modelling of geophysical data. We
present MOJO , a Pareto-optimal multi-objective joint inversion algorithm. It is based on an advanced genetic algorithm, hence
it does not only calculate a single optimised model, but a distribution of possible models as final result. In contrast to common
approaches, MOJO treats data sets as a separate objectives, which avoids spurious weighting. Additionally, we use the output
of MOJO to calculate and evaluate curves of the trade-off between the different objectives. The shapes and evolutions of thesecurves yield a measure for the compatibility of the used data sets. Furthermore, the evaluation of the resulting model distribution
provides valuable uncertainty estimates.

Keywords: Multi-objective optimisation, Joint-inversion, Data set compatibility, Model uncertainty, Magnetotellurics

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STT13-P03

Room:Convention HallTime:May 27 18:15-19:30

Interferometric location of microseismic events induced by gas storage operations
GRIGOLI, Francesco1∗ ; CESCA, Simone2
1

Institute of Earth and Environmental Sciences, University of Potsdam, Germany, 2 GFZ German Research Centre for Geoscience, Section 2.1 Earthquake and Volcano physics, Germany
Underground gas storage is a common industrial operation that consist in the injection of natural gas in aquifers, undergroundcavities or depleted hydrocarbon reservoirs. Gas is seasonally injected or extracted to meet the market demand. Although theseoperations rarely stimulated seismicity, a recent case at the Castor platform in the offshore Spain was accompanied by a significant seismic sequence, culminated with a magnitude Mw 4.3 earthquake (Cesca et al. 2014). Ad hoc microseismic monitoring
networks are nowadays extensively used to analyse the induced microseismicity generated by these industrial operations. However, in many cases, the lack of local microseismic monitoring networks limits the performance of the standard data analysis
procedures. In such cases, non conventional methods need to be established. Within this context, we extend here the analysisof triggered seismicity related to gas injection at the Castor platform, in September-October 2013, where standard location procedures failed for magnitudes below Ml 2. In this work we relocate these low magnitude events using an interferometry based
location method (Snieder and Vrijlandt, 2005). This technique exploits slight changes in the coda waves between two seismic
events within a cluster. We proof that microseismic events can be classified in different families by combining a waveform correlation analysis and a clustering technique. Clustered events are characterized by a high similarity of waveforms, which implies asimilarity in both source mechanism and location. In these conditions, the analysis of seismic coda recorded at a single receivercan be used to infer a measure of the spatial separation between two seismic sources. Coda waves are radiated in all directions
with a radiation pattern determined by the source mechanism and a small change in the source position affects the interference
pattern of the scattered waves that constitute the coda. This change in the coda waves is used to constrain the interevent distancefor each events pair. Absolute locations can be then retrieved by considering all interevent distances, a procedure which requires
at least three reference locations. We discuss the potential of the coda interferometry location approach to monitor triggered andinduced seismicity, by relocating about 1000 seismic events of the September-October 2013 seismic sequence offshore Spain,
close to the Castor project injection platform. Results are used to discuss the possible proposed faults scenarios.
References:
Cesca, S. et al. 2014, The 2013 September?October seismic sequence offshore Spain: a case of seismicity triggered by gasinjection?, Geophys, Jour. Int., doi: 10.1093/gji/ggu172
Snieder, R., and Vrijlandt, M., 2005. Constraining the source separation with coda wave interferometry: Theory and application to earthquake doublets in the Hayward fault, California, J. Geophys. Res., doi:10.1029/2004JB003317

Keywords: Induced seismicity, Microseismic monitoring, Seismic event location, Seismic interferometry

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STT13-P04

Room:Convention Hall

Time:May 27 18:15-19:30

Retrieving Focal Mechanism using Double Couple - Tensile Constrained Inversion : Methodand Synthetic Tests
IIDA, Shuhei1∗ ; KIM, Ahyi11

Yokohama City University

Understanding the source characteristics of hydraulic fracturing induced microearthquakes is expected to provide better understanding of both the fracturing process and the influence of pre-existing structures on the distribution of events. However, detailsof the source characteristics of the microearthquakes remain largely unknown. One controversial issue is whether a significant
volumetric change occurs because of the stimulation. Answering this question is important to provide the insight of the future
effective and safer hydraulic fracturing. Although the moment tensor inversion is one of the best approaches for studying source
mechanisms, it is often biased due to uncertainty of geologic model, sparse receiver coverage and low signal to noise ratio forthis types of induced earthquake analyses. So it is necessary to quantify the factors which bias the solution. Herein, to address
this question, we modified the double couple focal mechanism inversion method using the body-wave amplitude ratio and polarities developed by Snoke (2003) for stimulation induced microearthquake analysis. Since our goal is to figure out whether it
is possible to identify volumetric change with limited receiver azimuthal coverage, it is necessary to incorporate the non-double
couple mechanism in the inversion. Since the mechanism of the microearthquakes can be considered as 1) shear slip along thepre-existing fracture surface, 2) an tensile displacement which increase the porosity, or 3) a combination of these mechanisms,
we added tensile crack component to the original method. In this manner, we have smaller degree of freedom to stabilize theinversion than that of full moment tensor case. In this study we will examine the performance of the method with synthetic tests
under various conditions.
Keywords: induced seismicity, hydraulic fracturing, focal mechanism, tensile crack

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STT13-P05

Room:Convention Hall

Time:May 27 18:15-19:30

Extraction of body waves from seismic ambient noise
KIM, Eun mi1∗ ; KANG, Tae seob1 ; KIM, Tae sung2 ; KIM, Min wook1
1Pukyong National University, 2 Korea Institute of Geoscience and Mineral Resources

Ambient noise cross-correlation is used in seismology to obtain the part of the surface waves and applied to the theoreticalresearches and various experiments. Obtaining the part of body waves from the ambient noise correlation is difficult to recognize
because of the feature decreasing body waves along the travel path. However, the travel times of body waves detected from
temporal and spacial events occurrence involve uncertainty of the epicenter and accompany temporal-spacial restriction. On theother hand, ambient noise is always occurred and is obtained at the all stations. So it can be applied to research of the internal
earth when the case of extracting the body waves using the cross-correlation is possible. This study shows that body waves
can be observed by analyzing the ambient noise recorded seismic data in South Korea. Using 42 broad-band three components
stations located on the South Korea. The data removed the mean and trend are filtered high-frequency band(0.5-2Hz). The noisecorrelations were calculated for all combinations of radial, transverse and veltical components, which required rotation of thehorizontal components for each station pair according to the azimuth at each station of the greatcircle between the two stations.
Removing the part of broad-band signals effected by occurring event, the part of standard deviations more than three times areremoved. And it applied spectral whitening to reduce effects of the surface waves. After data processing, all ambient noise
signals are cross-correlated and temporal stacked. We found the signals propagating from one station to another station, thissignals can be interpreted as the body waves distinguished surface travel-time in high-frequency band. From this analysis, wecan extract the body waves using ambient noise cross correlation of continuous data at the stations.
Keywords: ambient noise, cross correlation

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