بهبود مدل سرعتي لرستان با روش توموگرافي زمان سير تلفيقي

Improving velocity model using integrated travel time tomography method in the Lorestan region

توموگرافي زمان سير لرزه‌اي، روشي مناسب براي بازسازي مدل سرعتي زير سطح زمين است. همواره كمبود داده‌ و پوشش پرتو به اندازه كافي موجب ايجاد ابهاماتي در توموگرافي شده است. با استفاده از روش توموگرافي بازتابي و توموگرافي عبوري (استفاده از امواج مستقيم)، نه‌‌تنها سرعت‌ها تعيين مي‌شوند بلكه با تلفيق آنها يك مدل سرعتي دقيق ساخته مي‌شود. در اين تحقيق داده‌هاي زمان رسيد بازتابي كه در طول يك خط لرزه‌اي در منطقه لرستان برداشت شده و داده‌هاي زمان رسيد زمين‌لرزه‌‌‌هاي محلي ثبت شده در بازه زماني 2006 تا 2011 در منطقه مورد بررسي، با اِعمال فيلترهاي مختلف به منظور ايجاد محدوديت‌هايي در عمق و فاصله رو مركزي و آزيموت، براي اجراي مراحل توموگرافي زمان سير لرزه‌اي تلفيق شدند. مدل سرعتي جديدي از داده‌هاي تلفيق يافته به‌دست آمده و در حين مراحل پردازش داده‌هاي خام در امتداد خط لرزه‌اي مورد نظر، بكار گرفته شد. بعد از اِعمال هر دو مدل سرعت به‌دست آمده از داده‌هاي بازتابي و داده‌هاي تلفيقي، مقاطع بعد از مهاجرت هر دو به‌‌طور جداگانه مقايسه شد. در اين دو مقطع كه در همه مراحل به‌جز مدل سرعتي، مشترك هستند، بازتاب‌‌كننده‌ها با وضوح و پيوستگي بيشتري در مقطع حاصل از داده‌هاي تلفيقي مشاهده شدند.

Traveltime tomography is a suitable method to image underground structure of the Earth. A common problem in seismic tomography is the inadequate amount of data required for accurate traveltime inversion. In this study, seismic traveltime data along a linear array in the Lorestan region and filtered earthquake traveltimes data were combined together to perform traveltime tomography. The obtained velocity models from seismic and combined data were compared for the sections of their post-migration step. The results obtained from combined data indicate a better resolution and state of being more continuous for reflectors. Data of the earthquakes used here, is related to latitude of 32 to 35 and longitude 47 to 49 degree between years 2006 to 2011 in the Lorestan region. To make combined data, some corrections such as limiting the depth less than 8km, epicentral distance less than 100km and gap (between 0 to180) were performed on the data of mentioned earthquakes. Seismic data should be converted to the format of earthquake as ingoing data for doing travel time tomography methods. For this part, reflected data was corrected in order to acquire the one-way travel time (times in reflected data are two way times). To be able to halve the two-way travel time and obtain one-way travel time, dip move out corrections (DMO) for removing the effects of dip layers, were carried out. Then, a path of a selected reflector was followed and number of shots and corresponding times recorded in each point. On the basis of this information, CDPs were considered as the sources of earthquakes and geophones as stations. To compare the obtained velocity models from seismic and combined data, their post-migration sections were determined. For better results, these two migration sections were divided into some smaller parts and then compared with each other individually. As expected, the velocity model from combined data shows higher quality results. For proving this matter, velocity models produced from seismic and the combined data were compared in the sections of their post migration. The results obtained from combined data shows higher resolution and state of being more continuous for the reflectors of migration section, especially in the parts with more ambiguities. The mentioned method can be used in places where a good velocity model cannot be obtained as the structure of the earth which willnot let the waves pass through (like Gachsaran formation). Applying both reflected and earthquake data in traveltime tomography method would improve the velocity model, so processing and interpreting the reflectors will be easier.