Konya şehir alanı yüzey deformasyonlarının InSAR yöntemiyle incelenmesi
Analysis of surface deformation in the metropolitan area of Konya city using synthetic aperture radar interferometry
Yazar: NURDAN ŞİRECİ
Danışman: PROF. DR. ZİYADİN ÇAKIR
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Katı Yer Bilimleri Ana Bilim Dalı / Jeodinamik Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Jeoloji Mühendisliği = Geological Engineering

Özet:
Bu tezde Yapay Açıklık Radar İnterferometrisi (InSAR) tekniği kullanılarak Konya şehir merkezi ve civarındaki yüzey deformasyonları çalışılmıştır. Bunun için Avrupa Uzay Kurumu'na ait Envisat (2004-2010) ve Sentinel-1 A/B (2014-2020) uyduları ile Japon Uzay Ajansı'na ait ALOS-1 (2006-2010) uydularının Yapay Açıklık Radar (SAR) verileri kullanıldı. GMTSAR, ROI_PAC ve DORIS yazılımları ile elde edilen interferogramlar StaMPS yazılımı kullanılarak zaman serileri ve radar bakış yönü (LOS) hız haritaları hesaplandı. Kuzey ve güney yönlü yörüngelerden elde edilen Sentinel-1 LOS hız alanları kullanılarak yatay ve düşey hız alanları elde edildi. Elde edilen sonuçlar Konya şehir merkezindeki zemin oturmalarının 2004 yılından 2020 yılına kadar geçen zamanda zamansal ve mekânsal değişimini ortaya koymaktadır. 2004 yılından itibaren başlangıçta birkaç cm olan zemin oturmaları yıllık 11 cm'ye ulaşmaktadır. LOS ve düşey hız alanları şehri Alaaddin Tepesi'nden tam ortadan ikiye bölen kuzey-güney yönlü bir hattın her iki tarafında iki büyük deformasyon alanının varlığını göstermektedir. Batı ve doğu deformasyon lobları sırasıyla tren istasyonunun 500 m batısında ve tren istasyonunun 5 km doğusunda merkezlenmiştir. En yüksek hızda gelişen ve geniş alanları içeren batı deformasyon alanı Konya tren istasyonunun hemen batısında odaklanmakta ve yıllık 11 cm'ye ulaşan deformasyonlar gözlenmektedir. Şehrin doğu bölümünde bulunan deformasyon ise merkezden yaklaşık 3,5 km uzaklıkta bulunmaktadır.  Deformasyon alanlarını ayıran bu kuzey güney yönlü hattın varlığına neyin sebep olduğu kesin bilinmemekle birlikte, buna şehrin üzerinde oturduğu alüvyon sedimanlardaki heterojen yapı veya yeraltı sularına geçirimsiz bir zon oluşturan bir fayın varlığı sebep olabilir. Bu sorunun cevabını bulmak için jeofizik yöntemlere başvurmak gerekmektedir. Hız alanları deformasyonların alüvyon ile sınırlı olduğunu göstermektedir. Şehrin batısındaki yamacın önünde bulunan aktif Konya fayı boyunca yüzeyde herhangi bir deformasyon gözlenmemektedir. Yapılan elastik yerdeğiştirme modellemesi deformasyonun Konya fayı üzerinde olası bir asismik kaymadan kaynaklanma ihtimalinin düşük olduğunu göstermektedir. Çünkü çok sığ derinliklerde (4-5 km) çok yüksek kayma (90 cm/yıl) gerektirmektedir. 2019 itibariyle, şehrin batı tarafındaki deformasyon önemli ölçüde yavaşlarken doğu kısmında ise artmaya devam etmiştir. InSAR zaman serileri ile kuyu suyu seviyelerindeki değişimler arasında oldukça iyi bir korelasyon mevcuttur. Bu InSAR sonuçları ile gözlenen ve 11 cm/yıl'a ulaşan zemin oturmalarının yılda ortalama 6 m düşen yeraltı su seviyesi değişimlerinden kaynaklandığını göstermektedir. 2019 yılından başlayan deformasyon hızındaki azalmanın sebebinin büyük olasılıkla şehrin su ihtiyacının yeraltı suyu dışında başka kaynaklardan karşılanmasından dolayı olduğu düşünülmektedir. Poly3D sınır elemanları metodu modellemesi çalışmaları, 2014 ve 2020 yılları arasında Konya metropol alanı ve çevresindeki akiferde sıkışma nedeniyle yıllık yaklaşık 7.7x106 m3'e ulaşan hacim kayıplarının oluştuğunu göstermektedir.

Abstract:
In this thesis, spatiotemporal evolution of surface deformation in the Konya city and its vicinity is studied using advanced multi-temporal synthetic aperture radar techniques with SAR data acquired by Envisat, ALOS-1, and Sentinel-1 A/B satellites between 2004 and 2020. Land subsidence resulting from excessive groundwater extraction is one of the major environmental problems in the developing world as cities increase in population and water use in the absence of adequate pumping regulation and enforcement (Haghshenas and Motagh, 2019). The land subsidence due to over-extraction of groundwater from aquifer systems can cause serious damages to the urban components such as motorways, buildings and underground structures in long-term. Multiple studies have documented that many cities have undergone severe land subsidence in recent years as a result of groundwater extraction. This settlement phenomena mostly occurs when a large amount of groundwater has been extracted from unconsolidated alluvial or basin-fill aquifer systems as a result of ground compaction. Multiple groundwater-related land subsidence phenomena have been observed in cities in Indonesia (Du et al. 2018), Iran (Motagh et al., 2008; Khorrami et al., 2020), Italy (Peduto et al., 2015; Rosi et al., 2016), Mexico (Castellazzi et al., 2017; Figueroa-Miranda et al., 2018), the United States (Amelung et al., 1999; Bekaert et al., 2017; Riel et al., 2018), Spain (Fernandez et al., 2018) and Turkey (Aslan et al., 2019; Imamoglu et al., 2019; Orhan, 2021). 
Knowledge of the spatial and temporal extents of land subsidence is necessary for developing water management programs and establishing measures to mitigate hazards associated with land settlements and infrastructure (Haghshenas and Motagh, 2019). In Turkey, land subsidence due to groundwater extraction is reported for Istanbul, Afyon and Bursa cities (Aslan et al., 2019; Imamoğlu et al., 2019). 
Land subsidence in the Konya region has long been known and documented geodetically first by Üstün et al. (2010) using GPS measurements. Establishing and measuring 6 GPS benchmarks, they found subsidence with rates reaching up to 5.2 cm/yr. The maximum subsidence is found to be near the city, to the south of the city center. Since GPS measurements are pointwise measurements, they did not reveal a full picture of the pattern of subsidence. First InSAR observations in the study area were made again by Üstün et al. (2015) using limited amount of Envisat SAR imagery over the Karapınar region. 
The ongoing subsidence in the metropolitan parts of the city is poorly known since no detail or comprehensive studies on this subsidence has been reported in the literature. Çomut et al. (2016) were the first to use InSAR technique with Sentinel-1 A/B and CosmoSky-Med images between 2014 and 2015 and report the subsidence in the metropolitan area of Konya. They found a maximum of 6 cm/yr of subsidence centered near the railway station. However, this study is based on a very short observation period, that is, 1.5 years of Sentinel-1 data only, and hence does not reveal the entire pattern of deformation in the city. 
Recently, Orhan (2021) used Sentinel-1 A/B data between October 5, 2014 and February 04, 2018 and found a ground subsidence of up to 7.5 cm/yr. Both Çomut et al. (2016), and Orhan (2021) used only a single track, that is, ascending or descending. Other InSAR observations in the Konya plain are mostly focused around the Karapınar region where widespread sinkhole formations are present (Üstün et al., 2015; Caló, et al., 2017; Orhan et al. 2021).
In this study, we calculate time series of deformation with advanced multitemporal InSAR techniques using descending ENVISAT images between 2004 and 2010, ascending ALOS images between 2006 and 2010, and ascending and descending Sentinel-1 A/B TOPS images between 2014 and 2020 in order to reveal the spatiotemporal pattern of ground subsidence in the entire Konya city since 2004.  Using multi temporal advanced InSAR techniques (MT-InSAR), the time series of surface deformation and mean line of sight (LOS) maps are calculated. The mean LOS velocity maps derived from Sentinel-1 data between 2014 and 2020 are decomposed in to vertical and E-W horizontal component following the approach given by Motagh et al. (2017). The E-W horizontal velocity maps reveal the horizontal motion due to the subsidence around the edges of deformation lobes.
MT-InSAR analysis of SAR images acquired by 3 different spaceborne sensors spanning different time periods shows that the city of Konya has been subsiding since 2004. Subsidence is confined to the Quaternary alluvium while there is almost no deformation in the hills to the west of the city where the active Konya fault is located. The pattern of subsidence shows two main lobes of rapid subsidence to the west and east of the city center with a N-S line with no deformation that divides the city in to two nearly equal halves. The western and eastern lobes are centered 500 m west, and 5 km east of the train station, respectively. What separates the subsidence of the city into two halves roughly along a north-south line is unknown. It may be due to lithological differences in the alluvium on which the city is built on or the presence of a fault that may be sealed and hence act as a barrier to the aquifer system or the rate of extractions on both sides which are not sufficiently high for lobes to merge.  Between 2004 and 2019 the rate of subsidence increased from a few cm per year to 11 cm/yr. Although subsidence has slowed down over the last two years, particularly in the western side of the city, it is still taking place at rates reaching up to 6-7 cm/yr in the eastern side. 
We also use groundwater level changes recorded by the StateHydraulic Works (Devlet Su İşleri) over the last five years where lobes of rapid subsidence are centered. The results obtained from multi-sensor InSAR time-series are compared with groundwater levels over the last five years to understand how land subsidence evolves in response to the water table fluctuations.
Spatiotemporal variation of subsidence and its strong correlation with change in water table level confirm that subsidence in the metropolitan area of Konya is due to over drafting of the ground water used for urban needs.  Over the last two years or so, the demand appears to have decreased as subsidence rate has slowed down dramatically or stopped at some locations in the city. This appears to be due to the so called "Blue Channel" project that transfers drinking water to the city from several recently built dams to the south. 
We model the surface deformation deduced from Sentinel-1 data using elastic dislocations on triangular and rectangular surfaces (Okada, 1985). In order to check whether or not subsidence can be partly explained by creep on down dip section of the Konya fault beneath the city, we have modelled the subsidence with elastic dislocations on a rectangular fault that coincides with the Konya fault. Taking a forward approach in which fault parameters are estimated by trial and error, we model the vertical deformation observed between 2004 and 2017 with the Sentinel-1 data. Modelling results show that an unrealistic rate of aseismic slip (about 90 cm/yr) is required on a small portion of the fault (about 6 km) at depths between 4 and 6 km. While the western lobe can be approximated to some extent, the eastern lobe cannot be modelled satisfactorily. Thus, it is highly unlikely that the subsidence can be attributed to aseismic slip on the Konya fault. 
In addition to rectangular dislocation, we have also modelled the subsidence with triangular dislocations in order to calculate volume loss due to the compaction of the aquifer as a result of excessive water extraction (Thomas, 1993).  A near flat surface at a depth of 150 m below surface where the water table is approximately found is constructed using triangular elements. A negative opening of these triangular elements is inverted using the Poly3Dinv code (Maerten et al., 2005) using the vertical deformation observed between 2004 and 2017. The results show that every year a volume loss of ~7.7x106 m3 is happening due to groundwater extraction.  This amount is valid only for the Konya city center and its vicinity. The volume loss is much greater where agricultural activities are intensive. If the ground water extraction is totally stopped in the future, InSAR monitoring could answer the question of how much of this compaction is recoverable.
Although there is no reported incidence directly linked with the ongoing deformation, subsidence taking place for decades must have caused significant damage to buildings and infrastructure in Konya, which needs to be investigated. The damage is probably visible in the form of fractures and tilts in engineering structures that we assumed are attributed to the low quality of buildings and materials. If over pumping of the ground water continues the infrastructure and building will eventually be damaged seriously.