Using Geophysical Techniques in Geomorphological Problems: A Case Study From Kavs Flat in Dicle Valley (South of Diyarbakır)
Around the city of Diyarbakir, many problems arise with regard to the evolution of the river drainage in the Tigris valley, in terms of regional geomorphology. Among the various approaches we used for understanding and reconstruting these dynamics in the past as well as for the present, we have performed morphometric analysis and field studies aiming at reconstructing the recent morphological evolution of the valley from coring, sediment interpretation, 14C dating, and stratigraphic correlations between sections and cored sequences, etc. One of the modern methods and techniques used in geomorphological studies and elucidation of geomorphological problems is geophysical prospections. Electric resistivity data are efficient in determining the underground distribution of geological structures, especially when there is sufficient lithological contrast between geological units. This method produces very useful results that provide series of vertical sections that can be important contributions to interpretating geological features on the field. Such an electric resistivity project has been applied in the Diyarbakır vicinity as a method aiming at understanding problems raised by the development of the Tigris river channel and meanders east and south of the city of Diyarbakır. Within the frame of this programme, we present here the results of a geophysical survey of the Kavs Plain on the left bank of the Tigris river below the Diyarbakir City wall.
Jeomorfolojik Problemlerde Jeofizik Teknikler: Diyarbakır Güneyinde Dicle Nehri’nin Menderesli Vadisinden Bir Vaka Çalışması
Diyarbakır kenti çevresinde, Dicle vadisinde nehir drenajının gelişmesiyle ilgili olarak bölgesel jeomorfoloji (kapma, genişleme, dirseklenme, daralma vb.) yanında yerel jeomorfoloji açısından da birçok problem ortaya çıkmaktadır (mendereslenme, teraslanma, kayma, dairesel form vs.). Bu dinamiklerin geçmişteki ve günümüzdeki oluşum süreçlerini ve gelişim seyrini anlayabilmek, vadinin son dönemdeki morfolojik evrimini aydınlatmak ve yeniden yapılandırmak için kullandığımız çeşitli yaklaşımlar arasında; karot sondajı, sedimentlerin analizi ve yorumlanması, 14C tarihlendirmeleri, stratigrafik kesitler, korelasyonlar yeniden yapılandırmada morfometrik analizler ve saha incelemeleri sayılabilir. Jeomorfolojik çalışmalarda ve jeomorfolojik problemlerin aydınlatılmasında kullanılan modern yöntem ve tekniklerden biri de jeofizik uygulamalarıdır. Elektrik özdirenç verileri, özellikle jeolojik birimler arasında yeterli litolojik kontrast olduğunda jeolojik yapıların yeraltı dağılımının belirlenmesinde etkilidir. Bu yöntem, sahadaki jeolojik özelliklerin yorumlanmasına önemli katkılar sağlayabilecek ve birimlerin dağılışını düşey yönde yorumlama imkanı verebilecek sonuçlar üretir. Bu tür bir elektriksel direnç projesi, Dicle nehri yatağının gelişmesiyle ortaya çıkan sorunları ve Diyarbakır kentinin doğu ve güney kesimlerinin şekillenmesini anlamayı amaçlayan bir yöntem olarak Diyarbakır civarında uygulanmıştır. Bu çalışmada, bir araştırma programı çerçevesinde, Dicle Nehri’nin sol yakasında ve Diyarbakır eski kent surlarının altındaki Hevsel Bahçeleri’ne simetrik olarak bulunan Kavs Düzlüğü’ndeki jeofizik çalışmalarının sonuçları sunulmaktadır.
One of the modern methods and techniques used in geomorphological studies and elucidation of geomorphological problems is geophysical prospections. Among these methods, electrical resistivity is nowadays widely used in geological research. Its data are efficient in determining the underground distribution of geological structures, especially when there is sufficient electrical contrast between geological units. This method produces very useful results that provide series of vertical sections that can be important contributions to interpretating geological features on the field. In recent years, higher-precision and faster resistivity devices have emerged, also using numerical methods during the data evaluation.
Such an electric resistivity project has been applied in the Diyarbakır vicinity as a method aiming at understanding problems raised by the development of the Tigris river channel and meanders east and south of the city of Diyarbakır.
Aims of the operation were:
- To establish the relationships between the Pliocene-Pleistocene alluvial terraces built up by the Tigris old and present courses. In the area studied, the top alluvial soil forming the valley bottom studied, cover either a basement formed by Upper Miocene-Lower Pliocene-aged Şelmo Formation, or Early Pleistocene coarse alluvial deposits, or similar or thinner alluvial Holocene sediments.
- To identify possible tectonic phenomena.
Geophysic prospection has been realized along the left bank limiting the southern part of the Tigris wide meander at the foot of the city wall. Data were evaluated by a two-dimensional inversion electrical resistivity process (1) evidencing contrasts of resistivity of the sediment units below the ground surface, and (2) allowing an electrical mapping of sections through the units and an analysis of the distribution of these units along the lines explored.
ERT and IPT measurements have been performed along five lines. Coordinates as well as topography records and geophysic data were collected in association with GPS at all measurrement points. ERT work was then carried out on the five lines of different lengths, and organized along different directions that were crossing each other.
The main results from the electrical resistivity tomography (ERT) and induced polarization tomography (IPT) measurements, together with field observations are the following:
- Along the vertical profile of the valley left slope, which cuts the Selmo formation and its Plio-Pleistocene coarse alluvial cover, topographic ruptures are associated with vertical erosion of differently resistant lithology of the bedrock formations. However, stratigraphic observation also suggested deformations underlined by some cliff lines. Geophysical data demonstrate that these deformations are associated with a fault zone composed of an array of small normal faults.
- Along the foot of the left scarp of the river valley, the faulted zone separates the Plio-Pleistocene bedrock from other formations accumulated in the valley bottom. The uppermost unit of this fill presents a high-medium resistivity. Its thickness varies approximately between 20 to 40 meters. The geophysic data point to a coarse-grained alluvial and flood material. Below, another unit presents generally low resistivity values pointing to a high clay content that may characterize the lower levels of the Miocene Selmo formation. If our identification of the geological formations concerned is correct, these results suggest:
- The presence of a high discontinuity of an erosive nature between the Mio-Pliocene clayey formation (that outcrops on the other side of the valley below the city walls) forming the bottom of the valley at this area, and the Plio-Pleistocene alluvial formation above.
- The most probable contribution of the fault zone evidenced in the ERT and IPT results in controlling the displacement of the river in this area.
- The small normal faults are NW-SE oriented in the western part of the valley course, while some other faults also emerge in the eastern part. The occurrence of the fault signal up to the surface suggests that the system may still be active.
In conclusion, the use of electric resistivity measurements has produced important data concerning discontinuities. These discontinuities have been interpreted as (1) faulted ruptures and (2) stratigraphic unconformities. Both these evidences from the Kavs plain are important contributions to our research programme on the reconstruction of the dynamic geomorphological evolution of the Tigris valley during the Quaternary, and especially its dynamics during the Late Quaternary. In particular, it seems evident that, in this area which is located at the foot of the meandering terraces over which the Hevsel Gardens, which are inscribed on the Unesco World Heritage List as part of a cultural landscape also including the Roman city walls, tectonic may contribute to today’s dynamics of the river, in particular its erosion activities.
