ارزیابی فرسایش تونلی در محدوده روستای چشمه‌لی در استان گلستان

یکی از انواع فرسایش‌ها، فرسایش تونلی است که با توجه به عملکرد پیچیده تشکیل و مشکلات اندازه گیری، در مقایسه با فرسایش های سطحی کمتر مطالعه شده است. فرسایش تونلی از جمله فرسایش های آبی است که وقوع آن به تغییرات بارزی در چشم انداز زمین و تخریب محیط زیست منجر می شود. روستای چشمه‌لی در 45 کیلومتری شمال شرق کلاله و در 27 کیلومتری جنوب غرب مراوه‌تپه در استان گلستان واقع شده است. مطالعات ژئوالکتریک در محدوده روستای چشمه لی، با هدف مطالعه فرسایش تونلی، چهار پروفیل به نام های ab، cd، ef و gh در این روستا طراحی شد و پس از برداشت داده های صحرایی، پردازش داده های به دست آمده از این مطالعات انجام شد. پژوهش حاضر با استفاده از تکنیک ها و روش های ژئوالکتریک و ژئوتکنیک به منظور آشکارسازی، ارزیابی و تحلیل بخش زیرسطحی زمین انجام شد. همچنین با توجه به اینکه مطالعه سطوح زیرین زمین در صورت کند و کاو هزینه بالایی دارد؛ استفاده از سایر تکنیک های دقیق از نیاز های اساسی به نظر می رسد. به همین دلیل، در پژوهش حاضر از تکنیک ژئوالکتریک از شاخه های ژئوفیزیک استفاده  شد و داده های به دست آمده با اطلاعات ژئوتکنیکی  مورد ارزیابی و دقت سنجی  قرار گرفت.  داده های مربوط به مقاومت الکتریکی پس از برازش و پردازش، در نرم افزار تخصصی res2dinv تحلیل و نتایج آن به صورت مقاطع ژئوالکتریکی و نقشه های هم مقاومت ارائه شد. مقادیر مقاومت ویژه به دست آمده در ‌پروفیل های مورد مطالعه، عمدتاً متاثر از واحدهای لسی منطقه است. روند تغییرات مقاومت الکتریکی در برخی مقاطع، به طور متناوب با افزایش یا کاهش همراه است. با توجه به میزان مقاومت ویژه به دست آمده و مقایسه آن با نتایج داده های ژئوتکنیکی، افزایش مقدار مقاومت ویژه اغلب با واحدهای سنگی (آهکی) در ارتباط است و در تحتانی ترین عمق مورد مطالعه آشکار می شود. مقاطع مقاومت الکتریکی و نقشه های هم مقاومت نشان داد که قرارگیری سطوح میانی با کمترین میزان مقاومت الکتریکی در بین بخش های نسبتاً تحکیم یافته سطحی و واحد های سنگی سطوح تحتانی، شرایط را برای ایجاد فرسایش های تونلی فراهم نموده است. توزیع شیب توپوگرافی، با جریان های زیرسطحی هم سو بوده و با توجه به مهیا بودن شرایط زمین، از عوامل مهم در رخداد پایپینگ و فرسایش تونلی است. روش ژئوالکتریک با توجه به حساسیت مقدار مقاومت الکتریکی به تغییرات ساختاری در سطوح زیرین، قابلیت مناسبی دارد و در صورت استفاده، به کاهش هزینه ها در سایر مطالعات اجرایی منجر می شود.  

assessment of tunnel erosion in cheshmeh lee village in golestan province

1- introductionsinkholes are processes of land deformation that create conical shapes with different dimensions in karst and loess areas, and if they occur in high frequency in one area, they form a specific topography. sinkholes in loesses occur for various reasons and the main cause is the phenomenon of collapsibility in loesses.  sinkholes create spatial landscapes after formation and make subsequent changes in the earth’s surface. in iran, a large amount of valuable soil is lost to erosion every year for various reasons. the amount of soil loss by this phenomenon is estimated at about 2 billion tons, which is estimated as 7.6% of the world’s total erosion, and a significant amount of soil enters the rivers as sediment every year. one type of erosions is tunnel erosion, which has been less studied compared to surface erosion due to its complex formation and measurement difficulties. tunnel erosion is one of the water erosions whose occurrence causes significant changes in the earth’s landscape and environmental degradation. this type of erosion is often caused by dissolution. cheshmeh lee village is located 45 km northeast of kalaleh and 27 km southwest of maraveh tappeh in golestan province.2- methodologygeoelectric studies in the cheshmeh lee village had been considered with the aim of studying sinkholes, hidden created channels and their path, soil horizons and geological units, possible fractures, surface and groundwater as well as bedrock structures. a total of four profiles named as ab, cd, ef, and gh were designed in this village, then the data were collected using dipole-dipole array with an electrode distance of 10 meters for different separation steps. the data obtained from the geoelectric studies were processed after the field data were collected.3- results ab profile  the range of variation of the resistivity value across the obtained section is not large. changes in the amount of resistivity indicate small changes in the type of geological strata that are often associated with area loess. however, in some parts, the amount of resistivity has slightly increased or decreased. the increase in resistivity is probably related to the compaction (density) of loess units, and the decrease in resistivity can be due to the presence of moisture, salt (due to its observation in downstream springs) or an increase in clay.cd profile  there is a similar trend in terms of change in the amount of resistivity in the ab profile. the amount of resistivity affected by loess units has changed in the area. however, in some parts of the cross-section, an increase or decrease in the specific resistance can be observed. the amount of resistivity has decreased in medium depths, especially in the middle of the section. given the amount of resistance obtained, probably indicates the beginning of the floor stone in this section. ef profile the highest amount of resistivity is distributed at higher depths. increased resistivity can indicate the beginning of the depth of the floor stone in this area.gh profile  as in the previous profiles, in this profile, the main changes of special resistance are related to the loess units of the region, and sometimes in some parts, an increase or decrease in the amount of special resistance can be observed. considering the residential location of the village along with the outcrops of sinkholes and erosion tunnels, 4 points were selected and introduced for drilling holes in order to study the geotechnical features of the area. this part of the study includes field operations such as drilling boreholes, and conducting field experiments, and laboratory experiments. also, this part of the research was conducted with the objectives of determining the soil type and thickness of soil layers, determining the stratification and geotechnical conditions of the soil and examining the physical and mechanical characteristics of the soil.4- discussion & conclusions   the results of inverse modeling and preparation of all-electrical resistance maps provided appropriate information about the general state of the region. in general, the values of resistivity for the studied profiles showed the trend of changes in the amount of resistivity, which are mainly affected by the loess units of the region. however, the trend of change has increased or decreased in some parts. increasing the amount of resistivity is mainly connected with stone units. the results of the analyses conducted show that the geoelectric technique has a good capability due to the sensitivity of the amount of electrical resistance to the structural changes in the lower levels, and if this technique is used, it leads to a reduction in costs in other executive studies. also, the speed of harvesting, analyzing and drawing conclusions from this technique is high compared to other executive methods.