بررسی‌های مغناطیس‌سنجی، روشی برای شناسایی و تعیین عرصه و حریم گورستان‌های باستانی؛ نمونۀ موردی: گورستان باستانی مهدی‌آباد اولیا در منظر فرهنگی بم

گورستان های باستانی در کنار استقرارگاه های انسانی شکل می گرفته اند، ولی پراکندگی گورها به جمعیت و مدت استقرار در این سکونت گاه ها بستگی داشته است. یکی از دشواری های پژوهش های باستان شناختی در شناسایی گورستان ها، تعیین عرصه و حریم در این محوطه هاست. به دلیل پراکندگی گورها در گورستان ها، تراکم گورها بسیار متفاوت بوده و بنابراین تعیین عرصه و پیشنهاد حریم در این محوطه ها همواره با احتیاط بسیار زیادی صورت می گیرد. در سالیان اخیر، با پیشرفت علم و کمک تخصص های مختلف به باستان شناسی، تعیین عرصه و حدود محوطه های باستانی نیز با دقت بیشتری انجام می شود. یکی از روش هایی که در مکان یابی محوطه ها و آثار باستانی و به تبع آن، تعیین عرصه و حدود محوطه ها بسیار کارآمد و موثر است، بررسی های آرکئوژئوفیزیک است؛ به خصوص در تعیین عرصۀ گورستان های باستانی که تراکم و پراکندگی گورها بسیار متفاوت است و فقط با بررسی باستان شناختی و گمانه زنی، نمی توان با اطمینان به تعیین عرصه و حدود در این محوطه ها اقدام نمود. در این پژوهش سعی بر آن شده است که به پرسش های مهمی در زمینۀ تعیین عرصۀ محوطه های باستانی به خصوص گورستان های باستانی دست یافت؛ این که آیا روش های معمول تعیین عرصه می توانند به طور دقیق حدود گورستان های باستانی را مشخص نمایند؟ روش های ژئوفیزیکی چگونه می توانند در آشکارسازی مکان گورهای باستانی، باستان شناسان را یاری نمایند تا بتوانند با اطمینان بیشتری به تعیین عرصۀ گورستان های باستانی اقدام نمایند؟ تعیین عرصۀ گورستان های باستانی که تا به امروز انجام شده، نشان داده است که در مواردی ممکن است که به دلیل مشخص نبودن مکان گورها، بعضی از قسمت های گورستان های باستانی در عرصۀ تعیین شده قرار نگیرد. روش های ژئوفیزیکی به دلیل آن که قادر به مشخص نمودن تمامی پدیدارهای زیر سطحی و به طبع مشخص نمودن مکان گورهای باستانی هستند، به خوبی می توانند روشی باشند که باستان شناسان را در مشخص نمودن عرصۀ گورستان های باستانی کمک نموده و روش بسیار مناسبی جهت حفاظت از محوطه های باستانی باشند. بررسی آرکئوژئوفیزیک با روش مغناطیس سنجی در محوطۀ باستانی مهدی آباد اولیا1 در منظر فرهنگی بم، با هدف تعیین میزان پراکندگی گورهای باستانی و مشخص نمودن عرصه و حدود این گورستان باستانی در سال 1397 ه‍.ش. انجام گرفت. این بررسی باعث آشکار شدن حدود 800 گور باستانی گردید و نقشه های مغناطیسی به خوبی عرصۀ این گورستان باستانی را مشخص نمودند.

Magnetic Surveys as a Method to Determine the Core Zone of Archaeological Cemeteries Case Study: Mehdiabade Olia in the Cultural Landscape of Bam

AbstractIn premodern societies, cemeteries were formed next to human settlements. The distribution of graves in these cemeteries depended on the size of the population and the use of these settlements. One of the difficulties in modern archaeological research in identifying cemeteries is determining their core and buffer zones. Due to the dispersion of graves, the density of graves in cemeteries can sometimes vary considerably. Determining the core and buffer zone in these areas is therefore always done with great caution. With the advancement of science and various new methods in archaeological fieldwork, the core and buffer zones of archaeological sites can now be done more precisely. One of the more efficient and effective methods in identifying, and consequently, determining the core and buffer zone of sites, is the geophysical survey. A geophysical survey with the magnetic method in the archeological site of Mehdiabade Olia in the cultural landscape of Bam was carried out to determine the distribution of archaeological graves and determine the core zone of this ancient cemetery in 2018.Keywords: Archaeological Cemetery, Geophysical Survey, Core Zone, Buffer Zone, Mehdiabade Olia Cultural Landscape of Bam.IntroductionArchaeological sites are spaces where physical and material artifacts of past human life entangle. These sites can be investigated by various types of archaeological fieldwork. Archaeologists identify the ancient sites by using remote sensing studies, aerial photography and satellite imagery, and pedestrian surveys. Most of the time, when we move from the center to the outside of the sites, the density of cultural finds such as sherds of pottery, artifacts, stone tools, bones, or architectural remains decreases. Sometimes, around the ancient sites, the topography, geomorphology, and landform, as well as sediments during different periods, cause cultural materials and artifacts to remain under these sediments and be hidden from view. Therefore, archaeologists often need to excavate to determine the presence or absence of materials below the surface. Areas, where the remains of ancient materials are present and can be reached with confidence by survey and excavation, are called the core zones of an archaeological site. From a legal point of view, it is forbidden to do any modern changes in the core zone of an archaeological site. A buffer zone is often added to protect ancient sites. The buffer zone is defined concerning the core zone and depends on the topography, geomorphology, environmental conditions, and elements related to the site. One field where the density of artifacts is very uncertain is the ancient cemeteries, which were generally built next to the settlements. The graves were mostly solitary and sometimes included secondary burial or mass burials.Sometimes, due to the lack of obvious evidence of these graves, it is difficult and impossible to identify them, and determining their density and dispersion and the core zone of the cemetery. Determining the core zone and suggestion of the buffer zone traditionally is by using archaeological surveys and digging experimental sondages around the ancient site with special methods. Sometimes in ancient cemeteries, determine the core zone is not accurate, because maybe the archaeologist dig the experimental sondages between two graves and he doesn’t identify the graves, and due to the lack of awareness of the existence of graves, he doesn’t put this sections in the core zone, and this part should be outside the scope of the laws of the core zone of the archaeological site, and due to the lack of awareness of the existence of graves, this section not be included in the core zone of the site and will be outside the scope of the laws of the core zone of the archaeological site. But by geophysical methods, archaeologists can identify the location of all the graves and determine exactly the core zone of the ancient cemetery. Identified TrecesThe ancient cemetery of Mehdiabade Olia is located 3 km south of the village of Mehdiabade Olia. The cemetery expands on both sides of a branch of the seasonal River of Bandenesa, 90 km from the city of Bam. In winter 2016, after a flood event, several pits with ancient pottery fragments were found. Based on a preliminary analysis of the ceramics by the archaeologists of the research center of the citadel of Bam they were able to identify these to be of Achaemenid or Parthian date. Most of the graves are rectangular and they are only about 90 to 120 cm below the surface. The large extent of the site is ideal for geophysical studies to determine the distribution of graves and identify the core zone of this ancient cemetery. A magnetic survey is a fast and efficient method for the first approach of an archaeological site. The principle of the magnetic method is to measure the local variations of the Earth magnetic field due to the presence of iron oxides in the soils and the archeological structures. Surface soil is magnetically stronger than underground soils. Its properties are further enhanced by human activities. This makes identification of different archaeological structures possible: graves, pits, and holes prove a higher magnetic record after they were filled with soil and surface sediments. The difference in the intensity of the magnetic field causes graves and pits can be identified as point anomalies with the surrounding context on magnetic maps.The magnetic survey on the site was carried out with a cesium gradiometer G858 with a mesh grid of 1 m x 0.10 m interpolated at 0.50 m. Our survey covered an area of 8 hectares and focused on both sides of the road and the river by which several graves had been found. The magnetic map showed several anomalies: linear anomalies and positive and bipolar point anomalies. The strong linear anomaly on the south of the map was linked to the recently made modern canal. Another linear anomaly corresponding to the road in the southwestern part of this area. Other linear anomalies were related to artificial water passages. The majority of point anomalies are related to the graves, however. Our magnetic map identified approximately 800 graves. Some of these point anomalies are located on the surface graves that were found in the flood. ConclusionAccording to the magnetic map, the density of point anomalies decreases in the east, south, and southeast of the area surveyed. This means that the limit of the cemetery is specified in this section and the core zone line of the site can be well defined here. The highest density of cemetery graves is in the central part of the studied area. The anomalies of graves continue in the north and west of the section, but the density of these anomalies decreases, and the line of the core zone of the cemetery can be identified in this part of the area. Our magnetic survey shows that the core zone of the ancient cemetery is well recognizable without actual excavation. Any excavation and opening of sondages in this region are dangerous and would pave the way for the looting of these sites. A geophysical survey, however, will protect this ancient cemetery. After our survey, an Iranian archaeological mission directed by Shahram Zare found a large Achaemenid building near 650 m west of the cemetery. Archaeological research on this site is continuing.