استفاده از روش هاي ژئوالكتريك در بررسي وضعيت نشت آب از محدوده سد رسوب گير معدن مس ميدوك

Geoelectrical investigation of seepage beneath the tailing dam areas of Miduk Copper Mine

با توجه به اثرات سوء زيست‌محيطي سدهاي باطله و ناپايداري مكانيكي آن‌ها به دليل فرسايش داخلي ناشي از نشت‌هاي احتمالي، ديد‌باني عملكرد اين سدها بسيار ضروري است و عاملي براي مديريت بهتر فعاليت‌هاي معدني به حساب مي‌آيد. در اين پژوهش به منظور بررسي وضعيت نشت آب از سد رسوب‌گير معدن مس ميدوك و شناسايي عامل نشت، از روش‌هاي ژئوفيزيكي توموگرافي مقاومت‌ويژۀ الكتريكي و پتانسيل خودزا استفاده شده است. برداشت توموگرافي مقاومت‌ويژۀ الكتريكي بر روي 3 پروفيل 200 متري با فاصلۀ الكترودي 10 متر و 1 پروفيل 400 متري با فاصلۀ الكترودي 20 متر و برداشت پتانسيل خودزا با اندازه‌گيري 208 نقطه بر روي 7 پروفيل با فاصلۀ الكترودي 10 متر، در پايين‌دست سد رسوب‌گير انجام گرفت. لازم به ذكر است به‌علت ساختار سنگي سد و پوشش تاج آن با مصالح سنگي درشت‌دانه، برداشت‌هاي ژئوفيزيكي بر روي تاج امكان‌پذير نبود. نتايج برداشت توموگرافي مقاومت‌ويژۀ الكتريكي در پايين‌دست سد رسوب‌گير، دو زون رسانا، يكي در يال شرقي و ديگري در يال اصلي سد رسوب‌گير را نشان مي‌دهد كه بيانگر خردشدگي بستر سد در اين محل‌هاست. در يال شرقي در حال حاضر، نشت از روي بستر سد قابل مشاهده است. پيش‌بيني مي‌شود با افزايش سطح آب و رسوبات در پشت سد، نشت از بستر يال اصلي نيز اتفاق بيفتد. نتايج نشان مي‌دهد عوامل زمين‌شناسي و وجود زون خرد شده در بستر سد، عامل اصلي نشت است. همچنين در نتايج برداشت پتانسيل خودزا، وجود آنومالي مثبت پتانسيل خودزا در پايين‌دست سد، دو مسير نشت زير سطحي را نشان مي‌دهد كه تأييدي بر نتايج توموگرافي است. انطباق نتايج حاصل از دو روش، نشان‌دهندۀ وجود نشت از بستر سد در يال شرقي و اصلي آن است. با افزايش سطح آب و رسوبات در پشت سد، امكان افزايش دبي نشت از زون‌هاي مشاهده شده و ايجاد مسيرهاي جديد وجود دارد. اين موضوع علاوه بر كاهش مقدار آب برگشتي به كارخانه فرآوري، مخاطرات زيست‌محيطي نيز در پي خواهد داشت.

A tailing dam or confining embankment is constructed to enable the deposited tailings to settle and retain processed water. Tailing dams are susceptible to different kinds of pressures such as water pressure and the load of the tailings themselves. Miduk tailing dam was originally constructed with a fine silty sand layer to retain water covered by coarse grains. It was constructed in stages according to the downstream method, filter and support fill. Tailing dams and downstream areas must be monitored as they undergo internal erosion, during which, the fine grains in the core of a dam are flushed away by seeping water and, as a consequence, the hydraulic conductivity in the remaining material increases. High velocity flows through the dam embankment can cause progressive erosion and piping. Moreover, the saturation of embankment soils, abutments, differential settlements in foundations, local stress relaxation in the soil and locally increased hydraulic gradient generally reduce soil strengths. The seepage issue in a tailing dam is the cause of reservoir loss to groundwater . Furthermore, it causes environmental problems such as the diffusion of heavy metals, acid drainage and so forth. Reversed water from the tailing dam is particularly important in desert areas. Resistivity and self-potential (SP) monitoring has been widely applied for solving environmental and engineering problems of embankment dams by studying the changes in the subsurface properties with time. SP changes are caused by water movements through (or under) the dam and resistivity changes reflect the changes in the electrical properties of the dam materials. Self-potential (SP) is a method where naturally occurring electrical potentials are measured. There are a number of different electro-chemical processes that can create such potentials. The type that is of interest in dam investigations is the so-called streaming potential which is the voltage difference parallel to the direction of flow. The streaming potential is manifested by a shearing of the diffuse layer caused by the hydraulic gradient. The field equipment for SP measurements is simple and inexpensive. It requires a pair of non-polarized electrodes, a high impedance voltmeter and t cables to connect them. Electrode drifts were controlled during SP measurements. Electrode drift is primarily caused by variations in temperature or soil moisture or by contamination of the electrolyte by ions introduced from the soil. Changes in the telluric currents induce substantial changes in the potential distribution in the subsurface, an effect accounted for by making regular measurements of the SP difference between the reference point and the base point within the survey area. The Resistivity method involves the measurement of the apparent resistivity of soil and rocks as a function of depth or position. The resistivity of the ground is measured by injecting a current with two electrodes and measuring the resulting potential difference with two other electrodes. The readings are usually converted into an apparent resistivity of the sub-surface. From these measurements, the true resistivity of the subsurface can be estimated. The investigated volume can be changed by moving the electrodes. The data are usually inverted to a vertical resistivity section, assuming a 2D geometry perpendicular to the profile. Most commonly, the local variability is minimized, resulting in smooth models compatible with the measured data, meaning that sharp resistivity borders such as the ground water surface is visualized as a smooth transition in such inverted sections. The principle objective of the present study was to evaluate the electrical resistivity and the self-potential methods used to detect anomalous seepage through mine tailing dams. In this regard, field measurements of resistivity and self-potential were carried out on the downstream grounds of tailing dam so as to identify the SP-responses related to seepage. The hydro-stratigraphy was mapped with the resistivity data (4 profiles of ERT) and groundwater flow patterns were specified with self-potential data (208 SP measurement points). The groundwater flow pattern was controlled by the geological and tectonic history of bedrock and the preferential flow pathway existing beneath the dam.