We show the results of a geophysical survey carried out in the Alalobeda (Alalobad) geothermal field (Afar, Ethiopia). The site is located on the western margin of the NW-SW Tendaho Graben, at the intersection with the NNE-SSW Main Ethiopian Rift.

The survey included 121 Magnetotelluric (MT) and Time Domain Electromagnetics (TDEM) soundings, and 300 gravity records. We applied 3D MT and gravity inversion to target a previously inferred high-T (220 °C) near neutral pH liquid-dominated chloride reservoir.

Geophysical modelling identified the graben basin structure, with normal fault stepping in the Afar Stratoid basalts (2485 kg/m³), underlying a low-density (2035 kg/m³) and conductive (1 Ohm m) sedimentary cover. Along the graben shoulder, a density anomaly (+200 kg/m³) suggests a propylitization zone corresponding to the reservoir, underlying the typical conductive clay cap updomings and downdomings of high-T geothermal systems.

A major conductive plume, without an appreciable gravity signature, occurs in the graben shoulder. Integration with the geochemical results allows to interpret it as a vapor-cored system. The hot spring water CO₂, H₂S and Cl content indicates the contamination of magmatic gases into the reservoir that produce fluid acidification while ascending from a deep source. The local bulk resistivity (10 Ohm m) is explainable by assuming conductive permeating acid fluids (10 S/m), an enhanced porosity (20%) and a significant gas saturation (20%). The absence of an appreciable density increase is also justified by the hypothesised porosity and the gas fraction. Since the acidic fluids are neutralised by interaction with the host rock, the sampled waters are nearly pH-neutral.

Our results suggest the first occurrence of a vapor-cored system in divergent plate tectonics, such as the Tendaho Graben sector of the Afar rift zone.

我们展示了在阿拉贝达（阿拉巴德）地热田（埃塞俄比亚阿法尔）进行的地球物理调查的结果。该遗址位于 NW-SW Tendaho Graben 的西缘，与 NNE-SSW 主埃塞俄比亚裂谷的交汇处。

调查包括 121 次大地电磁 (MT) 和时域电磁 (TDEM) 测深，以及 300 次重力记录。我们应用 3D MT 和重力反演来针对先前推断的接近中性 pH 值液体为主的氯化物储层的高温度 (220 °C)。

地球物理模型确定了地堑盆地结构，在阿法尔层状玄武岩（2485 kg/m ³）中具有正断层阶梯，位于低密度（2035 kg/m ³）和导电（1 Ohm m）沉积覆盖层之下。沿着地堑肩部，密度异常 (+200 kg/m ³ ) 表明与储层相对应的青岩化带，位于典型的导电粘土帽隆起和高温地热系统的隆起下方。

没有明显的重力信号的主要导电羽流出现在grab住的肩部。与地球化学结果的整合允许将其解释为气芯系统。温泉水中的CO ₂，H ₂ S和Cl含量表明岩浆气体进入储层时会受到污染，从而在从深水源上升时产生流体酸化。局部体电阻率 (10 Ohm m) 可以通过假设导电渗透酸性流体 (10 S/m)、增强的孔隙率 (20%) 和显着的气体饱和度 (20%) 来解释。假设的孔隙率和气体分数也证明没有明显的密度增加是合理的。由于酸性流体通过与主岩相互作用而被中和，因此采样水的 pH 值接近中性。

我们的结果表明，在发散的板块构造中首次出现了气芯系统，例如阿法尔裂谷带的 Tendaho Graben 部分。