Three-dimensional inversion of magnetotelluric data : geological/geothermal interpretation of Asal Geothermal Field, Djibouti

Abstract

This research study is aiming at becoming acquainted with the resistivity method and different ways of performing interpretation of MT data for deep lying subsurface investigations. The MT data collected from Asal geothermal area were used; the comparison between the results of 1-D inversion carried out previously and 3-D inversion is performed. A total of 105 MT soundings were considered in this research project and the same number of corresponding TEM soundings collected at nearby sites. To allow the static shift correction in the 1-D inversion, the MT data were jointly inverted with TEM data. Shift correction was then applied to the two polarizations for 3-D inversion. The modern computing systems have made 3-D modelling of MT data achievable and it is now becoming common and useful for detailed subsurface surveys in geothermal industries as well as in other fields including ground water, oil and natural gas and mineral exploration. The WSINV3DMT code was used to perform 3-D inversion of the static shift corrected off-diagonal impedance tensor elements. Three different initial models were considered in order to test the inversion robustness and 31 periods evenly distributed on logarithmic scale from 0.003 to 300 s were used. The first initial model was compiled from the joint 1-D inversion of TEM and MT soundings; the second was a homogeneous earth of resistivity 10 Ωm and the third initial model was a homogeneous earth with a resistivity 50 Ωm. The RMS in all three different initial models was not of big difference with values of 1.44, 1.48 and 1.87, respectively.

The final models gave similar resistivity structures underneath Asal rift and are presented here as iso-depth resistivity maps and cross sections. The result of the interpretation shows four main resistivity structures below the geothermal area: A shallow lying thin high resistivity layer followed by low resistivity (conductive cap). Below there is a high resistivity layer (resistive core) underlain by a deep lying conductor. Lithology based on well data shows that the shallow thin high resistivity layer corresponds to dry basaltic rocks covering the surface, the conductive layer reflects saline fluids but correlates also with low temperature alteration (smectite and zeolites), the deep resistive core correlates with the high temperature alteration minerals (chlorite and epidote) whereas the deep seated conductive body is most likely connected to the heat source of the Asal geothermal system. At sea level high resistivity dominates the NE part of Asal rift towards Lake Asal in the vicinity of Ardoukoba volcano and the SE part of the rift around Baddikoma region. An updoming conductive cap intersects the high resistivity, running NE-SW and reflects presumably alteration within the geothermal system. It covers the central part of the Asal rift including the Fiale explosion crater (Lava Lake). This is the same area where the fumaroles and hot springs in the Asal rift are located.

Fjallað er um mismunandi aðferðir við túlkun MT gagna. Alls voru túlkaðar þrívítt 105 MT mælingar frá Asal jarðhitasvæðinu í Djibouti. Niðurstöður eru bornar saman við einvíða túlkun frá fyrri tíð. Sami fjöldi TEM mælinga var gerður nærri MT mælingunum til þess að leiðrétta hliðrun MT gagnanna. Niðurstöður túlkunarinnar sýna í meginatriðum fjögur viðnámslög í jarðhitakerfinu. Grunnt og þunnt háviðnámslag ofan við lágt viðnám (lágviðnámskápu). Þar fyrir neðan er viðnám hátt (háviðnámskjarni) og enn neðar er djúpliggjandi lágviðnám. Berggerð samkvæmt borholugögnum sýnir að þunna háviðnámið fellur saman við þurrt basalt sem þekur yfirborðið. Lágviðnámskápan endurspeglar saltan vökva en fellur einnig saman við lághitaummyndun (smektít og zeólíta), en háviðnámskjarninn fellur saman við háhitaummyndun (klórít og epidót). Djúpliggjandi lágviðnámið tengist að öllum líkindum hitagjafa Asal jarðhitakerfisins. Við sjávarmál er hátt viðnám ríkjandi á NA-hluta sprungusveimsins í áttina að Asal vatni og við Ardoukoba eldfjall, og eins á SA-hluta sveimsins á Baddikoma svæðinu. Lágviðnámskápan rís nærri yfirborði og gengur þvert á háa viðnámið í stefnu NA-SV og endurspeglar væntanlega ummyndun jarðhitakerfisins. Hún þekur miðhluta Asal sprungusveimsins þ.m.t. Fiale sprengigíginn (Lava vatn). Á þessu svæði finnast gufuhverir og laugar sprungusveimsins.