Supplemental Electrical Resistivity Surveys Ascension Island South Atlantic Ocean

This Report Describes An Electrical Resistivity Survey Completed In September Of 1984 As An Extension Of The Phase Ii Evaluation Of The Geothermal Potential Of Ascension Island. This Electrical Resistivity Work Had Been Recommended To Assist In Siting A Deep Temperature-Gradient Hole After Phase Ii Thermal Gradient Results Were Found To Be Permissive And Favorable For The Occurrence Of A High-Temperature Geothermal Resource. Reconnaissance Electrical Resistivity Measurements Had Been Completed Earlier Which Covered The Central 35 Sq. Km Of The Island. A Broad Zone Of Low Apparent Resistivity Was Mapped Which Trends North-Northeast From Devils Riding School Through Thistle Hill. Areas West Of Spoon Crater And Northeast Of Thistle Hill Showed The Lowest Apparent Resistivities Within This Broad (2 Km By 5 Km) Zone. Two Dipole-Dipole Profiles Which Were Completed In June 1983 Have Refined The Location, Depth And Intrinsic Resistivity Of The Low Resistivity Zones. The Profiles Showed High Resistivities Above Sea Level, With Some Resistive Masses Extending To Considerable Depth. Low-Resistivity Zones At Depth Are Thought To Result From Sea Water Incursion Within A Few Kilometers Of The Coast. Geothermal Brines Probably Contribute To Low-Resistivity Zones Further Inland, I.E. From Devil Is Riding School To Mcturks Culvert, And South Of Thistle Hill. The Low-Resistivity Zones Were Incompletely Defined Due To Rough Topography, Limited Access And Surface Rocks Of High Electrical Impedance. The Low-Resistivity Areas As Defined By The Survey Work Were Recommended For Thermal Gradient Test Drilling In A Subsequent. Dipole-Dipole Lines 4 And 5 Have Further Delineated The Low-Resistivity Zone And Will Contribute To The Optimum Siting Of Deep Thermal Gradient Tests And A Production Well. Numerical Modeling Of Lines 2, 4, And 5 And An Integration Of Pre-Existing Data, Suggest A Zone Of 5-10 Ohm-M Resistivities Which May Be Associated With Thermal Fluids At Approximately Sea Level Depths. Egi Reference Number Gl04132_1

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Citation Date 1984-10-26T00:00:00

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Original ID 628C9A929A0F44E9850D510323584C86
Index Date 2014-06-17T13:17:43
Original Format ISO-USGIN
Original Version 1.2


Name Howard P. Ross, Claron E. Mackelprang, And Sharrif F. Dajany

Geographic Extent

North Bound 78
South Bound -54.45
East Bound 28.65
West Bound -80.6