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Electromagnetic Applications in Hydrogeological Investigations : A Feasibility Study. Hydrogeological report 80/5.



Electromagnetic Applications in Hydrogeological Investigations : A Feasibility Study. Hydrogeological report 80/5.


Doherty, J.


E-Publications; E-Books; PublicationNT; Report no. 15/1981







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Northern Territory Government

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Report no. 15/1981

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Attribution International 4.0 (CC BY 4.0)

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Northern Territory Government



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Technical Report WRD81015 Viewed at 14:07:36 on 29/07/2010 Page 10 of 138. -b at"t.l-ibu'tabJ etc' chang8s ir. t!l{? conouctjv(;: "Larg8'l l~yer t.1"Je:J1 1..:. changes in the l.::ycrs enclosing it.. The Sal:lC v:,ill 0:;JP1y tc..' hi ah-f l,Leouency-cl omctT ic or :rc=ouencv soundings v:hE::re 1 aterc1 inhomoaer;eities -" in resistive la~ers be;:tv.1 een tral1smittina ano - - -receiving dipoles will have a ~iminished effect witll respect ~o their effect O~ I8Sistivity soundings over the same ground. 2.4 McMinn's Area In March/April 1980, a resistivity survey was conducted over the l'lcI>1inli I s Area near Darto:in I of interest due to the possibilit:" of supplementing Darwin's water supply with additional bores in this area. A typical geoelectric section over areas of subcropping calomi te is as follov.?s: .4-000 n-m 3" tr, l JOoo Jl-m ?O:M i , ~ 00 !l-rn ~o~ 1 "3000 1'(.. - fT"'I The base of layer 1 corresponds to the water table; layer 2 represents Cretaceous sandstones and siltstones from \olhich flo,,is usually poor, as indicated by the high resistivity of this laYE The third layer represents an old weathering surface above the lower Proterozoic dolomite, the lowermost layer. The zones of hydrogeological interest, those from which flows are highest, are the third and fourth. A thick, conductive third layer and/or a conductive fourth layer indicate more intense weathering in the old land surface, and cavernous zones in the dolomote, both of which are conducive to high water storage and ext~"action capacity_ As with Magella Creek, the ability of EN measurements to determined. lal1er parameters are varied, detect these variations is '- - ...!..- _. ~ variations in thickness of the third layer. and The variations shown in fig 8 are easily detected ,,-ith both EM frequency sounding magnitude measur~~ents and Schlumberger resistivity measurements, with the former showing greater relative differences (at the steep descent) per change in thickness. Variations in resistivity of the third layer. As ~ig 9 shows, EH frequency sounding is more sensitive than Schlumberger VES in this regard. S Equivalence. " ~L'""r' ,-, . o~n ' .. ~: -, ~ .: ... ,rs "":.y- ..... "':l'at or'C'l'f"""'-C p"-:.r"l,~.l,l' ~-. .I_! __ : r:.'. ":.'. _, __ "',..tl...-::.\ _'... ,,~_'-' L.J ... _ ..... <.-,- _,I,"",:, __ -. L:; , .; ~ ... ~ -'-'-", ,._.,'"..:;;.:, 9 ,"",", c ... .:; .-t':; < ,~."l -..... J., __ - .. lHl::'h.~l~l.'~.:\::"lJ"':~; ',il ,1 ' .. ~ -.:~,..:. ... ;t:)JCy ,"'U 1.,U..l...! ....... .:;:.1. ..... .::>'- .... l~SU_:.;,J .,l' ......... ".')1 cases ~~ich ftre indjstillguishabJe using resistivity sounding. Differences of between 1% and 4% exist over 7 of the 17 data Doin"'ts sho\r.'n in the It',agni tude spectriJ .. 1TI of fig 10.. Ho~~ever f if ;TIec:'sureffients can be :made to only 2-3% f the t.v.~o cases are p:r:osably not distillguishable. The actual deter:.!ination of field errors will have to await field measurements_ The phase me2..5UreJnents sho\-.~n in figlO have a sirnilar sensitivity. _ / 9

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