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Exhibit 2.8 P.G.S. MEXICANA. S.A. de C.V. SUPPLEMENTAL INDENTURE SUPPLEMENTAL INDENTURE (this Supplemental Indenture), dated as of 31 August 2004, among
P.G.S. Mexicana. S.A. de C.V. (incorporated in Mexico) (the Guaranteeing Subsidiary), a
subsidiary of Petroleum Geo-Services ASA (the Company), the other Guarantors (as defined in the
Indenture referred to herein) and Law Debenture Trust Company of New York, as trustee under the
indenture referred to below (the Trustee). W I T N E S S E T H WHEREAS, the Company has heretofore executed and delivered to the Trustee an indenture, dated
as of November 5, 2003 (the Original Indenture), as supplemented by the First Supplemental
Indenture (the First Supplemental Indenture and together with the Original Indenture, the
Indenture) and providing for the issuance of an aggregate principal amount of up to $745,948,810
of 10% Senior Notes due 2010 (the 10% Senior Notes due 2006) and $250,000,000 of 8% Senior Noted
due 2006 (the 8% Senior Notes due 2006); WHEREAS, the Indenture provides that under certain circumstances the Guaranteeing Subsidiary
shall execute and deliver to the Trustee a supplemental indenture pursuant to which the
Guaranteeing Subsidiary shall unconditionally guarantee all of the Companys obligations under the
Notes and the Indenture on the terms and conditions set forth herein and in Article X of the
Original Indenture (the Note Guarantee); and WHEREAS,
pursuant to Section 9.01 of the Indenture, the Trustee is authorized to execute and
deliver this Supplemental Indenture. NOW THEREFORE, in consideration of the foregoing and for other good and valuable
consideration, the receipt of which is hereby acknowledged, the Guaranteeing Subsidiary and the
Trustee mutually covenant and agree for the equal and ratable benefit of the Holders of the Notes
as follows: 1. Capitalized Terms. Capitalized terms used herein without definition shall have
the meanings assigned to them in the Indenture. 2. Agreement to Guarantee. (a) Along with all Guarantors named in the Indenture, the Guaranteeing Subsidiary, in
accordance with, and subject to the terms and conditions set forth in Article X of the Original
Indenture, hereby jointly and severally guarantees, to the Trustee for its own benefit and the
benefit of the Holders from time to time (i) the full and prompt payment of the principal of and
premium, if any, on the Securities and the indebtedness represented thereby, when and as the same
shall become due and payable, whether at the Maturity thereof, by acceleration, call for redemption
or otherwise, (ii) the full and prompt payment of interest and any Additional Amount on the
Securities when and as the same shall become due and payable and (iii) all other monetary
obligations of the Company under this Indenture (including under Section 7.07 of the Original
Indenture) and the Securities (the guarantee in clauses (i), (ii) and (iii), collectively referred
to as the Guarantee). Notwithstanding the foregoing this Guarantee shall be limited to the
extent that any Foreign Investment National Commission of Mexico prior approval is required
pursuant to the Foreign Investment Law of Mexico and the applicable legislation and International
Treaties to which the United Mexican States is a party in accordance with Article 133 of the
Political Constitution of Mexico. (b) This Guarantee shall not be discharged except in accordance with the terms of the
Indenture, and the Guaranteeing Subsidiary accepts all obligations of a Guarantor under the
Indenture. 3. Execution and Delivery. The Guaranteeing Subsidiary agrees that its Guarantee
shall remain in full force and effect notwithstanding any failure to endorse on each Note a
notation of such Guarantee. 1




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4. Headings. The headings in this Supplemental Indenture have been inserted for
convenience of reference only, are not to be considered a part hereof and shall in no way modify or
restrict any of the terms or provisions hereof. 5. Counterpart Originals. The parties may sign any number of copies of this
Supplemental Indenture. Each signed coy shall be an original, but all of them together represent
the same agreement. 6. THIS SUPPLEMENTAL INDENTURE AND THIS GUARANTEE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF
LAW. 7. The Trustee. The Trustee shall not be responsible in any manner whatsoever for or
in respect of the validity or sufficiency of this Supplemental Indenture or for or in respect of
the recitals contained herein, all of which recitals are made solely by the Guaranteeing
Subsidiary, the Company and the Guarantors. 2




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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly
executed and attested, all as of the date first above written. Dated: 31st August, 2004 P.G.S MEXICANA. S.A. se C.V.By:

/s/
Eric WersichName:
    
    Eric WersichTitle:
    
    President PETROLEUM GEO-SERVICES ASABy:
    /s/ Sam R. MorrowName:
    Sam R. MorrowTitle:
    Senior Vice President Finance and
Treasurer MULTIKLIENT INVEST ASBy:
    /s/ Svein RennemoName:
    Svein RennemoTitle:
    Director PGS EXPLORATION (UK) LTD.By:
    /s/ Sam R. MorrowName:
    Sam R. MorrowTitle:
    Senior Vice President Finance and
Treasurer of Petroleum Geo-Services ASA PGS GEOPHYSICAL ASBy:
    /s/ Svein RennemoName:
    Svein RennemoTitle:
    Director PGS SHIPPING (ISLE OF MAN) LTD.By:
    /s/ Svein RennemoName:
    Svein RennemoTitle:
    Director PGS RIO BONITO SABy:
    /s/ Sam R. MorrowName:
    Sam R. MorrowTitle:
    Senior Vice President Finance and
Treasurer of Petroleum Geo-Services ASA 3




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PGS INVESTIGACÀO PETROLIFERA LTD.By:
    /s/ Sam R. MorrowName:
    Sam R. MorrowTitle:
    Senior Vice President Finance and
Treasurer of Petroleum Geo-Services ASA PGS ONSHORE, INC.By:
    /s/ Svein RennemoName:
    Svein RennemoTitle:
    Director LAW DEBENTURE TRUST COMPANY OF
NEW YORK, as TrusteeBy:
    /s/ Patrick HealyAuthorized Signatory 4