<PAGE>   1
                                                                    EXHIBIT 10-L


                       FIRST AMENDMENT TO CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated as
of October 23, 1998, is between VERITAS DGC INC., a Delaware corporation
("Borrower"), each of the banks or other lending institutions which are or from
time to time may become parties to the Agreement [hereinafter defined] (such
banks and financial institutions, together with their respective successors and
assigns, the "Banks"), and BANK ONE, TEXAS, N.A. (the "Agent").

                                    RECITALS:

         A. The Borrower, the Agent and the Banks entered into that certain
Credit Agreement dated as of July 27, 1998 (the "Agreement").

         B. Pursuant to the Agreement, Veritas DGC Asia Pacific, Ltd., Digicon
(Malaysia) Sdn. Bhd., Euroseis, Inc., Digicon Geophysical Corp., Veritas DGC,
Ltd., Veritas DCG Land Inc., and Veritas Energy Services Partnership
("Guarantors") executed those certain General Continuing Guaranty agreements
dated as of July 27, 1998 (the "Guaranties") pursuant to which the Guarantors
guaranteed to the Agent and the Banks the payment and performance of the
obligations (as defined in the Agreement).

         C. The Borrower, the Agent and the Banks now desire to amend the
Agreement as herein set forth.

         NOW, THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

                                    ARTICLE I

                                   Definitions

         Section 1.01. Definitions. Capitalized terms used in this Amendment, to
the extent not otherwise defined herein, shall have the meanings given to such
terms in the Agreement, as amended hereby.




<PAGE>   2






                                   ARTICLE II

                                   Amendments

         Section 2.01. Amendments to Section 10.1. Effective as of the date
hereof, Section 10.1 of the Agreement is amended as follows:

           (a)     The phrase "clauses (b), (c) and (d) above" contained in
                   paragraph (d) of Section 10.1 of the Agreement is amended to
                   read "clauses (b) and (c) above".

           (b)     The phrase "in an aggregate principal amount not to exceed
                   $50,000,000.00" contained in paragraph (f) of Section 10.1
                   of the Agreement is amended to read "in an aggregate
                   principal amount not to exceed $60,000,000.00".

                                   ARTICLE III

                              Conditions Precedent

         Section 3.01. Conditions. The effectiveness of this Amendment is
subject to the receipt by the Agent of the following in form and substance
satisfactory to the Agent:

                   (a) Resolutions - Borrower. Resolutions of the Board of
          Directors of the Borrower certified by its Secretary or an Assistant
          Secretary which authorize the execution, delivery and performance by
          the Borrower of this Amendment and the other Loan Documents to which
          the Borrower is or is to be a party hereunder.

                   (b) Incumbency Certificate - Borrower. A certificate of
         incumbency certified by the Secretary or an Assistant Secretary of the
         Borrower certifying the names and signatures of the officers of the
         Borrower authorized to sign this Amendment and each of the other Loan
         Documents to which the Borrower is or is to be a party hereunder.

                   (c) Additional Information. Such additional documents,
         instruments and information as the Agent or any Bank may reasonably
         request.

         Section 3.02. Additional Conditions. The effectiveness of this
Amendment is also subject to the satisfaction of the additional conditions
precedent that (a) the representations and warranties contained herein and in
all other Loan Documents, as amended hereby, shall be true and correct as of the
date hereof as if made on the date hereof, (b) all proceedings, corporate or
otherwise, taken in connection with the transactions contemplated by this
Amendment and all documents, instruments and other legal matters incident
thereto shall be satisfactory to the Agent and

                                       -2-




<PAGE>   3






each Bank, and (c) no Event of Default shall have occurred and be continuing and
no event or condition shall have occurred that with the giving of notice or
lapse of time or both would be an Event of Default.

                                   ARTICLE IV

                 Ratifications, Representations, and Warranties

         Section 4.01. Ratifications. The terms and provisions set forth in this
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Agreement and except as expressly modified and superseded by this
Amendment, the terms and provisions of the Agreement are ratified and confirmed
and shall continue in full force and effect. The Borrower, the Agent and each
Bank agree that the Agreement as amended hereby shall continue to be the legal,
valid and binding obligation of such Persons enforceable against such Persons in
accordance with its terms.

         Section 4.02. Representations, Warranties and Agreements. The
Borrower hereby represents and warrants to the Agent and the Banks that (a) the
execution, delivery, and performance of this Amendment and any and all other
Loan Documents executed or delivered in connection herewith have been authorized
by all requisite corporate action on the part of the Borrower and will not
violate the certificate of incorporation or bylaws of the Borrower, (b) the
representations and warranties contained in the Agreement as amended hereby, and
all other Loan Documents are true and correct on and as of the date hereof as
though made on and as of the date hereof, (c) no Event of Default has occurred
and is continuing and no event or condition has occurred that with the giving of
notice or lapse of time or both would be an Event of Default, (d) the Borrower
is in full compliance with all covenants and agreements contained in the
Agreement as amended hereby, (e) the Borrower is indebted to the Banks pursuant
to the terms of the Revolving Credit Notes, (f) the liens, security interests,
encumbrances and assignments created and evidenced by the Loan Documents are,
respectively, valid and subsisting liens, security interests, encumbrances and
assignments and secure the Revolving Credit Notes, and (g) the Borrower has no
claims, credits, offsets, defenses or counterclaims arising from the Loan
Documents or the Agent's or any Bank's performance under the Loan Documents.

                                    ARTICLE V

                                  Miscellaneous

         Section 5.01. Survival of Representations and Warranties. All
representations and warranties made in this Amendment or any other Loan
Documents including any Loan Document furnished in


                                      -3-

<PAGE>   4






connection with this Amendment shall fully survive the execution and delivery of
this Amendment and the other Loan Documents, and no investigation by the Agent
or any Bank or any closing shall affect the representations and warranties or
the right of the Agent or any Bank to rely on them.

         Section 5.02. Reference to Agreement. Each of the Loan Documents,
including the Agreement and any and all other agreements, documents, or
instruments now or hereafter executed and delivered pursuant to the terms
hereof or pursuant to the terms of the Agreement, as amended hereby, are hereby
amended so that any reference in such Loan Documents to the Agreement shall mean
a reference to the Agreement, as amended hereby.

          Section 5.03. Expenses of the Agent and the Banks. As provided in the
Agreement, the Borrower agrees to pay on demand all costs and expenses incurred
by the Agent and the Banks in connection with the preparation, negotiation and
execution of this Amendment and the other documents and instruments executed
pursuant hereto and any and all amendments, modifications and supplements
thereto, including, without limitation, the reasonable costs and fees of the
Agent's and the Banks' legal counsel, and all costs and expenses incurred by the
Agent and the Banks in connection with the enforcement or preservation of any
rights under the Agreement, as amended hereby, or any other Loan Document,
including, without limitation, the reasonable costs and fees of the Agent's
and the Banks' legal counsel.

         Section 5.04. Severability. Any provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

         Section 5.05. Applicable Law. This Amendment and all other Loan
Documents executed pursuant hereto shall be deemed to have been made and to be
performable in Houston, Harris County, Texas and shall be governed by and
construed in accordance with the laws of the State of Texas.

         Section 5.06. Successors and Assigns. This Amendment is binding upon
and shall inure to the benefit of the Agent, the Banks and the Borrower and
their respective successors and assigns.

         Section 5.07. Counterparts. This Amendment may be executed in one or
more counterparts, each of which when so executed shall be deemed to be an
original, but all of which when taken together shall constitute one and the same
instrument.

         Section 5.08. Effect of Waiver. No consent or waiver, express or
implied, by the Agent or any Bank to or for any breach of or deviation from any
covenant, condition or duty by Borrower

                                       -4-




<PAGE>   5






shall be deemed a consent or waiver to or of any other breach of the same or
any other covenant, condition or duty.

         Section 5.09. Headings. The headings, captions, and arrangements used
in this Amendment are for convenience only and shall not affect the
interpretation of this Amendment.

         Section 5.10. ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER
INSTRUMENTS, DOCUMENTS, AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH
THIS AMENDMENT EMBODY THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO WITH
RESPECT TO THE SUBJECT MATTER HEREOF AND THEREOF AND SUPERSEDE ANY AND ALL PRIOR
COMMITMENTS, AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR
ORAL, RELATING TO THIS AMENDMENT AND THE OTHER INSTRUMENTS, DOCUMENTS AND
AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH THIS AMENDMENT, AND MAY NOT
BE CONTRADICTED OR VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT
ORAL AGREEMENTS OR DISCUSSIONS OF THE PARTIES HERETO. THERE ARE NO ORAL
AGREEMENTS AMONG THE PARTIES HERETO.

         Executed as of the date first written above.

                                        BORROWER:

                                        VERITAS DGC INC.


                                        By: /s/ ANTHONY TRIPODO
                                           -----------------------------------
                                            Anthony Tripodo
                                            Executive Vice President
                                            and Chief Financial Officer


                                        AGENT:

                                        BANK ONE, TEXAS, N.A. 


                                        By: /s/ MARC A. DUNMIRE
                                           ------------------------------------
                                            Marc A. Dunmire 
                                            Vice President 




                                      -5-

<PAGE>   6






                                        BANKS:
                                        -----

                                        BANK ONE, TEXAS, N.A.

                                        By: /s/ MARC A. DUNMIRE
                                           ------------------------------------
                                            Marc A. Dunmire
                                            Vice President

                                        COMERICA BANK

                                        By: /s/ REGINALD M. GOLDSMITH, III
                                           ------------------------------------
                                            Reginald M. Goldsmith, III
                                            Vice President

                                        BANQUE NATIONALE DE PARIS

                                        By: /s/ WARREN ROSS
                                           ------------------------------------
                                            Warren Ross
                                            Vice President

         Each of the undersigned Guarantors hereby consents and agrees to this
Amendment and agrees that the General Continuing Guaranty executed by such party
is in full force and effect and shall remain in full force and effect and shall
continue to be the legal, valid and binding obligation of such Guarantor,
enforceable against such Guarantor in accordance with its terms and shall
evidence such Guarantor's guaranty of the Revolving Credit Notes.

                                        DIGICON GEOPHYSICAL CORP.

                                        By: /s/ ANTHONY TRIPODO
                                           ------------------------------------
                                            Anthony Tripodo
                                            Vice President


                                       -6-





<PAGE>   7






                                        DIGICON (MALAYSIA) SDN. BHD.

                                        By:
                                           ------------------------------------
                                        Name:
                                             ----------------------------------
                                        Title:
                                              ---------------------------------


                                        EUROSEIS, INC.


                                        By: /s/ ANTHONY TRIPODO
                                           ------------------------------------
                                            Anthony Tripodo
                                            Vice President

                                        VERITAS DGC ASIA PACIFIC, LTD.

                                        By: /s/ ANTHONY TRIPODO
                                           ------------------------------------
                                            Anthony Tripodo
                                            Vice President

                                        VERITAS DGC LAND INC.

                                        By: /s/ ANTHONY TRIPODO
                                           ------------------------------------
                                            Anthony Tripodo
                                            Vice President

                                        VERITAS DGC, LTD.

                                        By:
                                           -----------------------------------
                                            Nicholas A.C. Bright
                                            Director

                                       -7-




<PAGE>   8
 





                                        VERITAS ENERGY SERVICES PARTNERSHIP, 
                                        by its partners:

                                        VERITAS DGC LAND LTD.

                                        Per:
                                            -----------------------------------
                                            Lawrence C. Fichtner
                                            Director

                                        VERITAS GEOSERVICES LTD.

                                        Per:
                                            -----------------------------------
                                            Lawrence C. Fichtner
                                            Director

                                        CANEX INFORMATION SERVICES LTD.

                                        Per:
                                            -----------------------------------
                                            Lawrence C. Fichtner
                                            Director

                                           

                                      -8-